# Exercise 4.13 - reverse the string s¶

## Question¶

Write a recursive version of the function reverse(s), which reverses the string s in place.

/* a recursive version of revese(s); the string reverse function */

#include<stdio.h>
#include<string.h>

#define MAXLINE 100

int mgetline(char line[], int lim);

void reverse(char s[]);

int main(void) {
char s[MAXLINE];

mgetline(s, MAXLINE);

reverse(s);

printf("%s", s);

return 0;
}

int mgetline(char line[], int lim) {
int i, c;

for (i = 0; i < lim - 1 && (c = getchar()) != EOF && c != '\n'; ++i)
line[i] = c;

if (c == '\n')
line[i++] = '\n';

line[i] = '\0';
}

void reverse(char s[]) {

static int i = 0;
static int len;

int j;
char c;

if (i == 0) {
len = strlen(s);
}

j = len - (i + 1);

if (i < j) {
c = s[i];
s[i] = s[j];
s[j] = c;
i++;
reverse(s);
}

// the algorithm has finished so we have to set i=0 again
else {
i = 0;
}
}

## Explanation¶

The main part of this program is the reverser function.

void reverser(char s[],int i,int len)
{
int c,j;

j = len - (i + 1);

if( i < j )
{
c = s[i];
s[i] = s[j];
s[j] = c;

reverser(s,++i,len);
}
}

The string to be reversed is taken in the character array s and the first invocation is called with i=0. The value len stands for the length of the string. During each invocation, j is calculated as len - (i+1), which is the character from the end which needs to be swapped and characters at i is swapped with j. And then reverser is called again with the next value of i, i.e, ++i. This whole operation is done till i (from left hand side of the string) is less than j (from the right end), i.e, i < j.