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.