Pointer as index of one array - C

Question

I'm having trouble to make the following algorithm work without index (with index it works). It's a small exercise for my university, and one of the requirements is that I can only use "pointer reference" to walk through my allocated array (frase), not index. What I've tried last to make it work is commented, and I give up because anything I try doesn't work... Thanks for the help.

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

int main(){
    char* frase;
    frase = malloc(sizeof(char) * 100);

    gets(frase);

    /*
    while(*frase != '\0'){
        printf("%c", *frase);
        frase++;
    }
    */
    int i = 0;
    while(frase[i]){
        printf("%c", toupper(frase[i]));
        i++;
    }

    printf("\n");
    free(frase);
}

What didn't work? The commented code seems OK. Except to the fact that you won't be able to free it afterwars, as you loose the original pointer. So you'll have to save it before incrementing. — Eugene Sh.
– Eugene Sh., Commented Sep 28, 2016 at 16:29
do while(*(frase + i)) and toupper(*(frase + i)). Change those to the ones I have written and try again :) — Derick Alangi
– Derick Alangi, Commented Sep 28, 2016 at 16:29
Eugene, with the commented code i was getting lot of crashs and my pc restarting, probably due to bad memory manage or something... — Willian Santos
– Willian Santos, Commented Sep 28, 2016 at 16:43

P.Bra · Accepted Answer · 2016-09-28 17:03:30Z

3

The problem is that you free a pointer which has changed (indeed, frase has been incremented using frase++) then free(frase) does not work.

edited Sep 28, 2016 at 17:03

answered Sep 28, 2016 at 16:41

P.Bra

2642 silver badges12 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Willian Santos Over a year ago

I agree that free is a problem, but my main problem was my while being a infinite loop until my pc crash... Anyway its solved now, thanks!

sokkyoku · Accepted Answer · 2016-09-28 18:19:59Z

3

By using "pointers only" you point to an element, process it and then increment the pointer to point to the next element:

char *pfrase = frase;
while(pfrase){
    printf("%c", toupper(pfrase));
    pfrase++;
}

edited Sep 28, 2016 at 18:19

sokkyoku

2,2411 gold badge22 silver badges22 bronze badges

answered Sep 28, 2016 at 16:42

Paul Ogilvie

25.3k4 gold badges25 silver badges43 bronze badges

1 Comment

Willian Santos Over a year ago

I used ur code within MD XF logic: while(*pfrase) , toupper(*pfrase) and *pfrase++, and its working well. Thanks!

R Sahu · Accepted Answer · 2016-09-28 17:00:00Z

The value of the argument passed to free must be a pointer that was returned by one of the malloc family of functions. If a different value is passed to free, the program has undefined behavior.

When you use:

frase = malloc(sizeof(char) * 100);

gets(frase);

while(*frase != '\0'){
   printf("%c", *frase);
   frase++;
}

free(frase);

You are calling free using a pointer whose value is different than the value returned by malloc. That is cause for undefined behavior.

One way to resolve this is what you have already done -- use an index to access the array.

Another way to resolve this is to use a different variable to iterate over the contents of the array.

frase = malloc(sizeof(char) * 100);

gets(frase);

// Don't modify frase. Use another variable to iterate
char* cp = frase;
while(*cp != '\0'){
   printf("%c", *cp);
   cp++;
}

free(frase);

Willian Santos · Accepted Answer · 2016-09-28 17:18:29Z

-1

With everyone responses, my algorithm is working now. Here is what its now:

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

      int main(){

           char* frase;
           frase = malloc(sizeof(char) * 100);

           gets(frase);

           char* pointer = frase;
           while(*pointer){
                printf("%c", toupper(*pointer));
                *pointer++;
                }


           printf("\n");
           free(frase);
           free(pointer);
           }

Both the problem of dont edit my "frase" array and the useless free problem seems to be solved. Also, i know about the gets problem, but this algorithm is for a exercise. Thanks everyone for the help!

answered Sep 28, 2016 at 17:18

Willian Santos

191 bronze badge

2 Comments

Paul Ogilvie Over a year ago

Ai!!! You can't free pointer because it was pointing to frase, and you freed frase already! - Remove the free(pointer);

Willian Santos Over a year ago

hmmm, now i see it. Thanks!

Collectives™ on Stack Overflow

Pointer as index of one array - C

4 Answers 4

1 Comment

1 Comment

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

1 Comment

1 Comment

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related