How do you reverse a string in place in C or C++?

Question

How do you reverse a string in C or C++ without requiring a separate buffer to hold the reversed string?

Answer 1

You can reverse a string in-place by swapping its first character with the last one, second character with the second last one and so on.

We can write a recursive function for that.

|-----rev----|

rev( A B C D E F G H ) ⇒ swap(A, H) + rev( B C D E F G ) ⇒ H + rev( B C D E F G ) + A

|____swap____|

void reverse(char *string, int start, int end) {

/* base case: when string has length ≤ 1 i.e. start ≥ end*/

if (start >= end)

return;

/* swap the characters pointed to be start and end indices in-place */

char ch = string[start];

string[start] = string[end];

string[end] = ch;

/* call the function reverse recursively on the rest of the string */

return reverse(string, start + 1, end - 1);

}

We can call this recursive function as follows

int main(void) {

char str[] = "abcdefgh";

printf("%s -> ", str);

reverse(str, 0, strlen(str) - 1);

printf("%s\n", str);

return 0;

}