assignment of pointer...

Hi ,

I am unable to debug what is going with this program of reversing a string.

I had another doubt that is the assignment
*a = *p
valid because when i try to debug the program it gives error at this line in the program below that access violation writing at location whatever address is contained in a.


insert

Code:

---


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

void xstrrev(char *);

int main(){

        int i;
        static char *s[] = {
                "To err is human...",
                "But to really mess things up...",
                "One needs to know C!!"
        };

        for(i=0;i<3;i++){
                xstrrev(s[i]);
                printf("\n\r%s", s[i]);
        }
        return 0;
}


void xstrrev(char *a){

        int len,j;
        char *p, temp1, temp2;

        //printf("\n\r%c %d", *a,a);
        len = strlen(a);
        //printf("\n\r%d %d", len, len/2);
        p = a + len -1;
        //printf("\n\r%d %c", p, *p);
        for(j = 1;j<= len/2;j++){
       
                temp1 = *a;
                temp2 = *p;
                printf("\n\r %c %c",*p, *a);
                *a = *p;
                printf("\n\r1\t%c\t%c",*a,*p);
                // *p = temp;
                //printf("\n\r2\t%c\t%c", *p, temp);
                ++a;
                printf("\n\r3");
                --p;
                printf("\n\r4");
        }
}

---

You can't modify string literals, you've been told this before.

i.e, your "s[]" array contents cannot be legally modified in most implementations. Thus you should be making it constant. As for the other parts, "What the!?".

You are getting an access violation because your strings are stored in a non-writable part of memory. You need to actually make an array of C strings instead of an array of pointers to C strings. That way your C strings will be copied into the array.
Try this:

Code:

---

        char s[3][80] = {
                "To err is human...",
                "But to really mess things up...",
                "One needs to know C!!"
        };

---

For you reversal routine remember to uncomment your *p = temp and change it to *p = temp1. You have the right idea that you need to store *a before overwriting it with *p.

EDIT:
Also, when testing, use data that is easy to check, and make sure you test the strange "corner" cases. For example:

Code:

---

        char s[5][80] = {
                "1234",
                "123",
                "12",
                "1",
                ""
        };

---

Test with a 0 length string, test with a length of 1, test with an odd length, test with an even length. If it works for these, it will work for others.


Question, can someone provide a more elegant syntax for the char s array?

a printf("\n") is sufficient to generate a newline.

Quote:

---

Originally Posted by roaan

Hi ,

I am unable to debug what is going with this program of reversing a string.

I had another doubt that is the assignment
*a = *p
valid because when i try to debug the program it gives error at this line in the program below that access violation writing at location whatever address is contained in a.

---

I've explained this question here. After going through my post #20 if you still have any problem, then post the updated code and your query.

I am really in distress now after having spent so much time. Okay i tried using this modified approach but still it does not work . I don't understand what i am doing wrong now. Please ignore the printf statements as i was just trying to understand what it is doing at each step.

insert

Code:

---


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

void xstrrev(char *);

int main(){

        int i;
        static char *s[] = {
                "To err is human...",
                "But to really mess things up...",
                "One needs to know C!!"
        };


        for(i=0;i<3;i++){

                printf("\n\rBefore reversal%s", s[i]);
                xstrrev(s[i]);
        }
        return 0;
}

void xstrrev(char *a){

        int len,i,j;
        char *p, temp1, temp2;

        //printf("\n\r%c %d", *a,a);
        len = strlen(a);
        printf("\n\r%d %d", len, len/2);
        p = malloc(len+2);
        printf("\n %d", p);
               
        //printf("\n\r%d %c", p, *p);
        while(*a != '\0'){
                printf("\n %c", *a);
                a++;
        }
        for(i = len;i>=0;i--){
               
                *p = *a;
                printf("\n %c", *p);
                printf("\n %d", p);
                //printf("\n %c", *a);
                p++;
                a--;
        }

        *p = '\0';
        printf("\n%c", *p);
        printf("\n %d", p);
        p = p - (len+1);
        printf("\n %d", p);
        printf("\n\rAfter reversal%s", p);
}

---

This can never work:

Code:

---

static char *s[] = {
                "To err is human...",
                "But to really mess things up...",
                "One needs to know C!!"
        };

---

The contents of s are constant and can never change. If you want to change them, you must use something else. See Zlatko's earlier post for an alternative.

You are just using an array of string literals.
And as has been pointed out to you, string literals cannot be changed. End of story.

No but now i am not trying to reverse the string literal now i am trying to reverse it in a new string p (I am copying the reverse characters in p with that pointed to in the original string so i am not at all modifying the original string literal)

You start with the first char in the source string and go back...

OK, take a deep breath. It's going to be OK. I see that you've changed your approach, and are copying the string a into the allocated memory p but backwards. We can work with that.

Look at this code

Code:

---

        while(*a != '\0'){
                a++;
        }

---

You've incremented a until it points to the '\0'
Then you're putting the '\0' into the first location pointed at by p. You don't want your reversed string to start with a '\0'. Thats the first problem. Adjust 'a' before going on.

Then, how many characters do you want to copy. len, right?

Code:

---

        for(i = len;i>=0;i--){

                *p = *a;
                p++;
                a--;
        }

---

How many is the above loop copying? It's copying len + 1 because its going from len through 0.
Fix that.

Then you have

Code:

---

        *p = '\0';
        p = p - (len+1);

---

You've null terminated p, thats good. You want to move p back to the beginning of the string, so you want to move it back len characters.
Imagine the string abc in memory. It lools like this
'a' 'b' 'c' '\0'
P is pointing to the '\0'. Len is 3. Move p back 1, it points to c, move p back 2, it points to b, move p back 3 (len), it points to a, where you want it.
Fix that, and you're done.

HURRAY :-)!!!!!!!!!!!!!!!!!!!!!!
Its working now I am really thankful to you guys for helping me out.
And with that last post i have learnt how to debug using small example first and then using it on bigger things later.

Thanks !!!!!!!!!!!!!!!!!!!!!!!

Tonight, I'm having a drink for you!

Quote:

---

Originally Posted by tabstop

This can never work:

Code:

---

static char *s[] = {
                "To err is human...",
                "But to really mess things up...",
                "One needs to know C!!"
        };

---

The contents of s are constant and can never change. If you want to change them, you must use something else. See Zlatko's earlier post for an alternative.

---

Why not? We can assign a pointer a chunk of memory using malloc and then using strcpy copy the original string to the pointer and then it's no longer a "constant". I've done this using this approach.

Tabstop refers to that you can never change s. If you want to change s, you must use something else.
Of course you can copy it elsewhere and modify it.