Exercise 1-18

We use getline to read our lines into line and store their lengths into len. After reading in a line, we execute a loop that runs as long as line[len-2]—the last character before the newline—is a space or a tab and len is greater than 1. To delete the trailing whitespace, during every iteration, we replace line[len-1] (previously a newline) with \0, replace line[len-2] (previously a space or tab) with a newline, and decrement len. Once we exit the loop, we only print the line if its length is greater than 1.

    #include <stdio.h>
    
    #define MAXLINE 1000    /* maximum input line size */
    
    int my_getline(char line[], int maxline);
    
    main()
    {
        int len;            /* current line length */
        char line[MAXLINE]; /* current input line */
    
        while ((len = my_getline(line, MAXLINE)) > 0) {
            while (len > 1 && (line[len-2] == ' ' || line[len-2] == '\t')) {
                line[len-1] = '\0';
                line[len-2] = '\n';
                --len;
            }
            if (len > 1)    /* ignore entirely blank lines */
                printf("%s", line);
        }
        return 0;
    }

Note: notice how in the while-loop that deletes the trailing spaces, we check if len > 1 first. This is important because if len was less than 1 and we did not check it first, line[len-2] would be accessing a negative index.