first_word — Display First Word of a String

Allowed functions: write

Write a program that takes a string and displays its first word, followed by a newline.

A word is a section of string delimited by spaces/tabs or by the start/end of the string.

If the number of parameters is not 1, or if there are no words, simply display a newline.

Examples

$> ./first_word "FOR PONY" | cat -e
FOR$
$> ./first_word "this        ...    is sparta, then again, maybe    not" | cat -e
this$
$> ./first_word "   " | cat -e
$
$> ./first_word "a" "b" | cat -e
$
$> ./first_word "  lorem,ipsum  " | cat -e
lorem,ipsum$

Solution

Download first_word.c

#include <unistd.h>

int main(int c, char **v) {

	if (c != 2) {

		write(1, "\n", 1);
		return 0;
	}

	char *s = v[1];
	while (*s == ' ' || *s == '\t')
		s++;
	while (*s && *s != '\t' && *s != ' ') {
		write(1, s, 1);
		s++;
	}

	write(1, "\n", 1);

	return 0;
}

How It Works

Goal: Display the first word of a string, skipping leading whitespace.

Approach: Use pointer iteration to skip spaces/tabs, then print characters until the next whitespace or end of string.

Step by step:

1. If argument count is not 2, print a newline and exit early.
2. Set a pointer to the start of the string and advance it past any leading spaces or tabs.
3. Loop through characters, writing each one, until a space, tab, or null terminator is reached.
4. Print a trailing newline.

Key concept: Pointer iteration and whitespace skipping — advancing a pointer through a string to bypass unwanted characters before processing.