1.5.4 Word Counting
The fourth in our series of useful programs counts lines, words, and characters, with the loose definition that a word is any sequence of characters that does not contain a blank, tab or newline. This is a bare-bones version of the UNIX program wc.
1 #include <stdio.h>
2
3 #define IN 1 /* inside a word */
4 #define OUT 0 /* outside a word */
5
6 /* count lines, words, and characters in input */
7 main()
8 {
9 int c, nl, nw, nc, state;
10
11 state = OUT;
12 nl = nw = nc = 0;
13 while ((c = getchar()) != EOF) {
14 ++nc;
15 if (c == '\n')
16 ++nl;
17 if (c == ' ' || c == '\n' || c = '\t')
18 state = OUT;
19 else if (state == OUT) {
20 state = IN;
21 ++nw;
22 }
23 }
24 printf("%d %d %d\n", nl, nw, nc);
25 }
Every time the program encounters the first character of a word, it counts one more word. The variable state records whether the program is currently in a word or not; initially it is "not in a word", which is assigned the value OUT. We prefer the symbolic constants IN and OUT to the literal values 1 and 0 because they make the program more readable. In a program as tiny as this, it makes little difference, but in larger programs, the increase in clarity is well worth the modest extra effort to write it this way from the beginning. You'll also find that it's easier to make extensive changes in programs where magic numbers appear only as symbolic constants.
The line
nl = nw = nc = 0;
sets all three variables to zero. This is not a special case, but a consequence of the fact that an assignment is an expression with the value and assignments associated from right to left. It's as if we had written
nl = (nw = (nc = 0));
The operator || means OR, so the line
if (c == ' ' || c == '\n' || c = '\t')
says "if c is a blank or c is a newline or c is a tab". (Recall that the escape sequence \t is a visible representation of the tab character.) There is a corresponding operator && for AND; its precedence is just higher than ||. Expressions connected by && or || are evaluated left to right, and it is guaranteed that evaluation will stop as soon as the truth or falsehood is known. If c is a blank, there is no need to test whether it is a newline or tab, so these tests are not made. This isn't particularly important here, but is significant in more complicated situations, as we will soon see.
The example also shows an else, which specifies an alternative action if the condition part of an if statement is false. The general form is
One and only one of the two statements associated with an if-else is performed. If the expression is true, statement1 is executed; if not, statement2 is executed. Each statement can be a single statement or several in braces. In the word count program, the one after the else is an if that controls two statements in braces.
Exercise 1-11. How would you test the word count program? What kinds of input are most likely to uncover bugs if there are any?
Exercise 1-12. Write a program that prints its input one word per line.