recursive string operation..

I need to build a function which gets two strings checks if their chars are alpha type
in ascending order.
If they both are then it needs to merge them in a third d string.

I cant call external functions.

The whole process must be in one function
and no iterative loops
and only in a recursive way.

I tried to build this function.

i think it works
but just after i input
abc in the first string and def into the second one

it does nothing and says that my program the EXE file
encountered a problem
and needs to be closed

why??

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", result);
    }
    else
    {
        printf("input is invalid.\n");
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
 {
      int res,index;
      int res2;
      //start checking apha chars for string 1
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
    {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res=merge_strings(str1, index++,str2,index2, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 1

      //start checking apha chars for string 2
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
      {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res2=merge_strings(str1, index,str2,index2++, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 2

    if ((res==1)&&(res2==1))
    {
        result[index3]=str1[index];
        if(index<255)
        {
            res=merge_strings(str1, index++,str2,index2, result, index3++);
        }

        result[index3]=str2[index2];
        if(index2<255)
        {
            res=merge_strings(str1, index,str2,index2++, result, index3++);
        }

    }
    return 1;
}

---

this function is without the ascending order check
i wanted to check one part

IMO, revisit the algorithm you have tried to to code because there are a few things you seem to have left out from the program implementation.
Also it's way better to compare chars instead of their value as in str1[index1]>= 'A' is the preferred way instead of str1[index1]>= 65. Just my 2c.

what things i leftout?

Run-Time Check Failure #3 - The variable 'index' is being used without being initialized.
Listen to your compiler warnings!

Code:

---

    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
    {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res=merge_strings(str1, index++,str2,index2, result, index3);
    }
    else
    {
      return 0;
    }

---

Could we not do this a bit simpler, since there is two exactly identical checks here...

--
Mats

Quote:

---

Originally Posted by Elysia

Run-Time Check Failure #3 - The variable 'index' is being used without being initialized.
Listen to your compiler warnings!

---

i have zero warnings.

index3 does not need to be initialized because it comes from
the signature of the function.

Quote:

---

Originally Posted by transgalactic2

what things i leftout?

---

For starters you need to loop through the string comparing each element with the next one and making sure that they are in alphabetically ascending order.
ATM you are not doing, that is, not comparing adjacent elements to make sure that each element is an alphabet and is lexically less than its successor.

index does not come from the signature of your function, and in merge_strings you most definitely use it without it being initialized.

Quote:

---

Originally Posted by transgalactic2

i have zero warnings.

---

Then turn up the warnings! Uninitialized variables are quite serious.

Quote:

---

index3 does not need to be initialized because it comes from the signature of the function.

---

Not talking about index3. Talking about index.

Quote:

---

Originally Posted by matsp

Code:

---

    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
    {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res=merge_strings(str1, index++,str2,index2, result, index3);
    }
    else
    {
      return 0;
    }

---

Could we not do this a bit simpler, since there is two exactly identical checks here...

--
Mats

---

The first "if" is the stop case were it reaches the end of the string.
The second "if" is for keep checking if the chars till now are correct and it checks the
next one.

i cant think of doing it in another way.

where is the problem in the algorithm?

Quote:

---

Originally Posted by transgalactic2

The first "if" is the stop case were it reaches the end of the string.
The second "if" is for keep checking if the chars till now are correct and it checks the
next one.

i cant think of doing it in another way.

where is the problem in the algorithm?

---

boc how do you make sure that the string is in ascending order? compare adjacent elements perhaps; and i think matsp is suggesting code modularization.

Quote:

---

Originally Posted by transgalactic2

The first "if" is the stop case were it reaches the end of the string.
The second "if" is for keep checking if the chars till now are correct and it checks the
next one.

i cant think of doing it in another way.

where is the problem in the algorithm?

---

The problem is that it's too complicated. So check the common thing first, then use a separate if-statement inside that to avoid rechecking four rather complicated conditions.

Also, you can't call any external function, but as I read it, you should be able to call YOUR OWN LOCAL functions. So perhaps you can write one that checks if a char is a letter A-Z, a-z. And please use character constants rather than 65, 90, 97, 122, etc. It makes it MUCH easier to read if some constant is 'A' than 65.

