Thread: regarding passing 2d array to functions

Hi all,

I have a general question.

This is regarding better understanding of representation/usage of 2D arrays.
consider the following.

Code:

#define MAX_COLS 4

int **first_array;
int second_array[][MAX_COLS];

Above, I have declared (defined) two 2D arrays namely first_array, and second_array.

Now, imagine there is a function that takes as an argument a 2D array.

Code:

void print_2D_array(int an_array[][MAX_COLS], int num_rows) {
     for (int x = 0; x < num_rows; x++) {
        for (int y = 0; y < MAX_COLS; y++) {
            printf(" %d ", an_array[x][y]);
        }
        printf("\n");
    }
    printf("\n");
}

Now, what is the correct way to pass "first_array" and "second_array" to the function "print_2D_array".

In my application, I am doing the following to get correct outputs.

Code:

print_2D_array(&first_array[0][0], num_rows);
print_2D_array(second_array, num_rows);

// another example
int m_hierarchy[13][4]; // part of structure
memcpy(reqs.m_hierarchy, first_array[0], num_rows*MAX_COLS*sizeof(int));

Also, for memcmp and memcpy functions how to correctly pass an array with each representation.

I am bit confused, for example, why not use

Code:

memcpy(reqs.m_hierarchy, first_array, num_rows*MAX_COLS*sizeof(int));

thanks,

zahid

Above, I have declared (defined) two 2D arrays namely first_array, and second_array.

Code:

#define MAX_COLS 4
 
int **first_array;
int second_array[][MAX_COLS];

Actually, you have not created two arrays.

The first is only a pointer to a pointer to an int, but not initialized.

The second cannot compile as you have not provided the first dimension!

You need to create a working program that demonstrates what you are asking about. Turn on and turn up the warning level on your compiler.

int **first_array implies that it has a dynamic number of columns (not necessaily a "ragged" array, but a fixed number of columns that is only able to be determined at runtime) as well as a dynamic number of rows. If it has a fixed number of columns determinable at compile time, then it should be:

Code:

int (*first_array)[NUM_COLS];

Note that NUM_COLS is a better name for a fixed number of columns. MAX_COLS implies that it could be less. Your print routine assumes that it always uses "MAX_COLS" columns.

If the number of columns is dynamic then your print function will not work properly in general.

Originally Posted by algorism

int **first_array implies that it has a dynamic number of columns (not necessaily a "ragged" array, but a fixed number of columns that is only able to be determined at runtime) as well as a dynamic number of rows. If it has a fixed number of columns determinable at compile time, then it should be:

Code:

int (*first_array)[NUM_COLS];

Note that NUM_COLS is a better name for a fixed number of columns. MAX_COLS implies that it could be less. Your print routine assumes that it always uses "MAX_COLS" columns.

If the number of columns is dynamic then your print function will not work properly in general.

thanks for reply.

I figured out, that normally, my second dimension was always fixed and I could always workout the size of first dimension. Therefore, I started to drop the pointer notation, and instead defining arrays as:

Code:

#define NUM_COLS 4
#define MAX_ROWS 13
int first_array[MAX_ROWS][NUM_COLS]

As noted, the first dimension can go to the maximum of 13, but it can be smaller.

I have a follow up question, with such notation, if I want to copy contents between two 2d arrays, is the following way using "memcpy" correct.

Code:

int p[MAX_ROWS][MAX_COLS];
int arr[3][4] = {
                       {1,2,3, 4}, 
                      {5, 6, 7, 8},
                     {9, 10, 11, 12}
                   };
    memcpy(p,arr,sizeof(int)*3*4);

And, also to use memcmp function for comparing the contents of the two arrays.

Code:

int array[2][3] = // some contents;
int array2[2][3]=// some contents;

memcmp(array, array2, sizeof(array));

thanks,

Code:

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

#define MAX_ROWS 13  // maximum number of rows
#define NUM_COLS  4  // fixed number of columns

void prn(int a[][NUM_COLS]) {
    for (int row = 0; row < MAX_ROWS; row++) {
        for (int col = 0; col < NUM_COLS; col++)
            printf("%2d ", a[row][col]);
        putchar('\n');
    }
    putchar('\n');
}

int main() {
    int p[MAX_ROWS][NUM_COLS] = {{ 0 }};
    int arr[][NUM_COLS] = {
        { 1,  2,  3,  4},
        { 5,  6,  7,  8},
        { 9, 10, 11, 12}
    };

    // This works, but only because both arrays have
    // the same number of columns.
    memcpy(p, arr, sizeof arr);
    prn(p);

    memset(p, 0, sizeof p); // zero out p again

    // This would work even if they had different column sizes.
    for (size_t row = 0; row < sizeof arr / sizeof *arr; row++)
        memcpy(p[row], arr[row], sizeof *arr);
    prn(p);

    return 0;
}