Array in C changing when another unrelated array is changed

Question

I am trying to create a 2D array where the values are taken as command line arguments.

The 2D array will always be a square with the side length being the first command line argument.

I can create the individual arrays fine, however it seems like creating one is getting rid of the previous one as when I check the array of arrays it is made up of the same array 3 times.

My code is as follows:

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

int main(int argc, char *argv[]) {
        int squareSize = atoi(argv[1]);
        int *matrix[sizeof(int) * squareSize];
        for(int i = 0; i < squareSize; i++) {
                int line[sizeof(int) * squareSize];
                int *lPointer = &line[0];
                for(int j = 0; j < squareSize; j++) {
                        *lPointer = atoi(argv[i*squareSize + j + 2]);
                        lPointer++;
                }
                printf("L: %d ", line[0]);
                printf("%d ", line[1]);
                printf("%d\n", line[2]);
                matrix[i] = line;
        }

        printf("M: %d ", matrix[0][0]);
        printf("%d ", matrix[0][1]);
        printf("%d ", matrix[0][2]);
        printf("%d ", matrix[1][0]);
        printf("%d ", matrix[1][1]);
        printf("%d ", matrix[1][2]);
        printf("%d ", matrix[2][0]);
        printf("%d ", matrix[2][1]);
        printf("%d\n", matrix[2][2]);

        return(0);
}

Command line input: ./filename 3 1 2 3 4 5 6 7 8 9

Expected output:

L: 1 2 3
L: 4 5 6
L: 7 8 9
M: 1 2 3 4 5 6 7 8 9

Actual output:

L: 1 2 3
L: 4 5 6
L: 7 8 9
M: 7 8 9 7 8 9 7 8 9

Does anyone know how to prevent new arrays that I create from replacing the last?

Your int *matrix[sizeof(int) * squareSize] is an array of some number of pointers. 2D array should contain squareSize² int elements. — dimich
– dimich, Commented Oct 15, 2022 at 20:22
You have matrix[i] = line; but line has gone out of scope before you report on matrix. Even if if didn't, the loop does not create independent/distinct line variables. — Weather Vane
– Weather Vane, Commented Oct 15, 2022 at 20:22
@dimich thank you, that makes sense. It doesn't fix the issue though — Cost Death
– Cost Death, Commented Oct 15, 2022 at 20:24
@WeatherVane nah, it's still within the scope. Line is declared inside the outer for loop — Cost Death
– Cost Death, Commented Oct 15, 2022 at 20:25

user3121023 · Accepted Answer · 2022-10-15 20:45:18Z

1

If squareSize is fairly small, use a simple Variable Length Array.
Use it the same as matrix[3][3].

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

int main(int argc, char *argv[]) {
    int squareSize = atoi(argv[1]);
    int matrix[squareSize][squareSize];
    for(int i = 0; i < squareSize; i++) {
        for(int j = 0; j < squareSize; j++) {
            matrix[i][j] = atoi(argv[i*squareSize + j + 2]);
        }
    }

    printf("M: %d ", matrix[0][0]);
    printf("%d ", matrix[0][1]);
    printf("%d ", matrix[0][2]);
    printf("%d ", matrix[1][0]);
    printf("%d ", matrix[1][1]);
    printf("%d ", matrix[1][2]);
    printf("%d ", matrix[2][0]);
    printf("%d ", matrix[2][1]);
    printf("%d\n", matrix[2][2]);

    return(0);
}

input: ./filename 3 1 2 3 4 5 6 7 8 9

output: M: 1 2 3 4 5 6 7 8 9

answered Oct 15, 2022 at 20:45

user3121023

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score 0 · Accepted Answer · 2022-10-15 21:14:05Z

The smallest change is to allocate a new line per loop iteration (free left out as exercise for reader):

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

int main(int argc, char *argv[]) {
    int squareSize = atoi(argv[1]);
    int *matrix[sizeof(int) * squareSize];
    for(int i = 0; i < squareSize; i++) {
        int *line = malloc(sizeof(int) * squareSize);
        int *lPointer = line;
        for(int j = 0; j < squareSize; j++) {
            *lPointer = atoi(argv[i*squareSize + j + 2]);
            lPointer++;
        }
        printf("L: %d ", line[0]);
        printf("%d ", line[1]);
        printf("%d\n", line[2]);
        matrix[i] = line;
    }
    printf("M: %d ", matrix[0][0]);
    printf("%d ", matrix[0][1]);
    printf("%d ", matrix[0][2]);
    printf("%d ", matrix[1][0]);
    printf("%d ", matrix[1][1]);
    printf("%d ", matrix[1][2]);
    printf("%d ", matrix[2][0]);
    printf("%d ", matrix[2][1]);
    printf("%d\n", matrix[2][2]);

    return(0);
}

and it now returns the expected result:

L: 1 2 3
L: 4 5 6
L: 7 8 9
M: 1 2 3 4 5 6 7 8 9

Here's a refactored version with error checking and just stores the data directory into a int matrix[squareSize][squareSize] variable and using separate function to print the matrix:

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

// offset into argv where the data starts
#define OFFSET 2

void printSquareMatrix(int n, int matrix[n][n]) {
    const char *delim = " \n";
    printf("M: ");
    for(int row = 0; row < n; row++)
        for(int col = 0; col < n; col++)
            printf("%d%c",
                matrix[row][col],
                delim[row + 1 == n && col + 1 == n]
            );
}

int main(int argc, char *argv[]) {
    if(argc < 2) {
        printf("usage: %s size data*(size^2)\n", *argv);
        return 0;
    }
    int squareSize = atoi(argv[1]);
    if(squareSize * squareSize != argc - 2) {
        printf("incorrect number of data items\n");
        return 1;
    }
    int matrix[squareSize][squareSize];
    for(int row = 0; row < squareSize; row++) {
        for(int col = 0; col < squareSize; col++) {
            matrix[row][col] = atoi(argv[OFFSET + row*squareSize + col]);
        }
    }
    printSquareMatrix(squareSize, matrix);
}

and the output is now just:

M: 1 2 3 4 5 6 7 8 9

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