--
Mats

Quote:

---

Originally Posted by tabstop

index does not come from the signature of your function, and in merge_strings you most definitely use it without it being initialized.

---

i changed

Code:

---

int index=0;

---

now when input abc in the first string and def in the second
it end without showing anything
??

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", result);
    }
    else
    {
        printf("input is invalid.\n");
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
 {
      int res,index=0;
      int res2;
      //start checking apha chars for string 1
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
    {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res=merge_strings(str1, index++,str2,index2, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 1

      //start checking apha chars for string 2
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
      {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res2=merge_strings(str1, index,str2,index2++, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 2

    if ((res==1)&&(res2==1))
    {
        result[index3]=str1[index];
        if(index<255)
        {
            res=merge_strings(str1, index++,str2,index2, result, index3++);
        }

        result[index3]=str2[index2];
        if(index2<255)
        {
            res=merge_strings(str1, index,str2,index2++, result, index3++);
        }

    }
    return 1;
}

---

i didnt check for ascending order i need this part to work so i will start working on the
ascending order part

Newsflash for you: stack overflow!
You do nothing but recursively call your function over and over again.

Quote:

---

Originally Posted by transgalactic2

i didnt check for ascending order i need this part to work so i will start working on the
ascending order part

---

gotcha!

i cant use my own external functions either

What is your exit condition for merging the strings?

--
Mats

i check index<255
if its larger then its not going further.

define your terminal condition when you reach the end of the string to prevent stack overflow.

My opinion is that you need to redo this whole code / function. It's way too complicated and has ands and ors put out without any ( and ), even. And what does all those indexes have to do with things?
Why don't you start by writing it all in pseudo code?

i changed the first if in the merging part to
index1<255
but its still gives me the same result

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", result);
    }
    else
    {
        printf("input is invalid.\n");
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
 {
      int res,index=0;
      int res2;
      //start checking apha chars for string 1
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
    {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res=merge_strings(str1, index++,str2,index2, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 1

      //start checking apha chars for string 2
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
      {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res2=merge_strings(str1, index,str2,index2++, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 2

    if ((res==1)&&(res2==1))
    {
        result[index3]=str1[index];
        if(index1<255)
        {
            res=merge_strings(str1, index++,str2,index2, result, index3++);
        }

        result[index3]=str2[index2];
        if(index2<255)
        {
            res=merge_strings(str1, index,str2,index2++, result, index3++);
        }

    }
    return 1;
}

---

Quote:

---

Originally Posted by transgalactic2

i cant use my own external functions either

---

I'm not talking about EXTERNAL functions - or do you mean to say that the assignment clearly says that this has to be done in ONE FUNCTION? If so, I'd say it's worse than learning to tie your shoe-laces with one hand - you are now having to do it with one hand and your thumb and forefinger tied together with a sticking plaster. That is, it is utter lunacy to do this in ONE FUNCTION.

Edit: I would double-check with your teacher (or whatever) that you are actually supposed to do all of this in ONE FUNCTION. And if that is the case, go look for another school and ask for your money back [assuming you paid for the course], as it is not at ALL a good course.

--
Mats

Quote:

---

Originally Posted by itCbitC

define your terminal condition when you reach the end of the string to prevent stack overflow.

---

i did define a terminal condition

its

Code:

---

if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
    {

        return 1;
    }

---

and

Code:

---

if(index1<255)

---

Quote:

---

Originally Posted by transgalactic2

i check index<255
if its larger then its not going further.

---

redo the code with proper indentation; index is a local variable and everytime merge_strings() is invoked index is reset to zero due to this int res,index=0; in merge_strings().

Eh, I give up. transgalactic2 is impossible to teach anyway. As usual.

Quote:

---

Originally Posted by matsp

I'm not talking about EXTERNAL functions - or do you mean to say that the assignment clearly says that this has to be done in ONE FUNCTION? If so, I'd say it's worse than learning to tie your shoe-laces with one hand - you are now having to do it with one hand and your thumb and forefinger tied together with a sticking plaster. That is, it is utter lunacy to do this in ONE FUNCTION.

