The controlled form specifies name, a control variable that is assigned an initial value (the result of expri, formatted as though 0 had been added) before the first execution of the instruction list. The variable is then stepped (by adding the result of exprb) before the second and subsequent times that the instruction list is processed.
The instruction list is processed repeatedly while the end condition (determined by the result of exprt) is not met. If exprb is positive or 0, the loop is ended when name is greater than exprt. If negative, the loop is ended when name is less than exprt.
The expri, exprt, and exprb options must result in numbers. They are evaluated only one time, before the loop begins and before the control variable is set to its initial value. The default value for exprb is 1. If exprt is omitted, the loop runs indefinitely unless some other condition stops it.
Do I=3 to -2 by -1 /* Displays: */
say i /* 3 */
end /* 2 */
/* 1 */
/* 0 */
/* -1 */
/* -2 */
The numbers do not have to be whole numbers:
I=0.3 /* Displays: */
Do Y=I to I+4 by 0.7 /* 0.3 */
say Y /* 1.0 */
end /* 1.7 */
/* 2.4 */
/* 3.1 */
/* 3.8 */
The control variable can be altered within the loop, and this may affect the iteration of the loop. Altering the value of the control variable is not usually considered good programming practice, though it may be appropriate in certain circumstances.
Note that the end condition is tested at the start of each iteration (and after the control variable is stepped, on the second and subsequent iterations). Therefore, if the end condition is met immediately, the group of instructions can be skipped entirely. Note also that the control variable is referred to by name. If (for example) the compound name A.I is used for the control variable, altering I within the loop causes a change in the control variable.
The execution of a controlled loop can be bounded further by a FOR phrase. In this case, you must specify exprf, and it must evaluate to a positive whole number or zero. This acts just like the repetition count in a simple repetitive loop, and sets a limit to the number of iterations around the loop if no other condition stops it. Like the TO and BY expressions, it is evaluated only one time—when the DO instruction is first processed and before the control variable receives its initial value. Like the TO condition, the FOR condition is checked at the start of each iteration.
Do Y=0.3 to 4.3 by 0.7 for 3 /* Displays: */
say Y /* 0.3 */
end /* 1.0 */
/* 1.7 */
In a controlled loop, the name describing the control variable can be specified on the END clause. This name must match name in the DO clause in all respects except case (note that no substitution for compound variables is carried out); a syntax error results if it does not. This enables the nesting of loops to be checked automatically, with minimal overhead.
Do K=1 to 10
...
...
End k /* Checks that this is the END for K loop */