malloc string array

Hi. Im quite new to c programming. Im trying to figure out how to use malloc. I saw a question similar to this in the forum but it was a good bit more complicated and I didnt really understand it.

I want to create a 1D array of ints from a file. I had a stab at it but the while loop isnt correct, am going about it the right way?

Code:

---

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



int main(void)
{
        FILE *fid;

        int * iarray;
        int n=0;
       
        fid = fopen("unsorted.txt", "r");                                                                       
        if (fid == NULL)
        {
                printf("Cannot open file\n");                                                       
                exit(EXIT_FAILURE);
        }
       
        iarray = (int *) malloc ( n * sizeof(int) );

        iarray[n] = fgets(fid);
        while( ( iarray[n] = fgets(fid) ) != EOF )
        /*or ?    while(fscanf(fid, "&#37;s", &iarray[n]) != EOF )*/
        {
       
                fscanf(fid, "%d", &iarray[n]);
                n++;
       
        }


       


        fclose(fid);
        printf("%d", iarray[n]);
       
        return(0);
}

---

You are using it correctly however for every element that you add you must malloc() storage which is being invoked only once.
n != 0 otherwise the call to malloc() will return a null pointer since you are essentially requesting no storage at all.
Changes to be made to the source are in red.

Code:

---

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



int main(void)
{
        FILE *fid;

        int * iarray;
        int n=0;        /* should not be initialized to zero */
       
        fid = fopen("unsorted.txt", "r");                                                                       
        if (fid == NULL)
        {
                printf("Cannot open file\n");                                                       
                exit(EXIT_FAILURE);
        }
       
        iarray = (int *) malloc ( n * sizeof(int) ); /* goes inside while loop because you need to store each new array element added */

        iarray[n] = fgets(fid);
        while( ( iarray[n] = fgets(fid) ) != EOF )
        /*or ?    while(fscanf(fid, "%s", &iarray[n]) != EOF )*/
        {
       
                fscanf(fid, "%d", &iarray[n]);
                n++;
       
        }


       


        fclose(fid);
        printf("%d", iarray[n]);
       
        return(0);
}

---

Thanks for your help. The problem now is I am making a string array and comparing this to an int. I messed around a lot and came up with the following line:

while( (a = atoi(fgets(iarray, 2, fid) )) != EOF )

It compiled with no errors but when i tried to run the program I got error while dumping....Could you help me with this while statement please?

What string array? You don't have a string array in your code anywhere. You have iarray, which is an array of integers that you are filling in an ... interesting way. You don't have a char array either.

Maybe you should start by figuring out what the program is supposed to do. If you're supposed to read integers from a file, you don't use fgets (unless you're ultra paranoid).

Ok.

Code:

---

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



int main(void)
{
        FILE *fid;

        int * iarray;
        int n=0;
       
        fid = fopen("unsorted.txt", "r");                                                               
        if (fid == NULL)
      {
                printf("Cannot open file\n");                                               
                exit(EXIT_FAILURE);
        }
       
       

        while( (fscanf(fid, "&#37;d", &iarray[n]) != EOF ) )
        {
                iarray = (int *) malloc ( n * sizeof(int) );
                /*fscanf(fid, "%d", &iarray[n]);*/
                n++;
       
        }
       

       


        fclose(fid);
        printf("%d\n", iarray[0]);  prints the last number in the file
        printf("%d\n", iarray[1]);  prints 0 ???
        printf("%d\n", iarray[2]);  prints 0 ???
        printf("%d\n", iarray[32]); prints 0 ???
        printf("%d\n",n);              33 - there are 33 ints in the file
       
        return(0);
}

---

What I take out of this is that the malloc part isnt working. My array will only take in 1 value, and keeps replacing this value with a new value until it exits the loop. Is this because you cannot update the array size after you have declared it once?

Each malloc gives you a new piece of memory, yes. If you want to resize the current piece of memory, you should use realloc.

I'm afraid itCbitC was a little off.
You cannot use a pointer before you allocate some memory!
The malloc should be before the loop and stored inside the pointer for the total number of elements you need. The formula is generally: n * sizeof(type), where n is the number of elements and type is the type of array.

I believe you will want to use realloc in the loop.
Note that at the moment you have a buffer overrun and a memory leak.

