ps-ctrlstruct.md

1. Conditional Execution

if and ifelse
PostScript uses if and ifelse for conditional logic. These operators expect a boolean value and one or two procedures on the stack.

if Example:

% Check if a number is greater than 50
/num 65 def
num 50 gt { (Number is greater than 50) show } if

In this example, if num is greater than 50, the message is shown.

ifelse Example:

% Check if a number is even or odd
/num 42 def
num 2 mod 0 eq
{ (Number is even) show }
{ (Number is odd) show }
ifelse

This script checks whether num is even or odd, displaying an appropriate message.

2. Loops

PostScript provides several looping constructs: loop, repeat, for, and forall. Each serves different purposes and is used based on the context.

loop
This is an infinite loop, only exiting upon a conditional break.

% Print numbers from 1 to 10
/counter 1 def
{
    counter 10 lt
    {
        counter pstack
        /counter counter 1 add def
    }{
        exit
    } ifelse
} loop

The exit operator is used to break out of the loop.

repeat
Executes a block of code a specific number of times.

% Print "Hello" 5 times
5 { (Hello\n) print } repeat

for
Iterates over a range of numbers, commonly used for index-based operations.

% Print numbers 0 to 9
0 1 9 {
    pstack
} for

This script iterates from 0 to 9, printing each number.

forall
Iterates over elements in an array or dictionary.

% Print all elements in an array
[10 20 30 40 50] {
    pstack
} forall

If used with a dictionary, forall passes each key-value pair to the block:

<< /a 1 /b 2 >> {
    exch pstack  % First pop value, then key
    pstack
} forall

3. Exception Handling

PostScript handles runtime errors via the stopped operator, which can catch exceptions thrown by operations inside a procedure.

Example:

{
    /num 0 def
    10 num div
} stopped {
    (An error occurred.) print
} if

In this example, division by zero would normally cause an error, but stopped catches it and prints an error message instead.