2D array pointer in C

Question

I have the function and main

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <sys/time.h>

setArray(double *thearray){
    *thearray[0][0] = 2.0;
    *thearray[0][1] = 2.0;
    *thearray[1][0] = 2.0;
    *thearray[1][1] = 2.0;
}

void main(){
    double myarray[2][2];
    setArray(&myarray);
}

I cannot specify the size of array on setArray function because I don't know what will it be. I need to fill up the array in this specific fucction but I can't. get errors:

test.c: In function ‘setArray’:
test.c:8:13: error: subscripted value is neither array nor pointer nor vector
test.c:9:13: error: subscripted value is neither array nor pointer nor vector
test.c:10:13: error: subscripted value is neither array nor pointer nor vector
test.c:11:13: error: subscripted value is neither array nor pointer nor vector
test.c: In function ‘main’:
test.c:16:1: warning: passing argument 1 of ‘setArray’ from incompatible pointer type [enabled by default]
test.c:7:1: note: expected ‘double *’ but argument is of type ‘double (*)[2][2]’

Answer 1

You can use a VLA:

void setArray(int m, int n, double arr[m][n])
{
    for (int r = 0; r < m; ++r)
        for (int c = 0; c < n; ++c)
             arr[r][c] = 2.0;
}

int main()
{
    double myarray[2][2];
    setArray(2, 2, myarray);
}

VLAs are supported in C99, and optional in C11. If your compiler does not support VLAs, then you cannot fulfil your requirements. However, you could pass the array as a 1-D array and find the right elements using arithmetic:

void setArray(int num_rows, int num_cols, double *arr)
{
#define ARR_ACCESS(arr, x, y) ((arr)[(x) * num_cols + (y)])
    for (int r = 0; r < num_rows; ++r)
        for (int c = 0; c < num_cols; ++c)
             ARR_ACCESS(arr, r, c) = 2.0;
#undef ARR_ACCESS
}

int main()
{
    double myarray[2][2];
    setArray(2, 2, (double *)&myarray);
}

Answer 2

Try this:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <sys/time.h>

void setArray(double **thearray){
    thearray[0][0] = 2.0;
    thearray[0][1] = 2.0;
    thearray[1][0] = 2.0;
    thearray[1][1] = 2.0;
}

void main(){
    int i;
    double **myarray = (double**) malloc(2 * sizeof(double*));
    for(i = 0; i < 2; ++i)
        myarray[i] = (double*) malloc(2 * sizeof(double));
    setArray(myarray);
}

Answer 3

First, your setarray should accept a 2D array, not a poniter. If you know the width of the array, you can define it like this:

void setArray(double (*thearray)[2]) //2D array decays into a pointer to an array

then just call:

setArray(myarray)

An array decays to a pointer only once, so a 2D array wouldn't decay to a pointer to pointer. if the width isn't fixed, use a pointer of pointer instead:

void setArray(double **thearray)
{
    ...
}

setArray((double **)myarray) //explicitly convert.

Answer 4

A 2D array has a double pointer ( ** ). When you are sending the array as an argument, you do not need to add the ampersand as the array without its brackets is an address.