[Problem] String repeat(String s, int n) that creates and returns a new string

Hi eveyone

Question is :-

String repeat(String s, int n) that creates and returns a new string that is made by concatenating
n copies of the parameter string s. For example, calling this method with the parameters “Hello”
and 3 would return the string “HelloHelloHello”. If n equals zero, the method should return the
empty string.

I am a beginner in c and trying my hand at recursion .This code doesn't work.
Please guide me and also tell any clever/advance/optimise steps that i can use to code as a pro.

Code:

---

#include<stdio.h>
#include<conio.h>
#include<string.h>


char* Repeat(char *s,int n)
{
char *c;
char *e;
       
  if(n==0)
 {
        return " ";

 }
 
if(n==1)
 {
        c=strcpy(c,s);
          return c;
 }

  if(n>1)
 {

  e=Repeat(s,n-1);

  e=strcat(e,s);
  return e;
 }

}




void main()
{
        char a[]="Hello";
        char *b;
        int n;
        printf("Enter the no of times u want to repeat\n");
        scanf("%d",&n);
        b= Repeat(a,n);
        printf("%s",b);
 
   
    getch();
   
}

---

:confused:

Why do you have c, and e just appearing like magic? You need some variables, and probably more importantly, you need some space to put these strings. Do you know malloc? Now might be a good time to learn.

1) Go into your compiler settings and turn the warning levels all the way up.
2) When you compile your code, treat each and every warning as an error to be corrected.

You'll be surprised how smart your compiler is about helping you write decent code.

For example... your variable e ... you just make that up, never declare it, don't initialize it and simply start using it... Your compiler should be complaining about that...

Quote:

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Originally Posted by tabstop

Why do you have c, and e just appearing like magic? You need some variables, and probably more importantly, you need some space to put these strings. Do you know malloc? Now might be a good time to learn.

---

yes i know malloc
but not getting the use here...
could please edit the function ...i will try to learn/understand by that code...

Quote:

---

Originally Posted by CommonTater

1) Go into your compiler settings and turn the warning levels all the way up.
2) When you compile your code, treat each and every warning as an error to be corrected.

You'll be surprised how smart your compiler is about helping you write decent code.

For example... your variable e ... you just make that up, never declare it, don't initialize it and simply start using it... Your compiler should be complaining about that...

---

I am using dev c++ can u tell me for that where and how to ?
and also please tell some compiler for win 7

Quote:

---

Originally Posted by tarunjain07

I am using dev c++ can u tell me for that where and how to ?
and also please tell some compiler for win 7

---

You really need to get a new compiler. Take a look at One of the many compiler threads. <------Click there

Nope, we can not edit the function. You have to do the learning and the coding. We just give you a nudge in the right direction when you're stuck. Here are a few pointers for you:

1. Work the problem out with paper and pencil first, and run through several examples by hand.
2. Count the characters in "Hello" (remember the null), then count the characters in "HelloHelloHello" (remember the null again). Try some other tests. What is the relationship between their lengths? That will tell you how to take your original string, and n, and malloc the right number of bytes for your new string.
3. Translate that into code, little by little, testing as you go. Perhaps start with an iterative (loop) solution, then make it recursive.
4. If you're still stuck, come back and post what you tried.

Quote:

---

Originally Posted by tarunjain07

I am using dev c++ can u tell me for that where and how to ?
and also please tell some compiler for win 7

---

First... if you want to know something about a specific IDE... that's what help files are for.

Dev C++ has been abandoned for quite some time. It's compiler is very out of date and you should look to udating to something current. For C-99 I generally recommend Pelles C <--- click the link!

Quote:

---

Originally Posted by tarunjain07

yes i know malloc
but not getting the use here...
could please edit the function ...i will try to learn/understand by that code...

---

Before you even start trying to use malloc, you need to review your C textbooks/tutorials for the correct way to declare and use variables. You cannot simply make them up as you go!

Quote:

---

Originally Posted by CommonTater

Before you even start trying to use malloc, you need to review your C textbooks/tutorials for the correct way to declare and use variables. You cannot simply make them up as you go!

---

ok :) i will try and willl post back

Quote:

---

Originally Posted by CommonTater

Before you even start trying to use malloc, you need to review your C textbooks/tutorials for the correct way to declare and use variables. You cannot simply make them up as you go!

---

i know malloc ..as i have read my textbooks and topics like link list and tree etc..now as i felt that my programming is very weak..
so i am working on this
block by block ..started with recursion ..

