Using strtok to break a line into fields
You can use the strtok function to break an input line read by
fgets into individual fields. The syntax of strtok is
char *strtok(char *src, char *pattern)
where:
- src is a pointer to the string you want to break into fields
- pattern contains one or more characters that denote the
delimiters for the fields. Common delimiters are a space
and a newline character.
Here is a list of pertinent facts you need to know about strtok:
- each call to strtok finds the next delimiter,
replaces it with a \0, and returns a pointer to the next field in the line.
- strtok returns NULL when it reaches the end of the line
- the first call to strtok for a line should contain a pointer to
src. Subsequent calls should pass NULL as the first argument.
Here is some example code that extracts each field from a line and
prints it:
next_word = strtok(line, " \n");
while (next_word != NULL) {
printf("word = %s\n", next_word);
next_word = strtok(NULL, " \n");
}
- Since fgets terminates most strings with \n, it is important to
include \n as a delimiter. Otherwise the last field returned
will be the string "\n", which is probably not what you want.
Example Program
Here is an example program that uses fgets and strtok to
read lines from a file, break each line into separate fields, and print
each field on a separate line, prefixed by its line number. For example, given
the input file:
The quick brown
fox jumped
the fence
the program will produce the output:
1: The
1: quick
1: brown
2: fox
2: jumped
3: the
3: fence
Here is the full text of the program:
<![CDATA[
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_LINE_SIZE 1000
int main(int argc, char *argv[]) {
FILE *input_file;
FILE *output_file;
int line_number = 0; // the line number of the current line read from input_file
char *next_field; // ptr to the next field extracted from the current line
char line[MAX_LINE_SIZE+1]; // ptr to the current input line
// check and make sure we have the proper number of arguments
if (argc != 3) {
fprintf(stderr, "usage: %s input_file output_file\n", argv[0]);
exit(1);
}
// open the input file and make sure that it is opened properly
input_file = fopen(argv[1], "r");
if (input_file == NULL) {
perror(argv[1]);
exit(1);
}
// open the output file and make sure that it is opened properly
output_file = fopen(argv[2], "w");
if (output_file == NULL) {
perror(argv[2]);
exit(1);
}
// read the file one line at a time. iterate through each line using strtok
// and print each field, prefixed with the current line number
while (1) {
fgets(line, MAX_LINE_SIZE, input_file); // read the line from input_file
// if we've reached the end of the input_file, break out of the loop
if (feof(input_file))
break;
// increment the line number because we are starting a new line
line_number++;
// initialize next_field to point to the first field on the line
next_field = strtok(line, " \n");
// iterate through the fields on the line until we have exhausted them. strtok
// will return NULL when there are no more fields to be read
while (next_field != NULL) {
// write the current field to the output file
fprintf(output_file, "%d: %s\n", line_number, next_field);
// get the next word on the line--remember to pass NULL as the first argument
// because only the first call to strtok on each line should receive a pointer
// to line
next_field = strtok(NULL, " \n");
}
}
// remember to close all files when we are done
fclose(input_file);
fclose(output_file);
}
]]>