1. Word Count Program

This example is a close adaptation of a tutorial example for Knuth and Levy's CWEB programming system, translated from CWEB to the Literate programming system. That example in turn is based on a program by Klaus Guntermann and Joachim Schrod [TUGboat 7 (1986), 134–137] for a version of the "word count" program from UNIX.

This example demonstrates literate programming in C, although the Literate programming system can be used in conjunction with any programming language. The level of detail in this document is intentionally high, for didactic purposes; many of the things spelled out here don’t need to be explained in other programs.

The purpose of wc is to count lines, words, and/or characters in a list of files. The number of lines in a file is the number of newline characters it contains. The number of characters is the file length in bytes. A "word" is a maximal sequence of consecutive characters other than newline, space, or tab, containing at least one visible ASCII code. (We assume that the standard ASCII code is in use.)

This version of wc has a nonstandard "silent" option (-s), which suppresses printing except for the grand totals over all files.

Most Literate programs share a common structure. It’s probably a good idea to state the overall structure explicitly at the outset, even though the various parts could all be introduced in a piecemeal fashion.

Here, then, is an overview of the file wc.c that is defined by this Literate program wc.lit:

▶ 1. Header files to include
▶ 2. Preprocessor definitions
▶ 3. Global variables
▶ 4. Functions
▶ 5. The main program

We must include the standard I/O definitions, since we want to send formatted output to stdout and stderr.

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

The status variable will tell the operating system if the run was successful or not, and prog_name is used in case there’s an error message to be printed.

#define OK 1    /* status code for successful run */
#define usage_error 1    /* status code for improper syntax */
#define cannot_open_file 2    /* status code for file access error */ 

int status = OK;    /* exit status of command, initially OK */
char *prog_name;    /* who we are */

Now we come to the general layout of the main() function.

int main(int argc, char **argv)
{
    ▶ 1. Variables local to main
    prog_name = argv[0];
    ▶ 2. Set up option selection
    ▶ 3. Process all the files
    ▶ 4. Print the grand totals if there were multiple files
    return status;
}

If the first argument begins with a '-', the user is choosing the desired counts and specifying the order in which they should be displayed. Each selection is given by the initial character (lines, words, or characters). For example, '-cl' would cause just the number of characters and the number of lines to be printed, in that order. The default, if no special argument is given, is '-lwc'.

We do not process this string now; we simply remember where it is. It will be used to control the formatting at output time.

If the '-' is immediately followed by 's', only summary totals are printed.

int file_count;    /* how many files there are */
char *which;    /* which counts to print */
int silent = 0;    /* nonzero if the silent option was selected */

Now we scan the remaining arguments and try to open a file, if possible. The file is processed and its statistics are given. We use a do ... while loop because we should read from the standard input if no file name is given.

argc--;

do
{
    ▶ 1. If a file is given, try to open *(++argv ); continue if unsuccessful
    ▶ 2. Initialize pointers and counters
    ▶ 3. Scan file
    ▶ 4. Write statistics for file
    ▶ 5. Close file
    ▶ 6. Update grand totals

} while (--argc > 0);

Here’s the code to open the file. A special trick allows us to handle input from stdin when no name is given. Recall that the file descriptor to stdin is 0; that’s what we use as the default initial value.

int fd = 0;

We will do some homemade buffering in order to speed things up: Characters will be read into the buffer array before we process them. To do this we set up appropriate pointers and counters.

char buffer[BUFSIZ];    /* we read the input into this array */
register char *ptr;    /* the first unprocessed character in buffer */
register char *buf_end;    /* the first unused position in buffer */
register int c;    /* current character or number of characters just read */
int in_word;    /* are we within a word? */
long word_count, line_count, char_count;   
    /* number of words, lines, and characters found in the file so far */

ptr = buf_end = buffer;
line_count = word_count = char_count = 0;
in_word = 0;

The grand totals must be initialized to zero at the beginning of the program. If we made these variables local to main, we would have to do this initialization explicitly; however, C’s globals are automatically zeroed. (Or rather, "statically zeroed.") (Get it?)

long tot_word_count, tot_line_count, tot_char_count; 
    /* total number of words, lines and chars */

The present section, which does the counting that is wc's raison d'être, was actually one of the simplest to write. We look at each character and change state if it begins or ends a word.

while (1)
{
	▶ 1. Fill buffer if it is empty; break at end of file

	c = *ptr++;
	if (c > ' ' && c < 177)    /* visible ASCII codes */
	{
	        if (!in_word)
		{
			word_count++;
			in_word = 1;
		}

		continue;
	}

	if (c == '\n')
		line_count++;
	else
	{
		if (c != ' ' && c != '\t')
			continue;
	}

	in_word = 0;    /* c is newline, space, or tab */
}

Buffered I/O allows us to count the number of characters almost for free.

if (ptr >= buf_end) {
    ptr = buffer;
    c = read(fd, ptr, BUFSIZ);
    if (c <= 0) break;
    char_count += c;
    buf_end = buffer + c;
}

It’s convenient to output the statistics by defining a new function wc_print(); then the same function can be used for the totals. Additionally we must decide here if we know the name of the file we have processed or if it was just stdin.

if (!silent) {
    wc_print(which, char_count, word_count, line_count);
    if (file_count) printf(" %s\n", *argv);    /* not stdin */
    else printf("\n");    /* stdin */
}

tot_line_count += line_count;
tot_word_count += word_count;
tot_char_count += char_count;

We might as well improve a bit on UNIX’s wc by displaying the number of files too.

if (file_count > 1 || silent) {
    wc_print(which, tot_char_count, tot_word_count, tot_line_count);
    if (!file_count) printf("\n");
    else printf(" total in %d file%s\n", file_count, file_count > 1 ? "s" : "");
}

Here now is the function that prints the values according to the specified options. The calling routine is supposed to supply a newline. If an invalid option character is found we inform the user about proper usage of the command. Counts are printed in 8-digit fields so that they will line up in columns.

void wc_print(char *which, long char_count, long word_count, long line_count)
{
    while (*which)
        switch (*which++) {
        case 'l': printf("%8ld", line_count);
            break;
        case 'w': printf("%8ld", word_count);
            break;
        case 'c': printf("%8ld", char_count);
            break;
        default:
            if ((status & 1) == 0) {
                fprintf(stderr, "\nUsage: %s [-lwc] [filename ...]\n", prog_name);
                status |= 1;
            }
        }
}