Test the word count program

K&R Exercise 1-11: say how you would test the word-count program, and which kinds of input are most likely to uncover bugs.

This is a testing exercise, not a new program. You already have Make escapes visible. Same getchar loop; this time you compile the word-count listing below and try to break it. You do not need the book.

What counts as a word

A word here is a run of characters that are not a space, a tab, or a newline. That is the whole definition. Punctuation stays glued to the letters. hello, is one word. don't is one word.

A line is a '\n'. A character is every getchar that is not EOF.

The program you are testing

Create a file called wc.c:

#include <stdio.h>

#define IN 1  /* inside a word */
#define OUT 0 /* outside a word */

int main(void)
{
    int c, nl, nw, nc, state;

    state = OUT;
    nl = nw = nc = 0;
    while ((c = getchar()) != EOF) {
        ++nc;
        if (c == '\n') {
            ++nl;
        }
        if (c == ' ' || c == '\n' || c == '\t') {
            state = OUT;
        } else if (state == OUT) {
            state = IN;
            ++nw;
        }
    }
    printf("%d lines, %d words, %d characters\n", nl, nw, nc);
    return 0;
}

state starts OUT. The first non-whitespace character bumps the word count and flips the state to IN. Further letters in that word do nothing to nw. The next space, tab, or newline puts you back OUT.

Keep c as an int. getchar() and EOF already showed why: getchar has to return every character and EOF.

Compile and run

In Terminal, from the folder that holds wc.c:

gcc wc.c -o wc
./wc

Type a line, press Return, then Ctrl+D on an empty line. Or pipe a string so the test is exact:

printf 'hello\n' | ./wc

You should see:

1 lines, 1 words, 6 characters

Five letters plus the newline. If gcc is missing, finish Getting your macOS ready for C first.

Inputs that hunt for bugs

Decide the expected counts before you run each case. The interesting tests sit at the edges: nothing, only whitespace, a missing newline, punctuation, and characters that look like spaces but are not.

Empty input

printf '' | ./wc

Or run ./wc and press Ctrl+D immediately. You should see:

0 lines, 0 words, 0 characters

If this hangs, the EOF test is wrong.

Only newlines

printf '\n\n\n' | ./wc

3 lines, 0 words, 3 characters

Three returns, no words. A program that treats “a line” as “anything you typed” would get this wrong.

Only blanks and tabs

printf ' \t \t  ' | ./wc

0 lines, 0 words, 6 characters

Whitespace is not a word. There is no '\n', so the line count stays 0 even though you typed something.

A word with no newline

printf 'hello' | ./wc

0 lines, 1 words, 5 characters

The file has a word and no '\n'. Line count is the number of newline characters, not “how many rows of text it felt like.”

Punctuation glued on

printf 'hello, world!\n' | ./wc

1 lines, 2 words, 14 characters

hello, and world! are two words. The comma and the ! are not separators. Hyphens and apostrophes behave the same: don't and well-known are one word each.

That is not a crash. It is the definition. A tester still writes it down, so nobody is surprised later.

Characters that look like spaces

printf 'one\rtwo\n' | ./wc

1 lines, 1 words, 8 characters

'\r' (carriage return) is not a space, tab, or newline, so one and two count as one word. Windows line endings (\r\n) will inflate the character count and glue words together if a lone '\r' appears. Form feed and vertical tab do the same.

What is most likely to fail

The cases above are the ones that uncover mistakes. Empty input and EOF. Whitespace with no words. A last line that never got a '\n'. Punctuation that humans think is a boundary. Characters that look blank on screen but are not ' ', '\t', or '\n'.

A huge file can also overflow int counters. You do not need to generate one today — know that nl, nw, and nc will wrap if the file is big enough.

If you stored getchar in a char instead of an int, that is a bug in the program, not in the input. High-bit bytes and EOF are what expose it. Keep int c.

What you should take away

The word-count program is only as good as its definition of a word. Test the edges of that definition: empty, whitespace-only, no trailing newline, punctuation, and “space-like” characters it does not treat as separators. Decide the numbers first, then run.

Next

This is the last of the Chapter 1 walkthroughs on this site so far. Chapter 1: A Tutorial Introduction lists Exercises 1-1 through 1-11. The book continues with 1-12.