Lambda functions

lambda operator or lambda function is a way to create small anonymous functions, i.e. functions without a name. These functions are throw-away functions, i.e. they are just needed where they have been created.

Lambda functions are mainly used in combination with the functions filter(), map() and reduce()

The general syntax of a lambda function is quite simple:

    lambda argument_list: expression

The argument_list consists of a comma separated list of arguments and the expression is an arithmetic expression using these arguments. You can assign the function to a variable to give it a name.

>>> f = lambda x, y : x + y
>>> f(1,1)
2

The map() function

The advantage of the lambda operator can be seen when it is used in combination with the map() function.

map() is a function with two arguments:

r = map(func, seq)

map() applies the function func to all elements of sequence seq. It returns a new list with the elements changed by func

Without lambda

    def fahrenheit(T):
        return ((float(9)/5)*T + 32)
    def celsius(T):
        return (float(5)/9)*(T-32)
    temp = (36.5, 37, 37.5,39)

    F = map(fahrenheit, temp)
    C = map(celsius, F)

With lambda

    >>> Celsius = [39.2, 36.5, 37.3, 37.8]
    >>> Fahrenheit = map(lambda x: (float(9)/5)*x + 32, Celsius)
    >>> print Fahrenheit
    [102.56, 97.700000000000003, 99.140000000000001, 100.03999999999999]
    >>> C = map(lambda x: (float(5)/9)*(x-32), Fahrenheit)
    >>> print C
    [39.200000000000003, 36.5, 37.300000000000004, 37.799999999999997]
    >>> 

Map can be applied on one or more lists.

• The lists have to have the same length.

• map() will apply its lambda function to the elements of the argument lists,

    >>> a = [1,2,3,4]
    >>> b = [17,12,11,10]
    >>> c = [-1,-4,5,9]
    >>> map(lambda x,y:x+y, a,b)
    [18, 14, 14, 14]
    >>> map(lambda x,y,z:x+y+z, a,b,c)
    [17, 10, 19, 23]
    >>> map(lambda x,y,z:x+y-z, a,b,c)
    [19, 18, 9, 5]
  

The filter() function

The function filter(f,l) needs a function f as its first argument. f returns a Boolean value, i.e. either True or False.

This function will be applied to every element of the list l. Only if f returns True will the element of the list be included in the result list.

Here the function will list the even fiboancei numbers

    >>> fib = [0,1,1,2,3,5,8,13,21,34,55]
    >>> result = filter(lambda x: x % 2, fib)
    >>> print result
    [1, 1, 3, 5, 13, 21, 55]
    >>> result = filter(lambda x: x % 2 == 0, fib)
    >>> print result
    [0, 2, 8, 34]

reduce()

Reduce function take the list as input and applies all the input. In other we reduce the list to single number applying a specific function.

>>> from functools import reduce
>>> reduce(lambda x,y: x+y, [47,11,42,14])
114
>>>

or

>>> f = lambda a,b: a if (a > b) else b
>>> reduce(f, [47,11,42,102,13])
102
>>> 

Reference:

• http://www.python-course.eu/lambda.php