How to reverse a string in C++
In C++, reversing a string, or completely flipping a string means flipping changing the order of the characters in it, such that it reads backward.
The illustration below shows what happens when we reverse a string.
How do you reverse a string?
There are multiple implementations that can be used to reverse a string, depending on the requirements of the programme you are writing. Let’s look at all of them in order.
1. Using the built-in reverse function
C++ has an in-built reverse function, that can be called to reverse a string. This function takes in two inputs;
- The iterator for the string start
- The iterator for string ends
The code snippet below shows how to use this function:
#include <iostream>//The library below must be included for the reverse function to work#include<bits/stdc++.h>using namespace std;int main() {string greeting = "Hello";//Note that it takes the iterators to the start and end of the string as argumentsreverse(greeting.begin(),greeting.end());cout<<greeting<<endl;}
2. Using a loop
Within the main body of the function, a loop can be written to reverse a string.
In the loop body, you will use the swap function, which is in-built, to change element positions. In case one is not allowed to use any in-built function, the second code tab shows how to carry out the reverse loop.
The third tab shows how to do the same reversal with a while loop using the swap method.
#include <iostream>using namespace std;int main() {string greeting = "Hello";int len = greeting.length();int n=len-1;for(int i=0;i<(len/2);i++){//Using the swap method to switch values at each indexswap(greeting[i],greeting[n]);n = n-1;}cout<<greeting<<endl;}
Note: The for loop only runs for half the length of the string as swaps are happening between the first and last positions and slowly moving inwards till both
iandnare the same. The while loop shows this condition more explicitly.
3. Using a function
A function can also be written to reverse a string using recursion.
The code snippet below shows how:
#include <iostream>using namespace std;void reverse_String(string& greet, int n,int i){if(n<=i){return;}swap(greet[i],greet[n]);reverse_String(greet,n-1,i+1);}int main() {string greeting = "Hello";cout<<"String before reversal: "<<greeting<<endl;reverse_String(greeting,greeting.length()-1,0);cout<<"String after reversal: "<<greeting<<endl;}
4. Creating a new string
A roundabout method for reversing a string is looping backwards through it and storing the elements in a new string of the same size.
To add elements to an empty string, we can the push_back method.
The code below shows how:
#include <iostream>using namespace std;int main() {string greeting = "Hello";string new_greeting;for(int n = greeting.length()-1; n >= 0; n--){new_greeting.push_back(greeting[n]);}cout<<"Original string: "<< greeting << endl;cout<<"New reversed string: "<< new_greeting << endl;}
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