10.23. List ComprehensionsΒΆ
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,
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 mylist
. 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 mylist
.
To write the primes_upto
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_upto(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
Note
This workspace is provided for your convenience. You can use this activecode window to try out anything you like.
Check your understanding
- [4,2,8,6,5]
- Items from alist are doubled before being placed in blist.
- [8,4,16,12,10]
- Not all the items in alist are to be included in blist. Look at the if clause.
- 10
- The result needs to be a list.
- [10].
- Yes, 5 is the only odd number in alist. It is doubled before being placed in blist.
list-22-3: 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)