Now my printfs dont print anything

Code:

---

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



int main(void)
{
        FILE *fid;

        int * iarray;
        int n=0;
       
        fid = fopen("unsorted.txt", "r");               
        if (fid == NULL)
      {
                printf("Cannot open file\n");                                       
                exit(EXIT_FAILURE);
        }
       
        iarray = (int *) malloc ( (n+1) * sizeof(int) );

        while( (fscanf(fid, "&#37;d", &iarray[n]) != EOF ) )
        {       
                iarray = realloc(iarray, (n+1)*sizeof (int));
                n++;
               
               
       
        }
       

       


        fclose(fid);
       
        printf("%d\n", iarray[0]);
        printf("%d\n", iarray[1]);
        printf("%d\n", iarray[2]);
        printf("%d\n", iarray[32]);
        printf("%d\n",n);

        free(iarray);
        return(0);
}

---

btw iMalc on your website I cant get your "best bubsort" to show source code

Once you free(iarray), it's gone (or could be gone, or may be gone -- we don't know, is the point). So you would have to printf before you free anything.

Why would you free an array then try to print it's members?

Still doesnt print....

Code:

---

iarray = realloc(iarray, (n+1)*sizeof int);

---

If i knew what was wrong with that line would it be there?

The point is that is what the line should be -- you need n+1 ints, not n+1 bytes.

Doesnt work, ima give up. Thanks for all of your help

It's completely unnecessary since you allocate the required memory before.
Do you need it? Otherwise, remove it.

Quote:

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Originally Posted by Tayls

Doesnt work, ima give up. Thanks for all of your help

---

Thats the spirit! This one is worth bookmarking.

Well I dont understand how to use malloc in this particular way. I dont need it for my assignment i just wanted to try it. Im more getting comments saying what are you doing this why would you do that.....than actual help. If you tell me I have a buffer overrun and a memory leak, I search to try and find out what a buffer overrun means, sort of understand it, but still dont know how to fix it in this particular case.

You should check to see if realloc fails.

Ok, well first off many people here escribe to the logic of "why should I write sample code?" Which is never my attitude. Secondly, a lot of times it helps to know why since sometimes we may know a better how. Don't let any of us discourage you through critiquing your work. Your program seems to be off to a fine start. Just be aware of what it is doing.

You are allocating a buffer with malloc(). Then within a loop reallocating the buffer with realloc(). Which is actually perfectly acceptable code. However, it may not function as fast as if you were to only realloc() when you know you absolutely have to. I think a lot of people get it etched into their brain that realloc() is slow somewhere along their learning C process to such an extent that they lose site of the fact that using it in a loop is not actually wrong, nor will it increase the probability of a NULL return. Its just realloc() is a slow function is all.

Thanks for your encouragement. I am checking if realloc fails. Still get error while dumping...

Quote:

---

Originally Posted by master5001

Ok, well first off many people here escribe to the logic of "why should I write sample code?" Which is never my attitude. Secondly, a lot of times it helps to know why since sometimes we may know a better how. Don't let any of us discourage you through critiquing your work. Your program seems to be off to a fine start. Just be aware of what it is doing.

You are allocating a buffer with malloc(). Then within a loop reallocating the buffer with realloc(). Which is actually perfectly acceptable code. However, it may not function as fast as if you were to only realloc() when you know you absolutely have to. I think a lot of people get it etched into their brain that realloc() is slow somewhere along their learning C process to such an extent that they lose site of the fact that using it in a loop is not actually wrong, nor will it increase the probability of a NULL return. Its just realloc() is a slow function is all.

---

While all that's true, I brought the point up because (a) you should check whether malloc and realloc fail, 'cause otherwise bad things happen and (b) on this Win machine with this program, realloc always fails the fifth time. Always. I don't know why, because I surely have more than twenty bytes of memory, but there it is.

Quote:

---

Originally Posted by Elysia

I'm afraid itCbitC was a little off.
You cannot use a pointer before you allocate some memory!
The malloc should be before the loop and stored inside the pointer for the total number of elements you need. The formula is generally: n * sizeof(type), where n is the number of elements and type is the type of array.

---

You're correct Elysia I got the order mixed up. Storage should be allocated before assigning the integer read from input. Moreover malloc() does not specify the order and contiguity of storage allocated by successive calls to it so printing iarray by incrementing it in units of 1 won't display the stored numbers as they may or may not be in successive memory locations.

Any other ideas?

Quote:

---

Originally Posted by tabstop

While all that's true, I brought the point up because (a) you should check whether malloc and realloc fail, 'cause otherwise bad things happen and (b) on this Win machine with this program, realloc always fails the fifth time. Always. I don't know why, because I surely have more than twenty bytes of memory, but there it is.

---

No disagreement there.

Quote:

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Originally Posted by Tayls

Any other ideas?

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The only other alternative I can think of is using a linked list since you have to have malloc().
Others may have better ideas.

A bst... I dunno. I see more problems with how you are going about the task rather than the mechanical process of your methods. Linked lists would work fine too.