Difference between revisions of "Lambda function"

From CS 61A Wiki
Jump to: navigation, search
[checked revision][checked revision]
m ({{Sufficient-class}})
(Added a formal definition for lambda. Add the lexical scoping tidbit when the class is ready.)
Line 2: Line 2:
 
A '''lambda function''' is an anonymous function; that is, it has no intrinsic name. It is a compact way to define a function inline without using the <tt>def</tt> statement. Use a lambda function if its body is a simple expression.  
 
A '''lambda function''' is an anonymous function; that is, it has no intrinsic name. It is a compact way to define a function inline without using the <tt>def</tt> statement. Use a lambda function if its body is a simple expression.  
  
 +
== Definition ==
 +
<syntaxhighlight lang="python">
 +
lambda args: return_expression
 +
</syntaxhighlight>
 +
The lambda expression above in Python behaves like the code below. 
 +
<syntaxhighlight lang="python">
 +
def <lambda>(args):
 +
    return return_expression
 +
</syntaxhighlight>
 +
Note that unlike with <code>def</code>, the lambda function does not automatically get bounded to a name in the [[Frame|frame]] (hence its anonymity). In an [[Environment diagram|environment diagram]], a new function is created in the heap when the lambda expression gets evaluated.
 +
 +
<!-- A lambda function is lexically scoped, so the new function object's parent is the current frame, the frame in which the lambda expression is evaluated, not the frame where any subsequent call expressions gets evaluated. -->
 
== Examples ==
 
== Examples ==
 
<syntaxhighlight lang="python">
 
<syntaxhighlight lang="python">
Line 25: Line 37:
 
True
 
True
 
</syntaxhighlight>
 
</syntaxhighlight>
 +
 +
==External links==
 +
*[http://docs.python.org/3.4/reference/expressions.html#lambda| Python 3.4 language reference for lambdas]

Revision as of 18:16, 8 June 2014

A lambda function is an anonymous function; that is, it has no intrinsic name. It is a compact way to define a function inline without using the def statement. Use a lambda function if its body is a simple expression.

Definition

lambda args: return_expression

The lambda expression above in Python behaves like the code below.

def <lambda>(args):
    return return_expression

Note that unlike with def, the lambda function does not automatically get bounded to a name in the frame (hence its anonymity). In an environment diagram, a new function is created in the heap when the lambda expression gets evaluated.

Examples

>>> def call_f(f):
...     return f()
... 
>>> call_f(lambda: 3)
3

You can bind a variable name to a lambda function if you want to refer to it in the future:

>>> even = lambda x: x % 2 == 0
>>> even(40)
True
>>> even(5)
False

You can also call lambda functions like regular functions:

>>> (lambda x: x % 2 == 0)(40)
True

External links