Reverse a String in C [3 Methods]

Summary: In this example, we will learn different ways to reverse a string in the C programming language.

Example:

If the user enters ‘Pencil‘, then the output should be ‘licneP‘.

Input:  Pencil
Output: licneP

Method 1: Using Loop

The idea is to assign characters of the input string to its reverse in reverse order using the ‘while’ or ‘for’ loop.

#include <stdio.h>
 
int main()
{
    char str[30];
    char rev[30];
    int i=0, l=0;
 
    printf("Enter a string \n");
    scanf("%[^\n]s",str);
 
    //computing length of the 'str'
    for(l=0; str[l] != '\0'; ++l);
 
    //assign characters to 'rev' in reverse order
    while(--l>=0){
        rev[i++] =str[l];
    }
    rev[i]='\0';
 
    printf("\n");
 
    printf("String: \t %s \n",str);
    printf("Reverse: \t %s",rev);
 
    return 0;
}

Output:

Using this method, we can also reverse a string with whitespace i.e., a whole sentence.

Method 2: Using Recursion

We can also reverse a string in C using the recursion algorithm.

We extract an character from the string on each recursive call and reassign them in reverse order during the traceback of the recursive calls.

#include <stdio.h>
 
//recursive function
void reverse(char *str, int i, int length){
    //Base condition - when index exceeds the string length
    if(i == length)
        return;
    
    //extract character
    char ch = str[i];
    //recusively call for next character
    reverse(str, i+1, length);
    //reassign in reverse order
    str[length-i-1] = ch;
}
 
int main()
{   
    char str[30];
    int length =0;
 
    printf("Enter a string \n");
    scanf("%[^\n]s",str);
 
    //computing length of the 'str'
    for(length=0; str[length] != '\0'; ++length);
    
    //reverse using the recusive function
    reverse(str, 0, length);
    
    //output the reversed string
    printf("%s",str);
    return 0;
}

Output:

Method 3: Using strrev()

string.h library has an inbuilt function called strrev() which in-place reverses the string passed as an argument.

Note: strrev() only works with non-linux compiler. strrev() method is not present in glibc compiler.

#include <stdio.h>
#include <string.h>
 
int main()
{
    char string[30];
 
    printf("Enter a string \n");
    scanf("%[^\n]s",string);
 
    printf("\n");
 
    printf("String: \t %s \n",string);
 
    //reversing string using inbuilt strrev() method
    strrev(string);
 
    printf("Reverse: \t %s",string);
 
    printf("\n");
 
    return 0;
}

Output:

These are three different ways we can use to reverse a string in a C programming language. Please comment below if you know any other methods.