CODEWITHSUNDEEP
carrayspointersmultidimensional-arraymalloc
Pete
Pete

Reputation: 1561

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

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

Upvotes: 1

Views: 81

Answers (1)

R Sahu
R Sahu

Reputation: 206707

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);

Upvotes: 1

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