Swapping

There are several ways to swap two elements in a data structure or two variables in general.

Using a Temp

char temp = s[i];
s[i] = s[j];
s[j] = temp;

Using Bitwise XOR Swap

This is mostly academic and not used for

// Let's assume a and b are int, and a != b, and &a != &b
a = a ^ b;
b = a ^ b;  // effectively b = (a ^ b) ^ b = a
a = a ^ b;  // effectively a = (a ^ b) ^ a = b
 
// If i != j
s[i] = s[i] ^ s[j];
s[j] = s[i] ^ s[j];
s[i] = s[i] ^ s[j];

C++ STL

Using std::swap

std::swap is usually the most straightforward and idiomatic in modern C++.

#include <string>
#include <utility>
using namespace std;
 
int a = 10, b = 20;
swap(a, b);  // Now a = 20, b = 10
 
string s = "Hello";
swap(s[0], s[4]); // Swap 'H' and 'o', s becomes "oellH"

Using std::exchange

std::exchange is sometimes handy for one-liners or if you need the “old value” while assigning a “new value” in the same expression.

#include <utility>
using namespace std;
 
int a = 10, b = 20;
a = exchange(b, a);
 
string s = "Hello";
s[0] = exchange(s[4], s[0]);

Using References in a Function

Sometimes you want a function that swaps two elements in an array, or two characters in a string, by passing them by reference:

void swapTwo(int& x, int& y) {
    int temp = x;
    x = y;
    y = temp;
}
 
// Usage:
int arr[] = {1, 2, 3, 4};
swapTwo(arr[0], arr[1]);  // arr is now {2, 1, 3, 4}