Table of Contents

• 1. where
  • 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. Feature Detection
    • 1.6.2. Safe Definition
    • 1.6.3. Module Loading
  • 1.7. Common Pitfalls
  • 1.8. Error Conditions
  • 1.9. Implementation Notes
  • 1.10. Comparison with known
  • 1.11. See Also

1. where

Determines which dictionary on the dictionary stack contains a given key.

1.1. Syntax

key where → dict true (if found)
          → false (if not found)

1.2. Stack Effects

Table 1. Before

Level	Object
0	key (key to search for)

Table 2. After (if found)

Level	Object
1	true
0	dict (dictionary containing key)

Table 3. After (if not found)

Level	Object
0	false

1.3. Description

where searches for key in each dictionary on the dictionary stack, starting with the topmost (current) dictionary.

• If found: returns the dictionary object and true

• If not found: returns false

This allows you to determine not just whether a key exists, but also which dictionary contains it.

1.4. PostScript Level

Level 1 and later

1.5. Examples

Finding a key

/pi 3.14159 def
/pi where {
  % Stack: dict true
  pop /pi get  % Get value: 3.14159
} {
  % Not found
  0
} ifelse

Conditional execution

/setpagedevice where {
  pop  % Level 2 available
  /A4 setpagesize
} {
  % Level 1 - use alternate method
  statusdict /a4tray known {
    statusdict begin a4tray end
  } if
} ifelse

Checking multiple dictionaries

/x 1 def  % In userdict

5 dict begin
  /x 2 def  % In local dict

  /x where {
    % Returns local dict (searched top-down)
  } if
end

1.6. Common Use Cases

1.6.1. Feature Detection

/languagelevel where {
  pop languagelevel 2 ge
} {
  false  % Level 1 (languagelevel not defined)
} ifelse

1.6.2. Safe Definition

/myvar where {
  pop  % Already defined, don't redefine
} {
  /myvar defaultValue def
} ifelse

1.6.3. Module Loading

/ModuleName where {
  pop  % Module already loaded
} {
  (module.ps) run  % Load module
} ifelse

1.7. Common Pitfalls

Returns Dictionary, Not Value - where returns the dictionary itself, not the value.

/x 42 def
/x where  % Returns: userdict true (not 42!)

True on Top - Boolean result is on top, dictionary below.

/key where {
  % Stack: dict true
  pop  % Remove true to access dict
  /key get
} if

Idiom for Getting Value - Common pattern to get value:

/key where { /key get } { defaultValue } ifelse

1.8. Error Conditions

Error	Condition
[invalidaccess]	Dictionary on stack has no-access attribute
[stackoverflow]	Not enough room for results
[stackunderflow]	No operand on stack
[typecheck]	Operand wrong type

1.9. Implementation Notes

• Searches dictionary stack top-down

• Stops at first match

• Returns actual dictionary object

• Fast operation (series of hash lookups)

1.10. Comparison with known

Aspect	known	where
Search scope	Specific dictionary	Entire dictionary stack
Returns	Boolean only	Dictionary + boolean
Syntax	dict key known	key where
Not found	false	false

1.11. See Also

• known - Check specific dictionary

• load - Get value from dictionary stack

• def - Define in current dictionary

• store - Store in dictionary stack

• get - Get from specific dictionary