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[]){

    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.

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