Python: Map, Filter, List

By Xah Lee. Date: . Last updated: .

Map Function to List

map(f, list)
Apply function f to all items in list. Return a iterator of the result. Does not modify the original list

Note: python 2, map, filter, return a list. In python 3, they return a iterator object that represent the list.

def ff(nn):
    return nn + 1

xx = [1, 2, 3, 4]

print(list(map(ff, xx)) == [2, 3, 4, 5])
map(f, li1, li2, etc)
Apply f to all lists in parallel. That is, first item of result would be f(e1, e2, e3) where its args are first elemet of each list. f should take same number args as there are lists. All lists must have same number of items (if short, None is used). Return the new iterator.
def ff(x1, x2):
    return str(x1) + str(x2)

s1 = [1, 2, 3]
s2 = ["a", "b", "c"]

print(list(map(ff, s1, s2)) == ["1a", "2b", "3c"])

Filter List

filter(f, list)
Return a new iterator, such that f(item) returns True.
# return True if divisible by 2
def ff(n):
    return n % 2 == 0

aa = [0, 1, 2, 3, 4, 5]

bb = filter(ff, aa)

print(list(bb) == [0, 2, 4])

Python, Data Structure

Python, Loop