--
Mats

---

I completely agree with you :)

I could write external function
and later merge it with this one.

but the point is the recursion
i cant see where i going wrong.

Quote:

---

Originally Posted by itCbitC

redo the code with proper indentation; index is a local variable and everytime merge_strings() is invoked index is reset to zero due to this int res,index=0; in merge_strings().

---

i changed the index=0 to index
now its giving me the exe error again

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", result);
    }
    else
    {
        printf("input is invalid.\n");
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
 {
      int res,index;
      int res2;
      //start checking apha chars for string 1
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
    {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res=merge_strings(str1, index++,str2,index2, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 1

      //start checking apha chars for string 2
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
      {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res2=merge_strings(str1, index,str2,index2++, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 2

    if ((res==1)&&(res2==1))
    {
        result[index3]=str1[index];
        if(index1<255)
        {
            res=merge_strings(str1, index++,str2,index2, result, index3++);
        }

        result[index3]=str2[index2];
        if(index2<255)
        {
            res=merge_strings(str1, index,str2,index2++, result, index3++);
        }

    }
    return 1;
}

---

Code:

---

      int res,index;
      int res2;
      //start checking apha chars for string 1
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))

---

index is not set to any value at this point...

--
Mats

Quote:

---

Originally Posted by itCbitC

redo the code with proper indentation; index is a local variable and everytime merge_strings() is invoked index is reset to zero due to this int res,index=0; in merge_strings().

---

i was told here that index=0 is a problem
so i removed it

do i need to put index=0
??

Quote:

---

Originally Posted by transgalactic2

i was told here that index=0 is a problem
so i removed it

do i need to put index=0
??

---

i didnt understand him correctly.
i will change the code

i changed it
like itCbitC said.
i dont get the exe error but i dont have any output either

??

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", result);
    }
    else
    {
        printf("input is invalid.\n");
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
 {
      int res,index=0;
      int res2;
      //start checking apha chars for string 1
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
    {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res=merge_strings(str1, index1++,str2,index2, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 1

      //start checking apha chars for string 2
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index==254))
      {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res2=merge_strings(str1, index1,str2,index2++, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 2

    if ((res==1)&&(res2==1))
    {
        result[index3]=str1[index];
        if(index1<255)
        {
            res=merge_strings(str1, index1++,str2,index2, result, index3++);
        }

        result[index3]=str2[index2];
        if(index2<255)
        {
            res=merge_strings(str1, index1,str2,index2++, result, index3++);
        }

    }
    return 1;
}

---

So, now the question is: Where does index change? And what do you actually use index for in your function?

--
Mats

i removed index. and i changed the code to the proper "index" from the signature

it doesnt give me an output at all

??

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", result);
    }
    else
    {
        printf("input is invalid.\n");
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
 {
      int res;
      int res2;
      //start checking apha chars for string 1
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index1==254))
    {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res=merge_strings(str1, index1++,str2,index2, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 1

      //start checking apha chars for string 2
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index1==254))
      {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res2=merge_strings(str1, index1,str2,index2++, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 2

    if ((res==1)&&(res2==1))
    {
        result[index3]=str1[index1];
        if(index1<255)
        {
            res=merge_strings(str1, index1++,str2,index2, result, index3++);
        }

        result[index3]=str2[index2];
        if(index2<255)
        {
            res=merge_strings(str1, index1,str2,index2++, result, index3++);
        }

    }
    return 1;
}

---

So, AGAIN: What is your exit condition. What do you have to do to get there?

--
Mats

this is my exit condition

Code:

---

if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index1==254))
    {

        return 1;
    }

---

it returns 1 when all the chars are ok.

in the merging part when the index reaches 255 it doesnt go to the if again
thus stopping the recursion and returning 1

Doesn't merging mean that you pick characters from one or another string so that the resulting string would also be in ascending order? Otherwise what is the purpose of checking whether the input strings are in ascending order?

