Lambda expressions

Presentation

A function can be expressed in a concise way called lambda expression. More generically, it's called an "anonymous function" because it doesn't have a name.

The function syntax is:

def the_name(parameter):
    return parameter * 2

The lambda syntax is:

lambda x: x * 2

As you see it has no name, and it's very concise.

It's particularly useful in order to give a minimalist function directly as a parameter of another function, typically, the builtin sorted.

>>> li = [('antoine', 18), ('julien', 42), ('kevin', 9)]
>>> sorted(li, key=lambda x: x[1])
[('kevin', 9), ('antoine', 18), ('julien', 42)]

In this exercise, you'll write a function named filtered, taking two parameters: an iterable, typically a list, and a filter, a lambda expression. Your function filtered should return an iterable (a list for example).

>>> items = [1, 2, 3, 4, 5, 6]
>>> even = filtered(items, lambda x: x % 2 == 0)
>>> odds = filtered(items, lambda x: x % 2 == 1)
>>> even
[2, 4, 6]
>>> odds
[1, 3, 5]

Obviously your filtered function should also work if passed a function name instead of a lambda expression.

items = [1, 2, 3, 4, 5, 6]

def is_even(x):
    return x % 2 == 0

def is_odd(x):
    return x % 2 == 1

even = filtered(items, is_even) # 2 4 6
odds = filtered(items, is_odd) # 1 3 5

(Now you see why we use lambda.)

After your filtered function, you'll write three lines using your filtered function and lambdas:

• print all numbers from 0 to 100 included that are multiple of 3
• print all numbers from 0 to 100 included that are multiple of 5
• print all numbers from 0 to 100 included that are multiple of 15

Numbers should be separated by comas, one list per line, such as:

0, 3, 6, 9, ...
0, 5, 10, 15, ...
0, 15, 30, 45, ...

You must allow anyone to import your file in order to use the filtered function without having to print the three lines of numbers. To do so, you'll have to put the instructions to print these three lines within an if statement: if __name__ == '__main__':.

This is very common statement in Python, used exactly for this: Allowing to import a file without executing a part of it. This part is typically used for running some tests, like you just did.