reverse-words.c

/* Reverse words delimeted by spaces or tabs */
#include <stdio.h>

static inline void swap(char *start, char *end)
{
	char temp = *start;
	*start = *end;
	*end = temp;
	return;
}

void reverse_chars(char *start, char *end)
{
	while (end > start) {
		swap(start, end);
		start++, end--;
	}
	return;
}

/*
 * Only spaces or tabs are treated as word delimeters.
 * Everything else is treated as part of a word.
 * The str argument is a NUL terminated C-string.
 */
void reverse_words(char *str)
{
	char *start, *end;
	start = str;

	/* make sure that 'start' points to a
   * non-space character */
	while (*start == ' ' || *start == '\t')
		start++;

	end = start;
	while (*end != '\0') {
		end++;
		if (*end == ' ' || *end == '\t') {
			reverse_chars(start, end - 1);
			end++;
			/* Skip to next non-space character */
			while (*end == ' ' || *end == '\t')
				end++;
			start = end;
		}
	}

	end--;
	reverse_chars(start, end);
	reverse_chars(str, end);
	return;
}

/* Test reverse words function */
int main()
{
	char input_string[128];

	fgets(input_string, sizeof(input_string), stdin);
	reverse_words(input_string);
	printf("%s\n", input_string);
	return 0;
}