Or is this task simply a collection of pretty arbitrary requirements (I don't see any relevance in the A...Z check) in hope that no other teacher has ever asked anybody to write such a bizarre program (and you can't search for the code on the internet)?

Quote:

---

this is my exit condition

---

This exit condition would appear to be always false (the last character in the max-length string shouldn't be anything other than '\0'), or you have already passed the end of the string and the result of that is undefined...

Also, could you for a start write this using a simple loop. It is usually trivial to turn a single loop into a recursion.

Quote:

---

Originally Posted by transgalactic2

this is my exit condition

Code:

---

if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index1==254))
    {

        return 1;
    }

---

it returns 1 when all the chars are ok.

in the merging part when the index reaches 255 it doesnt go to the if again
thus stopping the recursion and returning 1

---

And how long is the string you have entered?

--
mats

Quote:

---

Originally Posted by anon

Doesn't merging mean that you pick characters from one or another string so that the resulting string would also be in ascending order? Otherwise what is the purpose of checking whether the input strings are in ascending order?

Or is this task simply a collection of pretty arbitrary requirements (I don't see any relevance in the A...Z check) in hope that no other teacher has ever asked anybody to write such a bizarre program (and you can't search for the code on the internet)?



This exit condition would appear to be always false (the last character in the max-length string shouldn't be anything other than '\0'), or you have already passed the end of the string and the result of that is undefined...

---

i do need to pick up from the last place of the first string
thats why i put index3++ on both calls

ok i understand that str2 is written over str1 places

i dont know how to fix it

Quote:

---

Originally Posted by transgalactic2

i removed index. and i changed the code to the proper "index" from the signature

it doesnt give me an output at all

??

Code:

---

//start checking apha chars for string 2
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index1==254))

---

---

what do you think should go here? str1 or str2?
maybe that's why you are not getting any output at all.
post the entire code again, one more time (hopefully better indented).

So you want to copy first string and append the other? Can you do this with a loop? The other things like testing for order and character values is just a distraction that has little to do with the recursion (except perhaps you have to reserve the return value for signalling error).

Also, does it have to be all just one function, besides main?

Otherwise it might go something like this:

Code:

---


int recursive_copy_with_weird_extra_conditions(char** result, const char* input);

int funny_merge(const char* first, const char* second, char* result)
{
    return recursive_copy_with_weird_extra_conditions(&result, first) ||recursive_copy_with_weird_extra_conditions(&result, second);
}

---

Quote:

---

Originally Posted by anon

Also, does it have to be all just one function, besides main?

---

I asked that very same question.

I wrote a pretty simple piece of code that does the required work here - it's about 70 lines including main. But it uses 4 functions (which could be reduced a bit, since I have one function simply to check if it's a letter in the right range). It does ALL of the work using recursion (except for reading the string, where I cheated and used fgets()).

And to paraphrase Salem, this is like tying your shows using only your left hand - it's nothing useful to learn [unless someone has chopped off your right hand of course].

--
Mats

i did itCbitC change
its not giving me an output

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", result);
    }
    else
    {
        printf("input is invalid.\n");
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
 {
      int res;
      int res2;
      //start checking apha chars for string 1
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index1==254))
    {
          return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {
          res=merge_strings(str1, index1++,str2,index2, result, index3);
    }
    else
    {
        return 0;
    }
    //end checking apha chars for string 1

      //start checking apha chars for string 2
    if ((((str2[index2]>= 65) &&(str2[index2]<= 90))||((str2[index2]>= 97) &&(str2[index2]<= 122)))&&(index2==254))
    {

        return 1;
    }
    if (((str2[index2]>= 65) &&(str2[index2]<= 90))||((str2[index2]>= 97) &&(str2[index2]<= 122)))
    {

        res2=merge_strings(str1, index1,str2,index2++, result, index3);
    }
    else
    {
        return 0;
    }
    //end checking apha chars for string 2

    if ((res==1)&&(res2==1))
    {
        result[index3]=str1[index1];
        if(index1<255)
        {
            res=merge_strings(str1, index1++,str2,index2, result, index3++);
        }

        result[index3]=str2[index2];
        if(index2<255)
        {
            res=merge_strings(str1, index1,str2,index2++, result, index3++);
        }

    }
    return 1;
}

---

Quote:

---

Originally Posted by matsp

I asked that very same question.

