c pointers

andystanfordjason · 04-22-2007, 04:50 AM

hello, i am writing an implementation of gaussian elimination and have set up a matirx as follows: int matrix[n][n]. part of the algorithm requires swapping rows and was wondering, in order to be efficient, if the pointers to the rows could be swapped? is this possible? if so any cluse as to how to do it would be greatly appreciated. thanks a lot

jim mcnamara · 04-22-2007, 08:46 AM

If you do not ever want to restore the orginal matrix, it's fine.

Code:

int matrix[24][24]={0};
...........
void swap(int row1, int row2, int matrix[][24])
{
   int tmp24={0};
   memcpy(tmp, matrix[row1], 24 * sizeof(int));
   memcpy(matrix[row1], matrix[row2], 24 * sizeof(int));
   memcpy(matrix[row2], tmp, sizeof(int) *24);
}

You can also swap individual row pointers if you create a int **ptrs, which is probably faster, except that a block swap is fairly efficient on most platforms.
Then you have to refernece the array elements by indirection from then on.

andystanfordjason · 04-22-2007, 09:46 AM

thanks thats perfect

bartonski · 04-22-2007, 04:23 PM

as jim mcnamara said, you can also do the swap with **char; here's compilable code that does that (although I've used char *line[ROW] instead of char **line to avoid an extra malloc() ). I was also busy using chars instead of ints, because I wanted to be able to easily printf() a given line.

Code:

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

#define ROW 6
#define COL 10

char *line[ROW];

int main(void){
        char ch;
        char *placeholder;

        /* for each row in *line[], allocate space for 11 characters,
           set the first 10 characters to the current value of 'ch',
           put a null character at the end. */
        for( ch='a'; ch <= 'f'; ch++ ){
                line[ ch - 'a' ] = malloc( (COL + 1) * sizeof(char) );
                memset(line[ ch - 'a' ], ch, COL);
                line[ ch - 'a' ][COL] = '\0';
                printf("%s\n", line[ ch - 'a' ]);
        }

        /* Swap first and last lines*/
        placeholder = line['f' - 'a'];
        line['f' - 'a'] = line['a' - 'a'];
        line['a' - 'a'] = placeholder;

        /* print results */
        printf("=== Switcheroo ===\n");
        for( ch='a'; ch <= 'f'; ch++ ){
                printf("%s\n", line[ ch - 'a' ]);
        }

        return 0;
}