Permutation generator on C

Question

I need a simple Algorithm of permutation generator which could be apply on simple C language.

Answer 1

Permutes over numbers:

In order to do use each permutation, you have to hook up to the print function.

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

/**
   Read a number, N, from standard input and print the
   permutations.
 */

void print(const int *v, const int size)
{
  if (v != 0) {
    for (int i = 0; i < size; i++) {
      printf("%4d", v[i] );
    }
    printf("\n");
  }
} // print


void swap(int *v, const int i, const int j)
{
  int t;
  t = v[i];
  v[i] = v[j];
  v[j] = t;
}


void rotateLeft(int *v, const int start, const int n)
{
  int tmp = v[start];
  for (int i = start; i < n-1; i++) {
    v[i] = v[i+1];
  }
  v[n-1] = tmp;
} // rotateLeft


void permute(int *v, const int start, const int n)
{
  print(v, n);
  if (start < n) {
    int i, j;
    for (i = n-2; i >= start; i--) {
      for (j = i + 1; j < n; j++) {
    swap(v, i, j);
    permute(v, i+1, n);
      } // for j
      rotateLeft(v, i, n);
    } // for i
  }
} // permute


void init(int *v, int N)
{
  for (int i = 0; i < N; i++) {
    v[i] = i+1;
  }
} // init


int main()
{
    int *v = (int*) malloc(sizeof(int)*10);
    init(v, 10);
    permute(v, 0, 10);
    free(v);
}

Answer 2

I have, ahem, written a little about permutations and other combinatorial algorithms in C here:

Combinatorial algorithms

Answer 3

This is a classic algorithm found (among other places) in Knuth's TAOCP.

Here's an example I used for a project euler problem. It creates all the permutations of a string in lexicographical order and prints them to stdout.

#include<stdio.h>
int main()
{
        char set[10]="0123456789";
        char scratch;
        int lastpermutation = 0;
        int i, j, k, l;
        printf("%s\n",set);
        while (!lastpermutation)
        {
                //find largest j such that set[j] < set[j+1]; if no such j then done
                j = -1;
                for (i = 0; i < 10; i++)
                {
                        if (set[i+1] > set[i])
                        {
                                j = i;
                        }
                }
                if (j == -1)
                {
                        lastpermutation = 1;
                }
                if (!lastpermutation)
                {
                        for (i = j+1; i < 10; i++)
                        {
                                if (set[i] > set[j])
                                {
                                        l = i;
                                }
                        }
                        scratch = set[j];
                        set[j] = set[l];
                        set[l] = scratch;
                        //reverse j+1 to end
                        k = (9-j)/2; // number of pairs to swap
                        for (i = 0; i < k; i++)
                        {
                                scratch = set[j+1+i];
                                set[j+1+i] = set[9-i];
                                set[9-i] = scratch;
                        }
                        printf("%s\n",set);
             }
        }
        return 0;
}