Concepts of Programming Languages

368 Chapter 8 Statement-Level Control Structures

else

loopBody()

forLoop loopBody, (reps - 1);;

In this function, the parameter loopBody is the function with the body of the

loop and the parameter reps is the number of repetitions. The reserved word

rec appears before the name of the function to indicate that it is recursive. The

empty parentheses do nothing; they are there because in F# an empty statement

is illegal and every if must have an else clause.

8.3.2 Logically Controlled Loops

In many cases, collections of statements must be repeatedly executed, but the

repetition control is based on a Boolean expression rather than a counter. For

these situations, a logically controlled loop is convenient. Actually, logically

controlled loops are more general than counter-controlled loops. Every count-

ing loop can be built with a logical loop, but the reverse is not true. Also, recall

that only selection and logical loops are essential to express the control struc-

ture of any flowchart.

8.3.2.1 Design Issues

Because they are much simpler than counter-controlled loops, logically con-

trolled loops have fewer design issues.

• Should the control be pretest or posttest?


• Should the logically controlled loop be a special form of a counting loop
  or a separate statement?

8.3.2.2 Examples

The C-based programming languages include both pretest and posttest logi-

cally controlled loops that are not special forms of their counter-controlled

iterative statements. The pretest and posttest logical loops have the following

forms:

while (control_expression)

loop body

and

do

loop body

while (control_expression);