Weird results with swap and substr functions - c++

Question

I was trying to solve solve questions at TopCoder. I was getting weird results on test sections of TopCoder; however, when I submit my code I got AC(stands for accepted). The case was the same on TCHS SRM 47 Level one - 250 pt - Cards Shuffle. I used swap function, my code
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <string>
using namespace std;
int main() {
int f, l, t;
string c;
cin>>c;
scanf("%d%d%d", &f, &l, &t);
//while(t--) c=c.substr(f-1, l-f+1)+c.substr(0, f-1)+c.substr(l);
while(t--) for(int i=0, j=f-1; j<=l-1; i++, j++) swap(c[i], c[j]);
cout<<c;
return 0;
}
returns WA(stands for wrong answer) on test section of TC(stands for TopCoder), and AC when submitted on TC. Then I checked the code on ideone, with both substr and swap functions respectively. On first try substr function gave expected result, while swap function unexpected result. However, on second try it was vice versa. I dont know what is going on, whether my code has a bug, or ideone, or TopCoder testing system.

Answer 1

Your Algorithm
Your algorithm using swap is faulty. Let's walk through the steps for one iteration of the while loop. You have:
for ( int i = 0; j = f-1; j <= l-1; ++i, ++i )
swap(c[i], c[j]);
I don't know why you are using f, l, and t. It will be so much easier to read if you use first, last, and times.
for ( int i = 0; j = first-1; j <= last-1; ++i, ++i )
swap(c[i], c[j]);
Let's use the following input:
ABCDEFGHIJ
5 6 1
In the first iteration of the for loop,
i = 0;
j = 4;
after the swap, the new value of c is
EBCDAFGHIJ (A and E are swapped)
In the second iteration of the for loop,
i = 1;
j = 5;
after the swap, the new value of c is
EFCDABGHIJ (B and F are swapped)
The iteration stops here since the value of j becomes 6.
What you needed to end up with is:
EFABCDGHIJ
A Different Algorithm
If you want to minimize the number of strings created, you can use the following strategy.
For the given inputs, create the sub-string "EF" and store it. Then move "ABCDE" to the right by two. Then move "EF" to the start of the string. The following function does that. It changes c in place.
void fun(string& c, int first, int last)
{
// Convert first to a 0-based index for easier manipulation.
--first;
int delta = last-first;
string c1 = c.substr(first, delta);
for ( int i = first-1; i >= 0; --i )
{
c[i+delta] = c[i];
}
for ( int i = 0; i < delta; ++i )
{
c[i] = c1[i];
}
}