List Comprehensions
A Python list comprehension allows you to create a list from another sequence (i.e.,
something you can iterate over using a for loop). Note that a list comprehension
is a list defined by a for-loop inside square brackets, and that the loop iterates
over some sequence, creating a new list in the process. In the examples below, the output of
each print statement is shown in italics.
print [ x*2 for x in range(0, 10) ] [0, 2, 4, 6, 8, 10, 12, 14, 16, 18] print [ s[0] for s in ['apple', 'termite', 'elephant'] ] ['a', 't', 'e'] print [ n % 2 == 0 for n in [2, 4, 6, 1, 3, 5, 8] ] [True, True, True, False, False, False, True ] print [ n for n in [2, 4, 6, 1, 3, 5, 8] if n % 2 == 0 ] [2, 4, 6, 8]
Explain List Comprehensions
For the exercises below, first try using paper, pencil, discussion, and thinking (no computer) to solve them, before using the Python Interpreter to check your answers.
print [ i*i for i in range(1, 10) if i % 2 == 0 ]print [ x**2 for x in range(1, 10, 2) ]print sum([ 1 for p in range(1, 9) ])print [ s[1] for s in ["big", "brown", "bare", "stem", "pea"] ]print [ 'po'*i for i in range(0, 5) ]print len([ x for x in "ostentatiously" if "aeiou".find(x) > -1 ])
Generate List Comprehensions
Write a list comprehension within the call to the
function sum below that returns the only the odd integers
in a list of integers named nums. So for the value [1, 3, 2, 4, 5] your
list comprehension should create the list [1, 3, 5], so the entire expression evaluates to 9.
sum( )
Using a list comprehension and the list functions
sum and len, write an expression that returns
the average length of all the strings in a list of strings
named strs. You will
call two list functions: one with strs as an
argument
and one with a list comprehension using strs as an
argument: combining in one expression
sum(...) and len(...). For example,
the average length of the strings in the list below
is (3+3+4+5)/4 = 15/4 = 3.75
strs = ["cat", "dog", "bear", "tiger"]
