# Specifying Types of Expression in Patterns

You can tell a lot about what "type" of expression something is by looking at its head. Thus, for example, an integer has head Integer, while a list has head List.

In a pattern, _h and x_h represent expressions that are constrained to have head h. Thus, for example, _Integer represents any integer, while _List represents any list.

 x_h an expression with head h x_Integer an integer x_Real an approximate real number x_Complex a complex number x_List a list x_Symbol a symbol

Patterns for objects with specified heads.

This replaces just those elements that are integers:
 In:= Out= You can think of making an assignment for f[x_Integer] as like defining a function f that must take an argument of "type" Integer.

This defines a value for the function gamma when its argument is an integer:
 In:= The definition applies only when the argument of gamma is an integer:
 In:= Out= The object 4. has head Real, so the definition does not apply:
 In:= Out= This defines values for expressions with integer exponents:
 In:= The definition is used only when the exponent is an integer:
 In:= Out= 