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:

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#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);
        }
}

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Code:

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

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I don't think you can declaire a array of indeterment size within a function....

Code:

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

char *swich(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;
}

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;
}

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

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'switch' has special meaning, as for a "switch case".
I wasnt able to find out why the for loop was giving errors.

Hope that helps! : )

There are some problems with

Code:

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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;
}

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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;
}

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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);
  }
}

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Dave

Quote:

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Originally Posted by loopy

I wasnt able to find out why the for loop was giving errors.

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Some C compilers still complain if you don't put all variable declarations and initialization at the beginning of the function, so the original for loop statement:

Code:

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for(int nextchar = place; nextchar < strlen(topermute); nextchar++)

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could give problems.

I just declared nextchar at the beginning, to show that for part would work OK once we get beyond compiler quirks.

I would probably write it a little differently (to keep from calling strlen() every time), but this way works, and elegance is meaningless unless and until a basic understanding is in place.


Dave

I have the program bookmarked, I want to try to write my own. ; )