I wrote a pretty simple piece of code that does the required work here - it's about 70 lines including main. But it uses 4 functions (which could be reduced a bit, since I have one function simply to check if it's a letter in the right range). It does ALL of the work using recursion (except for reading the string, where I cheated and used fgets()).

And to paraphrase Salem, this is like tying your shows using only your left hand - it's nothing useful to learn [unless someone has chopped off your right hand of course].

--
Mats

---

what functions to write??

could i merge them later into 1?

Quote:

---

Originally Posted by transgalactic2

what functions to write??

could i merge them later into 1?

---

Anything that is ever repeated in code (that is more than one or two lines) can be made into a separate function. In this case, I would check if the character is alpha in a function, for example - or better yet, check if the whole string is alpha [that's what I did when I solved your task last night (it's no good talking about solutions unless you have actually solved the problem, and I don't think I've ever written string manipulation recursively before) - I did the check if the string is ALL alpha into a function, that also checks that the characters are in the right order (which I made a bit messy)].

--
Mats

i have written this
external function.

Code:

---

int check_valid(char ch)
{
    if (((ch>= 'a')&&(ch<='z'))||((ch>='A') &&(ch<='Z')))
    {
        return 1;
    }
    else
    {
        return 0; 
    }
}

---

i dont know how to integrate it into the big function in a recursive way.
if i could do loops then i simply

Code:

---

int status,flag=-1;
for(i=0;i<index1;i++)
{
    status=check_valid(str1[i]);
    if (status==0)
    {
        flag=0;
    }
}
if (flag==0)
{
    statut=0;  //i am using status again but for the use of putting the final result of a string
}
else
{
  status=1;
}

---

but its a recursion
i cant put this code in the loop.

Recursion IS a loop - it just uses a different mechanism to jump to the code...

--
Mats

how do i think of this mechanism

i built a function in the start of this post
i changed it later because there was some logical errors that i didnt mean to commit
but still i dont know what the problem in its algorithm

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", result);
    }
    else
    {
        printf("input is invalid.\n");
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
 {
      int res;
      int res2;
      //start checking apha chars for string 1
    if ((((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))&&(index1==254))
    {

        return 1;
    }
    if (((str1[index1]>= 65) &&(str1[index1]<= 90))||((str1[index1]>= 97) &&(str1[index1]<= 122)))
    {

        res=merge_strings(str1, index1++,str2,index2, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 1

      //start checking apha chars for string 2
    if ((((str2[index2]>= 65) &&(str2[index2]<= 90))||((str2[index2]>= 97) &&(str2[index2]<= 122)))&&(index2==254))
      {

        return 1;
    }
    if (((str1[index2]>= 65) &&(str2[index2]<= 90))||((str2[index2]>= 97) &&(str2[index2]<= 122)))
    {

        res2=merge_strings(str1, index1,str2,index2++, result, index3);
    }
    else
    {
      return 0;
    }
    //end checking apha chars for string 2

    if ((res==1)&&(res2==1))
    {
        result[index3]=str1[index1];
        if(index1<255)
        {
            res=merge_strings(str1, index1++,str2,index2, result, index3++);
        }

        result[index3]=str2[index2];
        if(index2<255)
        {
            res=merge_strings(str1, index1,str2,index2++, result, index3++);
        }

    }
    return 1;
}

---

If we split the problem into parts: Check that all letters are alphanumeric - how would you go about doing that, using recursion?

Think about it.
Start by thinking of the exit condition: When are you done?
The second step is to figure out how you split a check the letters into something you can do recurisvely - this can take the form "Test the first letter". How do you know if the rest of the string is alphanumeric?

By the way, have you checked with your tutor if you MUST do this with one function, or if you are allowed multiple functions as long as they fulfill the criteria of not calling outside functions and always using recursion?

--
Mats

i need to use do this whole operation in one function
using only recursion

i will try to build it again

i built this simple recursive function for checking only the first string

why its not working??