Quote:

---

Originally Posted by tarunjain07

ok :) i will try and willl post back

---

Looking at your edited code I see you are declaring your strings as char* ... nope, not going to work. A pointer is useless without space to store the data in...

AND you are attempting to return a string from your function... Be very careful with this, it doesn't do what you seem to think it does...

FWIW... please don't back edit like that... post a fresh copy, leave the original post alone. Others reading the thread will see the edited code and the conversation following won't make any sense to them at all.

Quote:

---

i know malloc ..as i have read my textbooks and topics like link list and tree etc..now as i felt that my programming is very weak..
so i am working on this
block by block ..started with recursion ..

---

Simply reading the textbook isn't the way to learn C... Work through it page by page, do all the exercises and quizzes, play with the code (it's just as important that you learn what does not work), repeat as necessary, then move on to the next section. When simply reading most people's retention of information (called "comprehension") runs about 60%... if you take the time and work with the code as you learn you can probably drive that up to about 90%... There is a very real difference between "knowing" and "understanding"....

Quote:

---

Originally Posted by CommonTater

Looking at your edited code I see you are declaring your strings as char* ... nope, not going to work. A pointer is useless without space to store the data in...

AND you are attempting to return a string from your function... Be very careful with this, it doesn't do what you seem to think it does...

FWIW... please don't back edit like that... post a fresh copy, leave the original post alone. Others reading the thread will see the edited code and the conversation following won't make any sense to them at all.

---

ok..i got what u mean to say...i have to allocate memory by malloc and then return that to char * for storage...:)
i know thats really foolish of me but i am new :)

Quote:

---

Originally Posted by AndrewHunter

Real men never define acronyms; they understand them genetically - How to write unmaintainable code

---

it redirects to a site..
boss how to use this site as this has quotes and other stuff

Quote:

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Originally Posted by tarunjain07

it redirects to a site..
boss how to use this site as this has quotes and other stuff

---

That is just part of my signature/tag line. It is not directed towards you.

finally got it :)

Code:

---

#include<stdio.h>
#include<conio.h>
#include<string.h>
#include<stdlib.h>


char* Repeat(char *s,int n)
{
        if(n==0){return " ";}
       
        char *a=(char*)malloc(2*sizeof(s));

        strcat(a,s);

       


 char* p=Repeat(a,n-1);
        strcat(a,p);
        return a ;

}




int main()
{
        char a[]="Hello";
        char *b;
        int n;
        printf("Enter the no of times u want to repeat\n");
        scanf("%d",&n);
        b= Repeat(a,n);
        printf("%s",b);
 
   
    getchar();
   
}

---

Nope, you don't got it:

Code:

---

#include<stdio.h>
#include<conio.h> // I sincerely hope you're not still using Turbo C.  If so, time to upgrade.
// Pelles C, Code::Blocks with MinGW or even MSVC++ Express are much better, free options
#include<string.h>
#include<stdlib.h>


char* Repeat(char *s,int n)
{
    if(n==0) {
        return " ";  // this is not the empty string, it's a string with one space.  "" is the empty string (two double " with nothing between them)
    }
   
    char *a=(char*)malloc(2*sizeof(s));  // This line mallocs a new string every time, so you throw away the old ones.  This is a memory leak.
    // You also mallco the wrong size.  s is a char *, which always has the same size (probably 4 or 8 bytes).
    // I can't reliably repeat a string like "ABCDEFGHIJKLMNOPQRSTUVWXYZ" more than a few times.  Try it yourself.
    // Your malloc here needs to be based on strlen(a) from main (the length of the original string to repeat)


    strcat(a,s);


    char* p=Repeat(a,n-1);
    strcat(a,p);
    return a;
}


int main(void)  // Can't hurt to be explicit here.
{
    char a[]="Hello";
    char *b;
    int n;
    printf("Enter the no of times u want to repeat\n");
    scanf("%d",&n);
    b= Repeat(a,n);
    printf("%s",b);


    getchar();


    return 0;  // Main is supposed to return an int, so make it do so.
}

---

There are copious other errors. Your code is still quite broken. I don't think you fully followed my advice in post #7. Did you work this out by hand and do several examples on paper?

Quote:

---

Originally Posted by AndrewHunter

That is just part of my signature/tag line. It is not directed towards you.

---

LOL... maybe you should make the above part of your signature...

