Python Map and Filter

Python map function

The Python map() function applies a function to a set of elements in an iterable, such as a list. Here’s an example of how to use it:

def multiply_by_two(x):
  return x * 2

numbers = [1, 2, 3, 4]
result = map(multiply_by_two, numbers)
print(list(result))

The output of this code would be [2, 4, 6, 8].

The map() function applies the multiply_by_two() function to each element in the numbers list, and returns a map object with the results. To see the results, you can convert the map object to a list, as shown in the print() statement. You can also use the map() function with multiple iterables. For example:

def multiply(x, y):
  return x * y

numbers1 = [1, 2, 3, 4]
numbers2 = [2, 3, 4, 5]
result = map(multiply, numbers1, numbers2)
print(list(result))

This would output [2, 6, 12, 20]. The map() function applies the multiply() function to each pair of elements (one from numbers1 and one from numbers2), and returns a map object with the results.

Python filter function

The filter() function in Python is used to filter a sequence of data, such as a list, by applying a function to each element in the list and only keeping the elements for which the function returns a True value. Here’s an example of how to use it:

def is_even(num):
    return num % 2 == 0

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

even_numbers = filter(is_even, numbers)

print(even_numbers)  # prints [2, 4, 6, 8, 10]

The filter() function takes two arguments: the function and the iterable (such as a list). It returns an iterator that produces the elements of the iterable for which the function returns a True value.

You can also use the filter() function with a lambda function (an anonymous function) instead of a named function like is_even():

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

even_numbers = filter(lambda x: x % 2 == 0, numbers)

print(even_numbers)  # prints [2, 4, 6, 8, 10]

You can also use the filter() function with other iterables, such as strings:

vowels = 'aeiou'

def is_vowel(char):
    return char in vowels

sentence = 'This is a sentence.'

vowels_in_sentence = filter(is_vowel, sentence)

print(vowels_in_sentence)  # prints 'i i a ee'