Filter

The goal of the filter(…) higher-order function is to create a new list with only certain items from the original list that all satisfy some criterion by applying a boolean function to each element (a function that returns either True or False) and only keeping an element if that function returns True for that element.

Below we provide two examples for this, one for a list of strings and one for a list of numbers. The first example uses a lambda expression that uses the string method startswith(…) to check whether or not a given string starts with the character ‘R’. Here is the code:

newList = filter(lambda s: s.startswith('R'), ['Building', 'ROAD', 'tree'])
print(newList)

Output
['ROAD']

In the second example, we use is_integer() from the float class to take only those elements from a list of floating point numbers that are integer numbers. Since this is a method, we again need to use the class name as a prefix (“float.”).

newList = filter(float.is_integer, [12.4, 11.0, 17.43, 13.0])
print(newList)

Output
[11.0, 13.0]

Lesson content developed by Jan Wallgrun and James O’Brien