Puzzled

[adam2016]

just how in the ... does this work,I have spent 2 hours literally drawing this out on paper and I just can't see how this code works

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

using namespace std;

/* Function to swap values at two pointers */
void swap(char *x, char *y)
{
    char temp;
    temp = *x;
    *x = *y;
    *y = temp;
}

/* Function to print permutations of string
   This function takes three parameters:
   1. String
   2. Starting index of the string
   3. Ending index of the string. */
void permute(char *a, int l, int r)
{
   int i;
   if (l == r)
      cout << a << endl;
   else
   {
       for (i = l; i <= r; i++)
       {
          swap((a+l), (a+i));
          permute(a, l+1, r);
          swap((a+l), (a+i)); //backtrack
       }
   }
}

/* Driver program to test above functions */
int main()
{
    char str[] = "JOE";
    int n = strlen(str);
    permute(str, 0, n-1);
    return 0;
}

this is the output

JOE
JEO
OJE
OEJ
EOJ
EJO

but when I draw it out on paper I just keep getting Joe printed,and only happens about 3 times I just don't understand how it works especially this block

void permute(char *a, int l, int r)
{
   int i;
   if (l == r)
      cout << a << endl;
   else
   {
       for (i = l; i <= r; i++)
       {
          swap((a+l), (a+i));
          permute(a, l+1, r);
          swap((a+l), (a+i)); //backtrack
       }
   }
}

L = 0 , r = 2, i = L = 0 so i = 0

what is the point of swapping (a+1) = 'e' with a+i which is also 'e'

then a recursive call to permute which essentially does the same

L will always be = to i until it hits 2 then the for loop doesn't execute

I just don't see how in the blue earth this works

very very very confused and that is an under statement everything I thought I knew about problem solving and programming is up in the air now,

I have even drawn out the stack and followed the function calls on paper and kept track of all the variables and their values but it's just not working

[cire]

what is the point of swapping (a+1) = 'e' with a+i which is also 'e'

It is not a + 1 it is a + l

One doesn't need to code a special case to avoid swapping when l and i are the same, simplifying the algorithm - that is the point.

You could do this:

       permute(a, l+1, r);
   	
       for (i = l+1; i <= r; i++)
       {
          swap((a+l), (a+i));
          permute(a, l+1, r);
          swap((a+l), (a+i)); //backtrack
       }

in place of the loop and get the same result.

[adam2016]

thanks cire much appreciated

I still can't figure out how and where the swapping occurs