How to return a 2D array from a C function?

Question

So I have to write a program that sorts a 2d array and then returns it but I just found out you cant return an array from a function. I tried researching this but nothing fixes the problem. Any help is appreciated, thanks.

Below is the function I wrote to sort the vertical rows.

int vertSort(int matrix[][51], int size)
{
    for(c=0;c<size;c++)
    {

        for(r=0;r<size-1;r++)
        {
            if(matrix[c][r]>matrix[c][r+1])
            {
                printf("Swap row %d column %d with row %d column %d", r, c, r, c)
                int tmp=matrix[c][r];
                matrix[c][r]=matrix[c][r+1];
                matrix[c][r+1]=tmp;


            }

        }
    }
    return matrix;


}

It can be returned as a 2D pointer. But still the r,c values are returned separately. — mahendiran.b
– mahendiran.b, Commented Oct 24, 2014 at 3:01
Even if you could return an array from a function, it wouldn't do you any good, since you can't assign to arrays in C. — Crowman
– Crowman, Commented Oct 24, 2014 at 3:25
for(r=0;r<size-1;r++) does not match with 51 in the signature; replace size by 51 or vice versa — M.M
– M.M, Commented Oct 24, 2014 at 3:25

M.M · Accepted Answer · 2014-10-24 03:24:01Z

3

Your code does not need to return anything, it modifies the caller's copy of the array. (Arrays cannot be passed by value in C). Change the int return type to void and all will be well.

answered Oct 24, 2014 at 3:24

M.M

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2 Comments

Dominic Dalessandro Over a year ago

Thank you for the help! I switched the int return type to void but now when I try and run the program I have an error at the bottom that says warning conflicting types for 'vertSort'

M.M Over a year ago

change it in the prototype also

user3386109 · Accepted Answer · 2014-10-24 03:40:22Z

The correct syntax is impossibly messy, so I suggest using a typedef as shown below

typedef int (*ptr51)[51];

ptr51 vertSort( int matrix[][51], int size )
{
    matrix[0][2] = 99;
    matrix[1][1] = 88;
    return( matrix );
}

int main( void )
{
    int test[2][51] = { { 10, 20, 30 }, { 40, 50, 60 } };

    ptr51 output = vertSort( test, 0 );

    for ( int row = 0; row < 2; row++ )
    {
        for ( int col = 0; col < 3; col++ )
            printf( "%d ", output[row][col] );
        printf( "\n" );
    }
}

Here is the impossibly messy syntax without a typedef

int (*(vertSort( int matrix[][51], int size )))[51]
{
    matrix[0][10] = 99;
    matrix[1][50] = 88;
    return( matrix );
}

int main( void )
{
    int test[2][51] = { { 10, 20, 30 }, { 40, 50, 60 } };

    int (*output)[51] = vertSort( test, 0 );

    for ( int row = 0; row < 2; row++ )
    {
        for ( int col = 0; col < 51; col++ )
            printf( "%d ", output[row][col] );
        printf( "\n" );
    }
}

doptimusprime · Accepted Answer · 2014-10-24 03:02:25Z

-1

function's return type should be int ** and typecast matrix to int **. It should work for you.

int** vertSort(int matrix[][51], int size)
{
     for(c=0;c<size;c++)
     {

        for(r=0;r<size-1;r++)
        {
             if(matrix[c][r]>matrix[c][r+1])
             {
                printf("Swap row %d column %d with row %d column %d", r, c, r, c)
                int tmp=matrix[c][r];
                matrix[c][r]=matrix[c][r+1];
                matrix[c][r+1]=tmp;


              }

         }
    }
    return (int **)matrix;


}

answered Oct 24, 2014 at 3:02

doptimusprime

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4 Comments

doptimusprime Over a year ago

I think C will show error when int [][51] is accessed as int **.

Ray Toal Over a year ago

To be pedantic, it's a warning, not an error (with clang, anyway), but you are of course right; it is bad practice to ignore warnings, so the explicit case is nice. +1

Crowman Over a year ago

It's bad practice to ignore warnings, but a worse practice to cast them away. If the thing passed to vertSort() actually is a two-dimensional array, returning it as an int ** and trying to access it via that pointer will be disastrous. If you actually are passing in an int **, you'll get the same problem in reverse. That warning is there for a good reason.

M.M Over a year ago

This is completely wrong, arrays are different to pointers. Silencing compiler warnings with a cast doesn't cause magic.

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