Multidimensional Array Malloc

**alvarito** · 09-30-2011

Hello

I am making my CPU burn hell and crash everytime I run this program, so there is some error either an infite loop or I dont know what, but it is meant to just assign on the fly from user input the number of rows, cols, and then i just fill content and print to see I did alright, but obviously, I didnt.

I d really appreciate if anybody could pinpoint where I am messing

Here it goes:

Code:

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






int main()  {
	
int **mat; // Pointer to pointer


int rows, cols, i, j, k;


printf("How many rows you want ");
scanf("%d", &rows);


mat = (int **)malloc(rows*sizeof(int*)); // array of number of rows


printf("How many cols ");
scanf("%d", &cols);
fflush(stdin);
for (i=0; i<rows; i++)  // for each row ...
{
 	
 	mat[i] = (int *)malloc (cols * sizeof(int)); // add these many cols
}


for (j = 0; j<=rows; j ++)
  {
 	
 	for (k = 0; k<=cols; k++)
 	{
 	
 	mat[j][k] = j * k;
 	
 	printf("so the values for the array ar %d: ", *(*(mat+j)+k));
 	
    }
    
 }
	getchar(); 
	
}

**Salem** · 09-30-2011

> fflush(stdin);
This is undefined behaviour - see the FAQ.
Also in the FAQ are some notes about not casting the result of malloc.

> for (i=0; i<rows; i++) // for each row ...
> for (j = 0; j<=rows; j ++)
What is different about these two loops?
Which one is correct for accessing the elements of an array?

**Adak** · 09-30-2011

Remove the = in your for loops stop check:

Code:

for(j=0;j<rows;j++)
   for(k=0;k<cols;k++) 
      //etc.

The last valid element of an array, is NumberOfElementsInThatDimension - 1. That is true for every dimension, of the array.

rows - 1
cols - 1
depth - 1 //if it were a 3D array
etc.

**alvarito** · 09-30-2011

Thank you, I overlooked that evidence. That avoided the crashing and seems to deliver the right values, I ll have a look at them but so far it pretty much allows me to examine results. Thanks a lot

best regards

Al

**alvarito** · 09-30-2011

Yes, I know what you mean, but in my development environment (windows and Dev C++, but I am moving to Linux asap) if I dont include that function, then the execution finishes right after the scanf, it does not go beyond and the console disappears before i get a chance to examine it.

**Adak** · 09-30-2011

Same basic program, just improved a bit.

Code:

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

int main(void)  {
   int **mat; // Pointer to pointer
   int rows, cols, i, j;

   printf("How many rows do you want? ");
   scanf("%d", &rows);
   getchar(); //remove newline from input stream

   //rows = 10;
   //cols = 10;

   mat = malloc(rows*sizeof(int*)); // array of number of rows

   printf("And how many columns? ");
   scanf("%d", &cols);
   getchar(); //removes the left over newline
 
   for (i=0; i<rows; i++) { // for each row ...
      mat[i] = malloc(cols * sizeof(int)); 
   }
   for (i = 0; i<rows; i++) {  //assign some test values
 	   for (j = 0; j<cols; j++) {
 	      mat[i][j] = i * j;
 	      //printf("%4d ", mat[i][j]); //same as below line of code
              printf("%4d ", *(*(mat+i)+j));
      }
      putchar('\n');
   }
   //free rows:
   for(i=0;i<cols;i++)
      free(mat[i];
   free(mat);     
   
   printf("\n\n"); 
   getchar(); //hold the terminal window open
   return 0;
}

**laserlight** · 09-30-2011

When you want a rectangular 2D array, another option to consider is to create a huge array of the objects that you want, then create another array of pointers to point to these objects at regular intervals. You then access this second array as a 2D array of the objects that you want. The advantage here is that you would only need to allocate memory twice. Building on Adak's example:

Code:

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

int main(void) {
    int *data;
    int **mat;
    int rows, cols, i, j;

    printf("How many rows do you want? ");
    scanf("%d", &rows);
    getchar(); /* remove newline from input stream */

    printf("And how many columns? ");
    scanf("%d", &cols);
    getchar(); /* removes the left over newline */

    /* Lazy, so assume that malloc always succeeds */
    data = malloc(rows * cols * sizeof(*data));
    mat = malloc(rows * sizeof(*mat));
    for (i = 0; i < rows; i++) {
        mat[i] = data + (i * cols);
    }

    /* Assign and print some test values */
    for (i = 0; i < rows; i++) {
        for (j = 0; j < cols; j++) {
            mat[i][j] = i * j;
            printf("%4d ", mat[i][j]);
        }
        putchar('\n');
    }

    /* Done, so cleanup */
    free(mat);
    free(data);

    printf("\n\n");
    getchar(); /* hold the terminal window open */
    return 0;
}

**Adak** · 09-30-2011

^^^^ Very interesting, Laserlight!

**BillyTKid** · 09-30-2011

Code:

    scanf("%d", &rows);
    getchar(); /* remove newline from input stream */
    /* this is not safe, test it, if you put a non-digit char: scanf will stop on this and getchar eat this, '\n' keeps in input buffer */
    /* better is always: */
    while(getchar()!='\n');
    ...

**anduril462** · 09-30-2011

Code:

      printf("so the values for the array ar %d: ", *(*(mat+j)+k));

This line is just begging for an error in pointer arithmetic. Just access you're 2-D array as a 2-D array:

Code:

      printf("so the values for the array ar %d: ", mat[j][k]);

Also note, you don't actually need k. It's a better idea to reuse i and j, keeping their usage consistent (i for rows, j for cols).

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