Precise Syntax for Accessing Elements in a 2D Array - by Pointer?

Question

I'm a CS student working on a homework assignment, and I need help with a C syntax issue. Yesterday in class, my professor said, "an int** pointer is a pointer to a 2D int array." This blew my mind.

Turns out, we have to write a C program which reads an int matrix from a file, then do operations on that matrix. For example, "matrix1.txt" might look like this:

 1   2   3   4   5
 6   7   8   9  10
11  12  13  14  15

...for a 5x3 matrix. I get the dimensions of the matrix from another file, which is a problem I've already solved. But the point is I have to dynamically allocate the matrix array using variables.

Here's my issue: Its easy enough to use an int** pointer to malloc() an Y-by-X array... but what's the syntax to access it? Here's my code:

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

int main(int argc, char *argv[]){

    char *myFile = argv[1];
    FILE *targetFile;
    targetFile = fopen(myFile, "r");

    if (targetFile == NULL)
        return -1;
    else {
        // Successfully opened "matrix1.txt" file
        int x, y;                                           // dimensions of the array, learned elsewhere...
        int i, j;
        char* data = NULL;

        int** matrix = (int**)malloc((x*y)*sizeof(int));    // allocate memory for an Y-by-X array

        for(i=0; i<y; i++){
            for(j=0; j<x; j++){
                fscanf(targetFile, "%c ", &data);
                matrix[i][j] = atoi(data);                  // program seg faults here
            }
        }
    }
    fclose(targetFile);
    return 1;
}

The issue is the "matrix[i][j] = atoi(data);" line; I'm either using the wrong syntax or I haven't initialized the array. I can't tell which - the program seg faults IMMEDIATELY once I hit this line in the GDB debugger.

I'm sure this is a C 101 kind of question... but I post this because I've been reading a lot of different posts on 2D arrays and pointers, yet I can't seem to find an example that fits my precise situation. Can anyone help me out with this?

 Thanks,
   -ROA

BTW, fscanf(targetFile, "%c ", &data); will lead to problems after you fix the memory allocation issue. Just use an int variable and %d format for reading from the file. int data; ... fscanf(targetFile, "%d", &data);. — R Sahu
– R Sahu, Commented Sep 27, 2016 at 19:25
Your prof is another of those who confuse a pointer and an array. I'd wonder what other missconceptions he teaches. Said that, there is no 2D array in your question, nor something that can represent one of point to one. A pointer is not an array. — too honest for this site
– too honest for this site, Commented Sep 27, 2016 at 19:30
@RSahu: That answer does not show a 2D array. Nor does the proposed dup show one! The question clearly is confused what a 2D array is, resp how to use a pointer to it. — too honest for this site
– too honest for this site, Commented Sep 27, 2016 at 19:32
See here for some info about 2D arrays: stackoverflow.com/a/35615201/4774918. — too honest for this site
– too honest for this site, Commented Sep 27, 2016 at 19:35
@RSahu Thanks, I appreciate the assist. The post you pointed me to actually was pretty confusing... but that post pointed to a second post which nicely walked through the logic of the int** pointer. This is a tougher topic than I thought... — Pete
– Pete, Commented Sep 27, 2016 at 20:21

R Sahu · Accepted Answer · 2016-09-27 19:50:06Z

1

The syntax used in

matrix[i][j] = atoi(data);

is not incorrect. It's the logic used to allocate memory that is wrong.

One way to allocate memory for the 2D array is:

// Allocate memory for y number of int*s.
int** matrix = malloc(y*sizeof(int*));
for(i=0; i<y; i++)
{
   // Allocate memory for x number of ints.
   matrix[i] = malloc(x*sizeof(int));
   for(j=0; j<x; j++)
   {
      // Assign to the ints
      matrix[i][j] = <some value>;
   }
}

For reading the data, use

int data;

and

fscanf(targetFile, "%d", &data);

Then, the inner loop above can be updated to:

   for(j=0; j<x; j++)
   {
      // Assign to the ints
      fscanf(targetFile, "%d", &data);
      matrix[i][j] = data;
   }

Make sure to add code to release the dynamically allocated memory.

// Free the memory allocated for the ints
for(i=0; i<y; i++)
{
   free(matrix[i])
}

// Free the memory allocated for the int*s
free(matrix);

answered Sep 27, 2016 at 19:50

R Sahu

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2 Comments

Pete Over a year ago

Ahhhhhhhh...! This is awesome, thanks! I read the other posts, but this is very, very clear. Thanks, I love this. If you're ever in NJ, look me up, I owe you a beer... :)

R Sahu Over a year ago

@Pete, I appreciate the sentiment. Thanks.

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