#include <stdio.h> 
#define MAXN 50

char a[MAXN];
int perm_no=0;
int verbose=0;

/* perm(i,n): gera todas as permutacoes dos elementos a[i],...,a[n-1].  
              se i=n-1, apenas imprime o vetor a[]; 
              caso contrario, permuta os elementos a[i],...,a[n-1] e, quando  
              termina, a trecho a[i..n-1] do vetor esta como originalmente  */

void perm(int i, int n)
{
  if (i==n-1) {
    int k;
    printf("Perm. number %3d: ", ++perm_no);
    for (k=0; k<n; k++)
      printf("%c", a[k]);
    printf("\n");
  } else {
    int k;
    int tmp;
    
    for (k=i; k<n; k++) {
      int l;
      tmp=a[i]; a[i]=a[k]; a[k]=tmp;
      perm(i+1,n);
      tmp=a[i]; a[i]=a[k]; a[k]=tmp;
      if (verbose) {
	printf("  Current permutation: ", k);
        for (l=0; l<n; l++)
	  printf("%c", a[l]);
	printf("\n");
      }
    }
  }
}

void set_up(int n)
{
  int k;
  char c='a';
  
  for (k=0; k<n; k++,c++)
    a[k]=c;
}

int scan_command(int argc, char *argv[], int *n)
{
  while (--argc) {
    if (sscanf(argv[argc],"-n%lu", n)==1) ;
    else if (strcmp(argv[argc],"-v")==0) verbose=1;
    else {
      printf("Usage: %s [-nN]\n", argv[0]);
      return -1;
    }
  }
  return 0; /* normal exit from ths function */
}

main(argc,argv)
  int argc; /* the number of command-line arguments */
  char *argv[]; /* an array of strings containing those arguments */
{
  int n=3; /* number elements to permute */

  if (scan_command(argc, argv, &n)==0) {
    int k;
    set_up(n);
    perm(0,n);
    if (verbose) {
      printf("\nFinal perm.: ");
      for (k=0; k<n; k++) 
        printf("%c",a[k]);
      printf("\n");
    }
    return 0; /* normal exit */
  }
}

====================================================================== 
<2>[jaca:/home/mac/yoshi/sgb/Mols] > all_perms2 -n4 -v
Perm. number   1: abcd
  Current permutation: abcd
Perm. number   2: abdc
  Current permutation: abcd
  Current permutation: abcd
Perm. number   3: acbd
  Current permutation: acbd
Perm. number   4: acdb
  Current permutation: acbd
  Current permutation: abcd
Perm. number   5: adcb
  Current permutation: adcb
Perm. number   6: adbc
  Current permutation: adcb
  Current permutation: abcd
  Current permutation: abcd
Perm. number   7: bacd
  Current permutation: bacd
Perm. number   8: badc
  Current permutation: bacd
  Current permutation: bacd
Perm. number   9: bcad
  Current permutation: bcad
Perm. number  10: bcda
  Current permutation: bcad
  Current permutation: bacd
Perm. number  11: bdca
  Current permutation: bdca
Perm. number  12: bdac
  Current permutation: bdca
  Current permutation: bacd
  Current permutation: abcd
Perm. number  13: cbad
  Current permutation: cbad
Perm. number  14: cbda
  Current permutation: cbad
  Current permutation: cbad
Perm. number  15: cabd
  Current permutation: cabd
Perm. number  16: cadb
  Current permutation: cabd
  Current permutation: cbad
Perm. number  17: cdab
  Current permutation: cdab
Perm. number  18: cdba
  Current permutation: cdab
  Current permutation: cbad
  Current permutation: abcd
Perm. number  19: dbca
  Current permutation: dbca
Perm. number  20: dbac
  Current permutation: dbca
  Current permutation: dbca
Perm. number  21: dcba
  Current permutation: dcba
Perm. number  22: dcab
  Current permutation: dcba
  Current permutation: dbca
Perm. number  23: dacb
  Current permutation: dacb
Perm. number  24: dabc
  Current permutation: dacb
  Current permutation: dbca
  Current permutation: abcd

Final perm.: abcd
<3>[jaca:/home/mac/yoshi/sgb/Mols] > 
======================================================================
#include <stdio.h> 
#define MAXN 50

int a[MAXN];
int subset_no=0;
int verbose=0;
int n=4;
int k=2;

void subsets(int i, int j)
{
  if (i==n) {
    int l;
    printf("%d-subset number %3d: ", k, ++subset_no);
    for (l=0; l<n; l++)
      printf("%d", a[l]);
    printf("\n");
  } else {
    if (n-i>j) {
      a[i]=0;
      subsets(i+1,j);
    }
    if (j>0) {
      a[i]=1;
      subsets(i+1,j-1);
    } 
  }
}

void set_up()
{
  int l;
  
  for (l=0; l<n; l++)
    a[l]=0;
}

int scan_command(int argc, char *argv[])
{
  while (--argc) {
    if (sscanf(argv[argc],"-n%lu", &n)==1) ;
    else if (sscanf(argv[argc],"-k%lu", &k)==1) ;
    else if (strcmp(argv[argc],"-v")==0) verbose=1;
    else {
      printf("Usage: %s [-nN][-kN][-v]\n", argv[0]);
      return -1;
    }
  }
  return 0; /* normal exit from ths function */
}

main(argc,argv)
  int argc; /* the number of command-line arguments */
  char *argv[]; /* an array of strings containing those arguments */
{
  if (scan_command(argc, argv)==0) {
    set_up();
    subsets(0,k);
    return 0; /* normal exit */
  }
}

======================================================================

<3>[jaca:/home/mac/yoshi/sgb/Mols] > k_subsets -n6 -k3
3-subset number   1: 000111
3-subset number   2: 001011
3-subset number   3: 001101
3-subset number   4: 001110
3-subset number   5: 010011
3-subset number   6: 010101
3-subset number   7: 010110
3-subset number   8: 011001
3-subset number   9: 011010
3-subset number  10: 011100
3-subset number  11: 100011
3-subset number  12: 100101
3-subset number  13: 100110
3-subset number  14: 101001
3-subset number  15: 101010
3-subset number  16: 101100
3-subset number  17: 110001
3-subset number  18: 110010
3-subset number  19: 110100
3-subset number  20: 111000
<4>[jaca:/home/mac/yoshi/sgb/Mols] >