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In computer programming, how do I Reverse a String?

Updated on September 4, 2015
Basketball, stored as a string with a zero-based index, is a collection of the individual characters.
Basketball, stored as a string with a zero-based index, is a collection of the individual characters.

What is a string?

In most programming languages, a string is simply an array of individual characters. This is common among all languages that support strings. The actual implementation differs between languages.

Some programming languages allow characters to be changed inside the string, while others do not. When a string can have its characters edited, it is called "mutable." When a string cannot have its characters edited, it is called "immutable." C++ allows the individual characters of a string to be edited, so it is mutable. C# does not allow the individual characters of a string to be edited, so it is immutable. Any edits to a string in C# results in the program allocating enough memory for the new string, and then creating the new string in that place.

Another thing to be aware of with your programming language of choice is how the environment indexes collections. Both C++ and C# will refer to the very first character as being in position 0. If a string had 10 characters, such as "Basketball", then the first character would be in position 0 and the final character in position 9. This is called "zero-based indexing." Other languages will refer to the first character as position 1, and the final character as position 10. This is called "one-based indexing." Visual Basic uses one-based indexing by default, but can be set to use zero-based indexing. SmallTalk and Lua both use one-based indexing as well.

How do I reverse a String? (Mutable)

Reversing a string is relatively simple and can be done in O(N) time, but the method is different depending on whether the string is mutable or immutable.

For a mutable string, characters can be swapped. Starting at the first character, swap the first and last character. Move to the second character. Swap the second and second-to-last character. Continue until you reach the halfway point in the string. Once all the swaps are completed, the string is reversed.

How to reverse a string with C++ (Mutable)

#include <iostream>
#include <string>

using namespace std;

void Reverse(string &str)
{
    char temp;
    uint length = str.length();
    
    for (uint x = 0; x < length / 2; x++)
    {
        if (x != length - 1 - x)
        {
            temp = str[length - 1 - x];
            str[length - 1 - x] = str[x];
            str[x] = temp;
        }
    }
}

int main()
{
    string input = "";
    
    cout << "Please enter any string: ";
    getline(cin, input);
    
    cout << "You entered: " << input << endl;
    
    Reverse(input);
    
    cout << "Input reversed: " << input << endl;
    
    return 0;
}

How do I reverse a String? (Immutable)

For an immutable string, you will have to build a new string. You cannot swap the characters in the original string. Create a new container, either a String or any other class that allows you to assemble a string, such as StringBuilder. From here, start at the final character in the original string, and add this character as the first to the new container. Keep moving over each character in the original string until you arrive at the first character. This addition will be the final character in the new string. Either return the new string from your method, or assign the value of the new string to the original string.

How to reverse a string with C# (Immutable)

using System;
using System.Text;

namespace StringReverse
{
    class Program
    {
        static void Reverse(ref string str)
        {
            StringBuilder rev = new StringBuilder();

            for (int x = str.Length - 1; x >= 0; x--)
            {
                rev.Append(str[x]);
            }

            str = rev.ToString();
        }

        public static void Main()
        {
            string input = "";

            Console.Write("Please enter any string: ");
            input = Console.ReadLine();

            Console.WriteLine("You entered: " + input);

            Reverse(ref input);

            Console.WriteLine("Input reversed: " + input);

            Console.Write("\nPlease press [ENTER] to exit...");
            Console.ReadLine();
        }
    }
}

How do I reverse a String by casting to an Array?

An alternate method supported by some languages is to cast the string as an array. This works with languages where the string data type is distinct from an array of characters and requires an explicit cast.

C# is one such language. First, explicitly cast the string to an array of characters (char data type). Then, use the Array.Reverse() method with your character array as the sole parameter. Finally, create a new string with the character array as the parameter, assigning the value to the old string reference. The original string is no longer referenced and will be garbage collected, while the new reversed string is the new value of the original reference.

How to reverse a string using an explicit cast to an array with C#

using System;
using System.Text;

namespace StringReverse
{
    class Program
    {
        static void Reverse(ref string str)
        {
            char[] strArray = str.ToCharArray();
            Array.Reverse(strArray);
            str = new String(strArray);
        }

        public static void Main()
        {
            string input = "";

            Console.Write("Please enter any string: ");
            input = Console.ReadLine();

            Console.WriteLine("You entered: " + input);

            Reverse(ref input);

            Console.WriteLine("Input reversed: " + input);

            Console.Write("\nPlease press [ENTER] to exit...");
            Console.ReadLine();
        }
    }
}

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