Editing Macro (computer science) (section)

==Parameterized and parameterless macro{{anchor|Parameterized macro|Parameterized|Parameterless}}==

A '''parameterized macro''' is a macro that is able to insert given objects into its expansion. This gives the macro some of the power of a [[function (computer science)|function]].

As a simple example, in the [[C (programming language)|C programming language]], this is a typical macro that is ''not'' a parameterized macro, i.e., a '''parameterless macro''':
  '''#define''' PI   3.14159
This causes <code>PI</code> to always be replaced with <code>3.14159</code> wherever it occurs. An example of a parameterized macro, on the other hand, is this:
  '''#define''' pred(x)  ((x)-1)
What this macro expands to depends on what [[argument (computer science)|argument]] ''x'' is passed to it. Here are some possible expansions:
  pred(2)    →  ((2)   -1)
  pred(y+2)  →  ((y+2) -1)
  pred(f(5)) →  ((f(5))-1)
Parameterized macros are a useful source-level mechanism for performing [[inline expansion|in-line expansion]], but in languages such as [[C (programming language)|C]] where they use simple textual substitution, they have a number of severe disadvantages over other mechanisms for performing in-line expansion, such as [[inline function]]s.

The parameterized macros used in languages such as [[Lisp programming language|Lisp]], [[PL/I]] and [[Scheme (programming language)|Scheme]], on the other hand, are much more powerful, able to make decisions about what code to produce based on their arguments; thus, they can effectively be used to perform [[run-time code generation]].