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.

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