Arrays and pointers in C

array_and_function.c

#include <stdio.h>
#include <stdlib.h>
#define NROWS 8
#define NCOLS 3
/*
 *
 * This sample code is taken from:
 *
 * http://www.ibiblio.org/pub/languages/fortran/append-c.html
 *
 *
 */

int func1();
int func2();
int func3();
int func4();
int func5();

int
main()
{
    short mat[NROWS][NCOLS],i,j;

    for ( i = 0; i < NROWS; i++ )
        for ( j = 0; j < NCOLS; j++ )
        {
            mat[i][j] = i*10 + j;
        }

    printf ( " Initialized data to: " );
    for ( i = 0; i < NROWS; i++ )
    {
        printf ( "\n" );
        for ( j = 0; j < NCOLS; j++ )
        {
            printf ( "%5.2d", mat[i][j] );
        }
    }
    printf ( "\n" );

    func1 ( mat );
    func2 ( mat );
    func3 ( mat );
    func4 ( mat );
    func5 ( mat );

    return 0;
}

/*
Method #1 (No tricks, just an array with empty first dimension)
===============================================================
You don't have to specify the first dimension!
*/

int func1 ( short mat[][NCOLS] )
{
    register short i, j;

    printf ( "Func1: Declare as matrix, explicitly specify second dimension: " );
    for ( i = 0; i < NROWS; i++ )
    {
        printf ( "\n" );
        for ( j = 0; j < NCOLS; j++ )
        {
            printf ( "%5.2d", mat[i][j] );
        }
    }
    printf ( "\n" );

    return 0;
}

/*
Method #2 (pointer to array, second dimension is explicitly specified)
======================================================================
*/

int func2 ( short ( *mat ) [NCOLS] )
{
    register short i, j;

    printf ( "Func2: Declare as pointer to column, explicitly specify 2nd dim: " );
    for ( i = 0; i < NROWS; i++ )
    {
        printf ( "\n" );
        for ( j = 0; j < NCOLS; j++ )
        {
            printf ( "%5.2d", mat[i][j] );
        }
    }
    printf ( "\n" );

    return 0;
}

/*
Method #3 (Using a single pointer, the array is "flattened")
============================================================
With this method you can create general-purpose routines.
The dimensions doesn't appear in any declaration, so you
can add them to the formal argument list.

The manual array indexing will probably slow down execution.
*/

int func3 ( short *mat )
{
    register short i, j;

    printf ( "Func3: Declare as single-pointer, manual offset computation: " );
    for ( i = 0; i < NROWS; i++ )
    {
        printf ( "\n" );
        for ( j = 0; j < NCOLS; j++ )
        {
            printf ( "%5.2d", * ( mat + NCOLS*i + j ) );
        }
    }
    printf ( "\n" );

    return 0;
}

/*
Method #4 (double pointer, using an auxiliary array of pointers)
================================================================
With this method you can create general-purpose routines,
if you allocate "index" at run-time.

Add the dimensions to the formal argument list.
*/

int func4 ( short **mat )
{
    short    i, j, *index[NROWS];

    for ( i = 0; i < NROWS; i++ )
        index[i] = ( short * ) mat + NCOLS*i;

    printf ( "Func4: Declare as double-pointer, use auxiliary pointer array: " );
    for ( i = 0; i < NROWS; i++ )
    {
        printf ( "\n" );
        for ( j = 0; j < NCOLS; j++ )
        {
            printf ( "%5.2d", index[i][j] );
        }
    }
    printf ( "\n" );

    return 0;
}

/*
Method #5 (single pointer, using an auxiliary array of pointers)
================================================================
*/

int func5 ( short *mat[NCOLS] )
{
    short i, j, *index[NROWS];
    for ( i = 0; i < NROWS; i++ )
        index[i] = ( short * ) mat + NCOLS*i;

    printf ( "Func5: Declare as single-pointer, use auxiliary pointer array: " );
    for ( i = 0; i < NROWS; i++ )
    {
        printf ( "\n" );
        for ( j = 0; j < NCOLS; j++ )
        {
            printf ( "%5.2d", index[i][j] );
        }
    }
    printf ( "\n" );
    return 0;
}