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

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

Answer 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);