Thread: String permutations help

I'm pretty new to c, so please excuse the basic question. I'm trying to make a program for string permutations, I couldint do it, so I looked at the solution at http://www.cprogramming.com/challenges/permutesol.html . Now even after using this code, and trying to change it to c (from c++), it still doesnt work Here is my code, what could be the problem?

Code:

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

void switch(char topermute[], int x, int y);
void permute(char topermute[], int place);

int main(int argc, char* argv[])
{
	if(argc!=2)
	{
		printf("Usage: ./permute string");
	}
	else
	{
		permute(argv[1], 0);
	}
	return 0;
}

void switch(char topermute[], int x, int y)
{
	char newstring[] = topermute[];
	newstring[x] = newstring[y];
	newstring[y] = topermute[x];
	return newstring;
}

void permute(char topermute[], int place)
{
	if(place == strlen(topermute) -1)
	{
		printf("%s", topermute);
	}
	for(int nextchar = place; nextchar < strlen(topermute); nextchar++)
	{
		permute(switch(topermute, place, nextchar),place+1);
	}
}

There are some problems with

Code:

char *swich(char *topermute, int x, int y)
{
	char *newstring;
        newstring = (char *) malloc (sizeof(*topermute));
        topermute = newstring; 
	newstring[x] = newstring[y];
	newstring[y] = topermute[x];
	return topermute;
}

First: the size of array newstring should be strlen(topermute)+1, so that it can hold all its characters and also the terminating '\0'.

(Note that sizeof(*topermute) is the size of a char.)

Second: Copy topermute into newstring.

Third: perform the character switch in newstring.

Fourth: return the pointer to the permuted string.

so, it could be something like:

Code:

char *swich(char *topermute, int x, int y)
{
	char *newstring;
        newstring = (char *) malloc (strlen(topermute)+1);
        strcpy(newstring, topermute);
	newstring[x] = newstring[y];
	newstring[y] = topermute[x];
	return newstring;
}

Now, this function is called many times, and allocates new storage every time. It is your responsibility to deallocate all memory before your program exits. So, function permute() should free() the memory after the call to swich().

Something like this should work:

Code:

void permute(char *topermute, int place)
{
  int nextchar;
  char *switched_string;

  if(place == strlen(topermute) -1)
  {
    printf("%s\n", topermute);
  }
  for(nextchar = place; nextchar < strlen(topermute); nextchar++)
  {
    switched_string = swich(topermute, place, nextchar);
    permute(switched_string, place+1);
    free(switched_string);
  }
}

Dave