Exercise 1.19 - reverse a string

Question

Write a function reverse(s) that reverses the character string s. Use it to write a program that reverses its input a line at a time.

Solution

/**
 * Exercise 1.19 - Write a function reverse that reverses the character
 * string s; use it to write a program that reverses its input a line at a time.
 *
 **/

#include <stdio.h>

#define MAXLINE 1000

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

void reverse(char rline[]);

int main(void) {
    int len;
    char line[MAXLINE];

    while ((len = _getline(line, MAXLINE)) > 0) {
        reverse(line);
        printf("%s", line);
    }

    return 0;
}

int _getline(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] = c;
        ++i;
    }
    line[i] = '\0';

    return i;
}

void reverse(char rline[]) {
    int i, j;
    char temp;

    for (i = 0; rline[i] != '\0'; ++i);

    --i;

    if (rline[i] == '\n')
        --i;

    j = 0;

    while (j < i) {
        temp = rline[j];
        rline[j] = rline[i];
        rline[i] = temp;
        --i;
        ++j;
    }
}

Explanation

A string in C is a character array which ends in 0. getline is a function in our program, which reads one character at a time using getchar and stores it in a character array called s[] and it returns the length of the array. We call the reverse function on our line. In the reverse function, we calculate the length of the line minus 0 and n if that is present. This determines the ultimate printable characters in the line from where we have to reverse.

We have to two incides, j=0 and i the last printable character and run through the program of swapping those characters till j < i. This reverses the contents of our string.

The crux of the program is this:

    j = 0;

while(j < i)
{
  temp = rline[j];
  rline[j] = rline[i];
  rline[i] = temp;
  --i;
  ++j;
}