Quote:

---

Originally Posted by anduril462

Nope, you don't got it:

Code:

---

#include<stdio.h>
#include<conio.h> // I sincerely hope you're not still using Turbo C.  If so, time to upgrade.
// Pelles C, Code::Blocks with MinGW or even MSVC++ Express are much better, free options
#include<string.h>
#include<stdlib.h>


char* Repeat(char *s,int n)
{
    if(n==0) {
        return " ";  // this is not the empty string, it's a string with one space.  "" is the empty string (two double " with nothing between them)
    }
   
    char *a=(char*)malloc(2*sizeof(s));  // This line mallocs a new string every time, so you throw away the old ones.  This is a memory leak.
    // You also mallco the wrong size.  s is a char *, which always has the same size (probably 4 or 8 bytes).
    // I can't reliably repeat a string like "ABCDEFGHIJKLMNOPQRSTUVWXYZ" more than a few times.  Try it yourself.
    // Your malloc here needs to be based on strlen(a) from main (the length of the original string to repeat)


    strcat(a,s);


    char* p=Repeat(a,n-1);
    strcat(a,p);
    return a;
}


int main(void)  // Can't hurt to be explicit here.
{
    char a[]="Hello";
    char *b;
    int n;
    printf("Enter the no of times u want to repeat\n");
    scanf("%d",&n);
    b= Repeat(a,n);
    printf("%s",b);


    getchar();


    return 0;  // Main is supposed to return an int, so make it do so.
}

---

There are copious other errors. Your code is still quite broken. I don't think you fully followed my advice in post #7. Did you work this out by hand and do several examples on paper?

---

i didnt get about conio.h ? whats wrong to use this ??
yes i am using Pelle now...
Yes i tried 1 or 2 ...should i try more on paper ?

thanxs a lot for such a reply...really good for me...someone pointing out mistakes ...:)

Right off... this is a bad choice for recursion. It would work far better as a loop....

Code:

---

#include <stdio.h>
#include <string.h>


char * repeat(char *str, int times)
  { char *ret = calloc(times, (strlen(str) + 1));
    while (times-- > 0)
      strcat(ret,str);
    return ret; }



 int main(void)
{  char a[]="Hello";
    char *b;
    int n;
    printf("Enter the no of times u want to repeat\n");
    scanf("%d",&n);
    b = Repeat(a,n);
    printf("%s",b);
    free (b);
    getchar();
    return 0; }

---

Quote:

---

Originally Posted by CommonTater

Right off... this is a bad choice for recursion. It would work far better as a loop....

---

Perhaps you forgot:

Quote:

---

Originally Posted by tarunjain07

I am a beginner in c and trying my hand at recursion .

---

It is a bad case for general use, but it's fine for practice, and will highlight some problems and issues that go along with dynamic memory allocation and recursion, as well as seeing just how slow lots of successive strcat calls can be. Neither of which the classic Fibonacci sequence example and the like will do.

Quote:

---

Originally Posted by anduril462

It is a bad case for general use, but it's fine for practice, and will highlight some problems and issues that go along with dynamic memory allocation and recursion, as well as seeing just how slow lots of successive strcat calls can be. Neither of which the classic Fibonacci sequence example and the like will do.

---

Point taken .... We need to know what does not work as well as what does.

Quote:

---

Originally Posted by tarunjain07

i didnt get about conio.h ? whats wrong to use this ??

---

Nothing terribly wrong with it, though I don't see any conio.h specific functions, so you don't need it. Also, much of the time I see it, it's a sign people are using the undead Turbo C, which is a piece of crap.

Quote:

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yes i am using Pelle now...

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Good, nothing to worry about.

Quote:

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Yes i tried 1 or 2 ...should i try more on paper ?

---

3-5 is probably better, but since you did try a few, I think your problem is not a full understanding of memory allocation or recursion. Here's some pseudo-code for what I would do:

Code:

---

// s is the string to repeat, n is how many times
char *repeat(char *repeated, char *original, int n)
{
    if n == 0
        base case, make an empty string
   
    strcat original onto repeated
    recurse with repeated, original and n-1
}

int main(void)
{
    char original[80];  // let the user enter a string to repeat, this makes testing easier too
    char *repeated;  // to hold the allocated memory with repeated string
    int n;

    print "Enter a string to repeat"
    read original
    print "Enter how many times to repeat"
    read n

    repeated = malloc enough for n copies of original + 1 for the null terminator, you work out the math
    repeat(repeated, original, n);

    print "The repeated string is", repeated

    free(repeated);  // free up memory when you're done

    return 0;
}

---

Read it carefully and study it. There is a way to do this with the malloc call in repeat(), but it is a bit more complicated, so don't bother with that yet until you understand this one 100%.