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", result);
    }
    else
    {
        printf("input is invalid.\n");
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
{
    int check;
    if (str1[index1]=='\0')
    {
          return 1;
    }

    if (((str1[index1]>='a')&&(str1[index1]<='z'))||((str1[index1]>='A')&&(str1[index1]<='Z')))
    {
        check=merge_strings(str1,index1++,str2,index2,result,index3);
        if (check==0)
        {
            return 0;
        }
        else
        {
            return 1;
        }
    }
    else
    {
        return 0;
    }


}

---

Try tracing your function by adding a printf() to see what is going into your merge_string() function. [Or if you have a debugger available, set a breakpoint at the beginning of the function.

--
Mats

ok this simple function works

the problem is with the return
i built it so it will return 1 or 0 depending if the string is correct

my return needs to be associated with the total resolt
how do i change it so i will get the resolt in one local variable??

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");

 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
       
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", input);
    }
    else
    {
        printf("%s is invalid.\n",input);
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
{
    int check;
    if (str1[index1]=='\0')
    {
          return 1;
    }

    if (((str1[index1]>='a')&&(str1[index1]<='z'))||((str1[index1]>='A')&&(str1[index1]<='Z')))
    {
        check=merge_strings(str1,index1+1,str2,index2,result,index3);
        if (check==0)
        {
            return 0;
        }
        else
        {
            return 1;
        }
    }
    else
    {
        return 0;
    }


}

---

on one hand I need a "return" to return the resolt for the next char
but on the other hand I need to remove it and put a local variable to collect the resolt
??

this input worked

i finished the ascii checking part its not returning anything just puting 1 into
the local variable
if all checks will be correct i will return 1.

how to do the ascending checking part ??

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");

 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
       
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", input);
    }
    else
    {
        printf("%s is invalid.\n",input);
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
{
    int check,check2;
    if (str1[index1]=='\0')
    {
          return 1;
    }

    if (((str1[index1]>='a')&&(str1[index1]<='z'))||((str1[index1]>='A')&&(str1[index1]<='Z')))
    {
        check=merge_strings(str1,index1+1,str2,index2,result,index3);
        if (check==0)
        {
            return 0;
        }
                else
                {
              check=1;
                }
    }
    else
    {
        return 0;
    }
////////////////////////////////////////////////////////
if (str2[index2]=='\0')
    {
          return 1;
    }

    if (((str2[index2]>='a')&&(str2[index2]<='z'))||((str2[index2]>='A')&&(str2[index2]<='Z')))
    {
        check2=merge_strings(str1,index1,str2,index2+1,result,index3);
        if (check2==0)
        {
            return 0;
        }
                else
                {
              check2=1;
                }
    }
    else
    {
        return 0;
    }



}//end function

---

i did the ascending part

i cant think of how to merge them recursevly

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");

 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
       
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", input);
    }
    else
    {
        printf("%s is invalid.\n",input);
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
{
    int check,check2,check3,check4;
    if (str1[index1]=='\0')
    {
          return 1;
    }

    if (((str1[index1]>='a')&&(str1[index1]<='z'))||((str1[index1]>='A')&&(str1[index1]<='Z')))
    {
        check=merge_strings(str1,index1+1,str2,index2,result,index3);
        if (check==0)
        {
            return 0;
        }
                else
                {
              check=1;
                }
    }
    else
    {
        return 0;
    }
////////////////////////////////////////////////////////
if (str2[index2]=='\0')
    {
          return 1;
    }

    if (((str2[index2]>='a')&&(str2[index2]<='z'))||((str2[index2]>='A')&&(str2[index2]<='Z')))
    {
        check2=merge_strings(str1,index1,str2,index2+1,result,index3);
        if (check2==0)
        {
            return 0;
        }
                else
                {
              check2=1;
                }
    }
    else
    {
        return 0;
    }
////////////////////////////////////////////////////////////////////////
if (str1[index1]=='\0')
    {
          return 1;
    }

    if (str1[index1]>=str1[index1-1])
    {
        check=merge_strings(str1,index1+1,str2,index2,result,index3);
        if (check==0)
        {
            return 0;
        }
                else
                {
              check3=1;
                }
    }
    else
    {
        return 0;
    }
////////////////////////////////////////////////////////////////
if (str1[index1]=='\0')
    {
          return 1;
    }

    if (str1[index1]>=str1[index1-1])
    {
        check=merge_strings(str1,index1+1,str2,index2,result,index3);
        if (check==0)
        {
            return 0;
        }
                else
                {
              return 1;
                }
    }
    else
    {
        return 0;
    }

