Thread: Dynamically Allocated Array Troubles

I have been assigned a project for my class which requires me to read in values from stdin then dynamically allocate an array "a" based upon the number of characters. After that, I must print the array backwards.

**NOTE**
The input is being redirected from a .txt file.


Code:

Code:

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

int main(void)
{

char *a;         // Declare the array pointer
int CharCount; //counts the # of characters in 
                    // input file
int i;             // used to loop through stdin

while (getchar()!=EOF) CharCount++;

This is where my problem is, I simply do not understand the functions of attaining data from the input stream.

For example: After I determine the amount of characters in the input stream, I would then proceed to dynamically allocate array "a" based upon CharCount.

Code:

a = malloc(CharCount * sizeof(a));

if(!=a)
{
printf("Allocation failed. Exiting.\n");
exit(EXIT_FAILURE);
}

At this point I have to save the characters from stdin to the array, I am stuck at re-retrieving the characters from the beginning of the input file.

Example:

Code:

for (i=0;i<c;i++)
a[i]=getchar();

Just not sure how to make it read from the beginning of the input file.
If I choose to print the array, I get random characters, which I interpret as a problem with getchar()'s position in the input stream. If I can solve that problem, I can easily print the characters in reverse order.

Help would be greatly appreciated, I'm new to this stuff.

P.S. I google'd "How to return getchar() to beginning of input stream" to no avail.

Or, you can dynamically keep reallocating the space you need, and adding on the new character.

Quzah.[/QUOTE]

Maybe something like this:

Code:

int CharCount=0;
char c;
char *a;

while ((c=getchar()) != EOF)
   {
	   if (CharCount==0) {
		   a=malloc((CharCount + 1) * sizeof(a));
		   a[0]=c;
	   }
	   else {
		   a=realloc((CharCount + 1) * sizeof(a));
		   a[CharCount]=c;
	   }
	   CharCount++;
   }

I'm not sure exactly how to use the realloc() function.

Originally Posted by laserlight

You should read up on realloc: your example does not even call it with sufficient arguments.

Also, getchar() returns an int, so c should be an int, not a char.

Code:

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

int main(void)
{
   char *a;
   int c;
   int CharCount=0;
   int i=0;

   c=getchar();
   a=malloc((CharCount+1) * sizeof(a)); // 1 added to prevent 0 bytes being allocated
   a[0]=c;
   CharCount++;

   if (!a) {
	   printf("Allocation failed. Exiting.");
	   exit(EXIT_FAILURE);
   }

   while ((c=getchar()) != EOF)
   {
	   *a=realloc(NULL, CharCount * sizeof(a));
	   a[CharCount]=c;
	   CharCount++;
   }

I'm getting closer, the first character is wrong, which maybe can be fixed by converting int to char (?), and getchar() is reading 3 extra characters after the final character from input. I know this can easily be fixed by deleting the extra 3 characters from the array before printing but I want it to function as efficiently as possible.

You've both been a great help so far, thanks.

Thanks again for the help on this, I'm going to sleep on it and most likely post more questions tomorrow. If I get it running I'll post the final source up here.