Control Flow

PostScript is an interpreted language. It reads operators from a program and executes them. Some of these operators allow the developer to control the flow of execution.

All control flow operators have a similar structure. They take as operands one or more procedures and a description when to execute it.

We will go through a few control flow operators, but don't hesitate to look for more in the reference.

if

The if operator takes as its two operands a boolean, which can be obtained with various operands like eq, and, not etc, and a procedure.

It will execute the procedure only if the boolean is true.

ifelse

With a little imagination one could guess what the ifelse operator does. It accepts three operands, a boolean and two procedures. Depending on the truth value of the boolean, either the first procedure is executed or else the second procedure is executed.

repeat

If you want to repeat a procedure a number of times, repeat is the operator you are looking for. Its operands, a number and a procedure, control the execution.

The number determines how many times the procedure is executed.

for

This is your standard for loop. It accepts a whooping four operands. E.g.:

0 1 10 { pop } for

The first three operands are numbers: initial, increment and limit. The fourth operand is the procedure to execute.

The procedure runs with values from initial by steps of increment to limit, inclusive. Each time around the loop, the index is pushed onto the stack for the procedure to consume.

Exercises

1. Imagine that the language designers forgot to provided ifelse, but they provided if. Implement ifelse in terms of if and the boolean operators.