}//end function

---

the problem with this asceding code
is that it checkson the first run cell -1

its a memory bug
??

how to fix it

why this code works????
i clearly need to have a buffer underrun
because in the first run i accsess cell -1

why its not giving me this bug??

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");

 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
       
    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", input);
    }
    else
    {
        printf("%s is invalid.\n",input);
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
{
    int check,check2,check3,check4;
    if (str1[index1]=='\0')
    {
          return 1;
    }

    if (((str1[index1]>='a')&&(str1[index1]<='z'))||((str1[index1]>='A')&&(str1[index1]<='Z')))
    {
        check=merge_strings(str1,index1+1,str2,index2,result,index3);
        if (check==0)
        {
            return 0;
        }
                else
                {
              check=1;
                }
    }
    else
    {
        return 0;
    }
////////////////////////////////////////////////////////
if (str2[index2]=='\0')
    {
          return 1;
    }

    if (((str2[index2]>='a')&&(str2[index2]<='z'))||((str2[index2]>='A')&&(str2[index2]<='Z')))
    {
        check2=merge_strings(str1,index1,str2,index2+1,result,index3);
        if (check2==0)
        {
            return 0;
        }
                else
                {
              check2=1;
                }
    }
    else
    {
        return 0;
    }
////////////////////////////////////////////////////////////////////////
if (str1[index1]=='\0')
    {
          return 1;
    }

    if (str1[index1]>=str1[index1-1])
    {
        check=merge_strings(str1,index1+1,str2,index2,result,index3);
        if (check==0)
        {
            return 0;
        }
                else
                {
              check3=1;
                }
    }
    else
    {
        return 0;
    }
////////////////////////////////////////////////////////////////
if (str1[index1]=='\0')
    {
          return 1;
    }

    if (str1[index1]>=str1[index1-1])
    {
        check=merge_strings(str1,index1+1,str2,index2,result,index3);
        if (check==0)
        {
            return 0;
        }
                else
                {
              return 1;
                }
    }
    else
    {
        return 0;
    }

}//end function

---

how do i say to it
skip the first cell

dont check the first cell [0] compared to the previous one?

how to do this ascending check
its not working

Code:

---

int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
{
if (str1[index1]=='\0')
        {
      return 1;
        }
        else
        {
      if(str1[index1]<=str1[index1+1])
          {
          check3=merge_strings(str1,index1+1,str2,index2,result,index3);
                  if (check3==0)
                  {
            return 0;
                  }
                  else
                  {
            printf("ok\n");
                        return 1;
                  }
          }
          else
          {
        return 0;
          }
        }
//end  ascending check
}//end function

---

First of all I think you're trying to cram too much into the same function, code modularization is what you need to do. matsp gave you the keys to the kingdom but you are not heeding his advice. Suggest you start all over again and do keep it simple.

i am keeping this simple thats why i am working only on the ascending part

Code:

---

int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
{
if (str1[index1]=='\0')
        {
      return 1;
        }
        else
        {
      if(str1[index1]<=str1[index1+1])
          {
          check3=merge_strings(str1,index1+1,str2,index2,result,index3);
                  if (check3==0)
                  {
            return 0;
                  }
                  else
                  {
            printf("ok\n");
                        return 1;
                  }
          }
          else
          {
        return 0;
          }
        }
//end  ascending check
}//end function

---

Can you post the entire updated code here.

actually i think i got the solution to the ascending part
i tried a various input and it gives me the correct answer
:)


the big problem is how to merge this two arrays into one
one after another into the big result array in a recursive way??

