exchange index in 2D array using pointer in C

Question

I have a problem to exchange the values of an index in a 2D array by using a pointer. My code for example has an two dimensional array with the initialization {{1,2},{2,3},{3,4},{4,5}} and I want to exchange the values of two indexes in the 2D array. I created a function named swap like this:

void swap(double points[20][2],int i, int j){
    double *a,*b;
    a= *(*(points +i)+ 0;
    *(*(points +i)+ 0 = points[j][0];
    *(*(points +j)+ 0 = a;
    b= *(*(points +i)+ 1;
    *(*(points +i)+ 1 = points[j][1];
    *(*(points +j)+ 1 = a;
}

i,j here mean I want to change index i to j and vice-versa so for example when I used swap(points,0,1) the 2D array points will change like this: {{2,3},{1,2},{3,4},{4,5}} Any help would appreciated.

Sergey Kalinichenko · Accepted Answer · 2020-03-22 11:16:10Z

2

You can treat 2D array as a pointer to 1D arrays, and do a swap using a temporary array, like this:

void swap(double points[20][2],int i, int j) {
    double tmp[2];
    memcpy(tmp, points+i, sizeof(*points));
    memcpy(points+i, points+j, sizeof(*points));
    memcpy(points+j, tmp, sizeof(*points));
}

demo

The implementation is the classic swap, i.e.

tmp = points[i];
points[i] = points[j];
points[j] = tmp;

but since arrays in C cannot be assigned as a unit, memcpy is used instead.

answered Mar 22, 2020 at 11:16

Sergey Kalinichenko

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2 Comments

Schrodinger Over a year ago

Thank you for your help. what is memcpy? memory copy right?

Sergey Kalinichenko Over a year ago

@Schrodinger Correct. More info on array assignment.

Vlad from Moscow · Accepted Answer · 2020-03-22 11:37:52Z

Here you are

void swap( double points[][2], size_t i, size_t j )
{
    double ( *a )[2] = points + i;
    double ( *b )[2] = points + j;

    for ( size_t i = 0; i < 2; i++ )
    {
        double tmp = ( *a )[i];
        ( *a )[i] = ( *b )[i];
        ( *b )[i] = tmp;
    }
}

Or entirely without the subscript operator

void swap( double points[][2], size_t i, size_t j )
{
    double ( *a )[2] = points + i;
    double ( *b )[2] = points + j;

    for ( size_t i = 0; i < 2; i++ )
    {
        double tmp = *( *a + i );
        *( *a  + i ) = *( *b  + i );
        *( *b + i ) = tmp;
    }
}

Here is a demonstrative program

#include <stdio.h>

#define N   2

void swap( double points[][N], size_t i, size_t j )
{
    double ( *a )[N] = points + i;
    double ( *b )[N] = points + j;

    for ( size_t i = 0; i < N; i++ )
    {
        double tmp = *( *a + i );
        *( *a  + i ) = *( *b  + i );
        *( *b + i ) = tmp;
    }
} 

int main(void) 
{
    double points[][N] =
    {
        { 1, 2 }, { 2, 3 }, { 3, 4 }, { 4, 5 }
    };

    const size_t M = sizeof( points ) / sizeof( *points );

    for ( size_t i = 0; i < M; i++ )
    {
        printf( "{ %.1f, %.1f } ", points[i][0], points[i][1] );
    }

    putchar( '\n' );

    swap( points, 0, 1 );

    for ( size_t i = 0; i < M; i++ )
    {
        printf( "{ %.1f, %.1f } ", points[i][0], points[i][1] );
    }

    putchar( '\n' );

    return 0;
}

The program output is

{ 1.0, 2.0 } { 2.0, 3.0 } { 3.0, 4.0 } { 4.0, 5.0 } 
{ 2.0, 3.0 } { 1.0, 2.0 } { 3.0, 4.0 } { 4.0, 5.0 }

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