for <==> while (an exception)

[Bill Vaughn]

(Stop me if you've seen this before.)

Section 3.5 of K&R (p. 56) states that the 'for' loop and 'while' loop
can be made equivalent i.e.

					expr1;
for (expr1; expr2; expr3)		while (expr2) {
	statement		<==>		statement
						expr3;
					}

Well ... almost.
As the following program demonstrates there is at least one exception:
If 'statement' contains a 'continue' statement, things may go awry.

Are there any other exceptions?

/*---------------------------------------------------------------------------
 * A program to show a problem with the
 * equivalence between for and while loops.
 */

#include <stdio.h>
#include <signal.h>

int i;

main()
{
	int message(), stop();

	signal(SIGALRM, message);
	signal(SIGINT, stop);

	/*
	 * Here's the for loop ...
	 */
	for (i=0; i<10; i++) {
		if (i > 0)
			continue;
	}

	printf("%d\n",i);
	printf("Hanging ...");
	fflush(stdout);
	alarm(5);

	/*
	 * Here's the equivalent while loop ..
	 * See K&R p. 56.
	 */
	i=0;
	while (i<10) {
		if (i > 0)
			continue;
		i++;
	}
}

stop()
{
	printf("\n%d\n",i);
	printf("The 'continue' statement 'breaks' the for/while equivalence.\n");
	exit(0);
}

message()
{
	printf("\t\tHit your interrupt key to stop this thing.\n");
}

/*----------------------------------------------------------------------*/

Bill Vaughn
{allegra,seismo,decvax}!rochester!ur-cvsvax!bill