i cant image it

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");

 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';

    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n", input);
    }
    else
    {
        printf("%s is invalid.\n",input);
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
{
    int check,check2,check3,check4;
    if (str1[index1]=='\0')
    {
          return 1;
    }

    if (((str1[index1]>='a')&&(str1[index1]<='z'))||((str1[index1]>='A')&&(str1[index1]<='Z')))
    {
        check=merge_strings(str1,index1+1,str2,index2,result,index3);
        if (check==0)
        {
            return 0;
        }
                else
                {
              check=1;
                }
    }
    else
    {
        return 0;
    }
////////////////////////////////////////////////////////
if (str2[index2]=='\0')
    {
          return 1;
    }

    if (((str2[index2]>='a')&&(str2[index2]<='z'))||((str2[index2]>='A')&&(str2[index2]<='Z')))
    {
        check2=merge_strings(str1,index1,str2,index2+1,result,index3);
        if (check2==0)
        {
            return 0;
        }
                else
                {
              check2=1;
                }
    }
    else
    {
        return 0;
    }
////////////////////////////////////////////////////////////////////////
if (str1[index1+1]=='\0')
    {
          return 1;
    }

    if (str1[index1]>=str1[index1-1])
    {
        check=merge_strings(str1,index1+1,str2,index2,result,index3);
        if (check==0)
        {
            return 0;
        }
                else
                {
              check3=1;
                }
    }
    else
    {
        return 0;
    }
////////////////////////////////////////////////////////////////
if (str2[index2+1]=='\0')
    {
          return 1;
    }

    if (str2[index2]>=str2[index2-1])
    {
        check=merge_strings(str1,index1,str2,index2+1,result,index3);
        if (check==0)
        {
            return 0;
        }
                else
                {
              return 1;
                }
    }
    else
    {
        return 0;
    }

}//end function

---

i wrote this merging code

but its putting only the first one
why??

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';

    printf("enter the second string \n");

 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';

    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n",result);
    }
    else
    {
        printf("%s is invalid.\n",result);
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
{
    int check,check2,check3,check4;

    if(str1[index1]=='\0')
    {
      return 1;
    }
    else
    {
        result[index1]=str1[index1];
        check=merge_strings(str1,index1+1,str2,index2,result,index3);
    }

    if(str2[index2]=='\0')
    {
      return 1;
    }
    else
    {
        result[index2]=str2[index2];
        check=merge_strings(str1,index1,str2,index2+1,result,index3);
    }
    return 1;
}//end function

---

because when you read the stream for the second string, there is a \n awaiting to be read, resulted by the 'enter' key you pressed when submitting the first string. so that is your first read character.

the getchar clean the buffers

there is no problem there

i put printf after i entered them

both of them were printed fine

this merging code is not working

??

Code:

---

#include <stdio.h>
int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3);
int main() {
    char input[255];
    char input2[255];
    char result[510];
    int index,flag,ch,lnd;
    printf("enter the first string \n");
 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input[index] = ch;
    }

    input[index] = '\0';
 printf("%s is valid.\n",input);
    printf("enter the second string \n");

 for (index = 0; index < 254 && (ch = getchar()) != '\n' && ch >=0; ++index)
    {
        input2[index] = ch;
    }

    input2[index] = '\0';
 printf("%s is valid.\n",input2);


    flag=merge_strings(input, 0,input2,0, result, 0);

    if (flag)
    {
        printf("%s is valid.\n",result);
    }
    else
    {
        printf("%s is invalid.\n",result);
    }

return 0;
}


int merge_strings(char str1[], int index1,char str2[],int index2, char result[], int index3)
{
    int check,check2,check3,check4;

    if(str1[index1]=='\0')
    {
      return 1;
    }
    else
    {
        result[index1]=str1[index1];
        check=merge_strings(str1,index1+1,str2,index2,result,index3);
    }

    if(str2[index2]=='\0')
    {
      return 1;
    }
    else
    {
        result[index2]=str2[index2];
        check=merge_strings(str1,index1,str2,index2+1,result,index3);

    }
  return 1;
}//end function

---