11. List Comprehensions¶
The previous example creates a list from a sequence of values based on some selection criteria. An easy way to do this type of processing in Python is to use a list comprehension. List comprehensions are concise ways to create lists. The general syntax is:
[<expression> for <item> in <sequence> if <condition>]
where the if clause is optional. For example,
my_list = [1,2,3,4,5]
your_list = [item ** 2 for item in my_list]
print(your_list)
The expression describes each element of the list that is being built. The for
clause iterates through each item in a sequence. The items are filtered by the if
clause if there is one. In the example above, the for
statement lets item
take on all the values in the list my_list
. Each item is then squared before it is added to the list that is being built. The result is a list of squares of the values in my_list
.
To write the primes_up_to
function we will use the is_prime
function to filter the sequence of integers coming from the range
function. In other words, for every integer from 2 up to but not including n
, if the integer is prime, keep it in the list.
def primes_up_to(n):
""" Return a list of all prime numbers less than n using a list comprehension. """
result = [num for num in range(2,n) if is_prime(num)]
return result
Check your understanding
What is printed by the following statements?
alist = [4,2,8,6,5]
blist = [num*2 for num in alist if num%2==1]
print(blist)