Exercise 1-19

The only way we can manipulate character strings using functions (at least for now) is by modifying the argument that is passed in. This means we have to reverse our string in place. We will first need to calculate the length of the string s. We do this by declaring the integer variable len and using a for-loop that increments len until s[len] is equal to \0. We then decrement len to exclude \0.

    /* reverse:  reverse character string s */
    void reverse(char s[])
    {
        int len;
    
        for (len = 0; s[len] != '\0'; ++len)
            ;
        --len;  /* exclude \0 */
    }

To reverse the string, we swap the first and the len^th element, the second and the len-1^th element, and so on, until i reaches len-i. We can achieve this using a for-loop that decrements len and increments i after every swap. We swap the elements as follows, where x and y are the elements we want to swap, and temp is a variable we declare to store the x's value:

    temp = x;
    x = y;
    y = temp;

Putting it all together, our reverse function looks like this:

    /* reverse:  reverse character string s */
    void reverse(char s[])
    {
        int len;
        int i, len, temp;
    
        for (len = 0; s[len] != '\0'; ++len)
            ;
        --len;  /* exclude \0 */
        for (i = 0; i < len; ++i) {
            temp = s[i];
            s[i] = s[len];
            s[len] = temp;
            --len;
        }
    }