Table of Contents

• 1. known
  • 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. Checking Before Access
    • 1.6.2. Optional Parameters
    • 1.6.3. Feature Detection
  • 1.7. Common Pitfalls
  • 1.8. Error Conditions
  • 1.9. Implementation Notes
  • 1.10. See Also

1. known

Tests whether a specific dictionary contains a given key.

1.1. Syntax

dict key known → bool

1.2. Stack Effects

Table 1. Before

Level	Object
1	dict (dictionary to check)
0	key (key to search for)

Table 2. After

Level	Object
0	bool (true if key exists, false otherwise)

1.3. Description

known returns true if there is an entry in dictionary dict whose key is key. Otherwise, it returns false.

The dictionary does not need to be on the dictionary stack - it can be any dictionary object.

1.4. PostScript Level

Level 1 and later

1.5. Examples

Checking for key

/mydict 5 dict def
mydict /total 0 put
mydict /total known   % Returns true
mydict /badname known % Returns false

Conditional definition

currentdict /myvar known not {
  /myvar defaultValue def
} if

Safe access

dict /key known {
  dict /key get
  % Use value
} {
  % Use default
} ifelse

1.6. Common Use Cases

1.6.1. Checking Before Access

/safeGet {  % dict key => value or null
  2 copy known {
    get
  } {
    pop pop null
  } ifelse
} def

1.6.2. Optional Parameters

/processOptions {  % dict =>
  dup /verbose known {
    /verbose get { (Verbose mode) print } if
  } if

  dup /output known {
    /output get setOutputFile
  } if

  pop
} def

1.6.3. Feature Detection

systemdict /setpagedevice known {
  % Level 2 features available
} {
  % Level 1 only
} ifelse

1.7. Common Pitfalls

Checks Specific Dictionary Only - known does NOT search the dictionary stack, only the specified dictionary.

/x 42 def  % In current dict

5 dict /x known  % Returns false (wrong dict!)
currentdict /x known  % Returns true

Different from where - known checks one dictionary; where searches the entire dictionary stack.

Use for Validation - Check before get to avoid [undefined] errors.

1.8. Error Conditions

Error	Condition
[invalidaccess]	Dictionary has no-access attribute
[stackunderflow]	Fewer than 2 operands on stack
[typecheck]	First operand not a dictionary

1.9. Implementation Notes

• Very fast operation (hash table lookup)

• Does not modify dictionary

• Can be used on any dictionary (not just those on dict stack)

• Returns immediately after checking

1.10. See Also

• where - Search dictionary stack for key

• load - Get value from dictionary stack

• get - Get value from specific dictionary

• def - Define key-value pair