Table of Contents

• 1. ifelse
  • 1.1. Syntax
  • 1.2. Stack Effects
  • 1.3. Description
  • 1.4. PostScript Level
  • 1.5. Examples
  • 1.6. Common Use Cases
    • 1.6.1. Value Selection
    • 1.6.2. Conditional Computation
    • 1.6.3. Default Values
  • 1.7. Common Pitfalls
  • 1.8. Error Conditions
  • 1.9. Implementation Notes
  • 1.10. Pattern: Multi-Way Branch
  • 1.11. See Also

1. ifelse

Conditionally executes one of two procedures based on a boolean value.

1.1. Syntax

bool proc1 proc2 ifelse → –

1.2. Stack Effects

Table 1. Before

Level	Object
2	bool (boolean condition)
1	proc1 (procedure if true)
0	proc2 (procedure if false)

Table 2. After

Level	Object
(varies)	Results from executed procedure

1.3. Description

ifelse removes all three operands from the stack, then executes proc1 if bool is true, or proc2 if bool is false.

The ifelse operator itself pushes no results, but the executed procedure may push results.

1.4. PostScript Level

Level 1 and later

1.5. Examples

Basic two-way branch

4 3 lt
  { (4 < 3: true) }
  { (4 < 3: false) }
ifelse
% Prints: 4 < 3: false

Returning different values

x 0 ge
  { x }
  { x neg }
ifelse
% Returns absolute value

Nested conditionals

x 0 gt {
  (positive)
} {
  x 0 lt {
    (negative)
  } {
    (zero)
  } ifelse
} ifelse

1.6. Common Use Cases

1.6.1. Value Selection

/max {  % num1 num2 => max
  2 copy gt { exch } if pop
} def

/min {  % num1 num2 => min
  2 copy lt { exch } if pop
} def

1.6.2. Conditional Computation

isColor {
  currentrgbcolor  % Get RGB
} {
  currentgray dup dup  % Convert gray to RGB
} ifelse

1.6.3. Default Values

/getValue {  % dict key defaultValue => value
  3 1 roll
  2 copy known {
    get exch pop
  } {
    pop pop
  } ifelse
} def

1.7. Common Pitfalls

Both Procedures Required - Unlike some languages, you cannot omit the false branch.

condition { true-code } ifelse  % Error: stackunderflow
condition { true-code } { } ifelse  % Correct (empty false branch)

Stack Balance - Both procedures should leave the stack in the same state.

% Bad: different stack effects
x 0 gt { x x mul } { } ifelse  % Unbalanced

% Good: both push one value
x 0 gt { x x mul } { 0 } ifelse

Use for Single Branch - If you only need one branch, if is simpler.

1.8. Error Conditions

Error	Condition
[stackunderflow]	Fewer than 3 operands on stack
[typecheck]	First operand not boolean, or second/third not procedures

1.9. Implementation Notes

• Only the selected procedure is executed

• No overhead for the non-executed branch

• Procedures execute in current context

• Very fast conditional evaluation

1.10. Pattern: Multi-Way Branch

For multiple conditions, nest ifelse or use dictionary dispatch:

% Nested ifelse
grade 90 ge {
  (A)
} {
  grade 80 ge {
    (B)
  } {
    grade 70 ge {
      (C)
    } {
      (F)
    } ifelse
  } ifelse
} ifelse

% Dictionary dispatch (often cleaner)
5 dict begin
  /red { 1 0 0 setrgbcolor } def
  /green { 0 1 0 setrgbcolor } def
  /blue { 0 0 1 setrgbcolor } def
  colorName load exec
end

1.11. See Also

• if - Single-branch conditional

• loop - Indefinite repetition

• repeat - Fixed repetition

• Comparison operators: eq, ne, lt, le, gt, ge

• Boolean operators: and, or, not