# 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 maxline);
void reverse(char s[]);

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

mgetline(s,MAXLINE);

reverse(s);

printf("%s",s);

return 0;
}

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

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

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

s[i]='\0';
}

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

reverser(s,0,strlen(s));
}

/* reverser: reverse string s in place; recursive */

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);
}
}

```

## 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.