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Iteration Statements

Limbo has three types of iteration statements that repeat execution of a statement or block of statements until a certain condition is reached: while, do, and for loops.

Each of the loop statements can be preceeded by a label that is used in conjunction with the jump statements break and continue. See Labels later in this section for more information.

The for Loop

The for loop in Limbo, like C and other languages, is flexible and powerful.

The general format of the for loop is:

	for (exp1; exp2; exp3)
		statement;

Inside the parentheses there are three expressions. These can be any valid Limbo expression. The following are the most commonly used expressions:

exp1 is an assignment or initialization for an index variable
exp2 is the test for loop continuation
exp3 is modification to the index variable, such as increment or decrement, for each loop iteration

The statement can be an empty statement, a single statement, or a statement block enclosed in braces ({...}).

For example, the following uses a for loop to print the numbers 1 to 100:

	i: int;
	for (i = 1; i <= 100; i++)
		sys->print("%d ", i);

Notice the declaration of the integer variable, i, before entering the for loop. The declaration and assignment (initialization) can also be done in the first expression of the for statement:

	for (i := 1; i <= 100; i++)
		sys->print("%d ", i);

Variations of the for Loop

None of the three expressions in the for statement are required. For example, you can create infinite loops, time delay loops, multiple control variable loops, and many others with the flexibility of the for loop.

For example, the for loop is the iteration statement conventionally used to create an infinite loop. The following example continuously reads one character from standard input until the user enters a carriage return (presses the Return or Enter key):

	stdin := sys->fildes(0);								#get FD for stdin
	buf := array[128] of byte;
	n : int;
	for(;;) {
		n = sys->read(stdin, buf, 1);
		#check for CR or ^D (break, n=0)
		if ((int(buf[0]) == 10) || (n <=0 ))
			break;
	}

For information about break used to terminate the loop, see The break Statement on page 2-40.

The while Loop

The while loop is a general iteration statement. The general format of the while loop is:

	while (condition)
		statement;

If the condition is true, then the statement is executed. The statement can be an empty statement, a single statement, or a statement block enclosed in braces ({...}).

For example, the following program fragment prints 1 to 100:

	n := 1;
	while (n <= 100)
		sys->print("%d ", n++);

The do...while Loop

Unlike for and while loops, the do...while loop tests the condition at the end of the iteration. This means that a do...while loop executes at least once.

The general form of the do...while loop is:

	do
		statement;
	while (condition);

The statement can be an empty statement, a single statement, or a statement block enclosed in braces ({...}). The statement is repeated if the condition is true.

For example, the following shows the do...while version of the counter:

	n := 1;
	do {
		sys->print("%d ", n++);
	} while (n <= 100);

Although the braces are not required when there is only one statement, they are generally used to improve readability.

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