Python Filter List

To filter a list, we use filter () method function. The filter () will test every element true or not in the sequence.

Syntax:

Filter(function, sequence)

Parameters:

• Function: Function that check and verifies if each number of a sequence true or not.
• Sequence: sequence has to filter numbers from the list, sets or tuples of any iterator.
• Returns: Returns already filtered numbers from an iterator.

Examples

• Example program for filtering vowels and sequence:

deffun(variable):
    alpha =['u', 'o', 'i', 'e', 'a']
    if(variable inalpha):
        returnTrue
    else:
        returnFalse
sequence =['z', 'a', 'e', 'm', 'k', 'x', 'v', 'r']
  
// using filter function  
Print('filtered elements:')
form infiltered:
    print(m)

Output:

Filtered elements:
a
e

• Example program for odd and even numbers:

sequence=[6,7,8,9,0,5]
  
// odd elements of the list
res =filter(lambdax: x %2!=0, seq)
print(list(res))
  
// even elements of the list
res =filter(lambdax: x %2==0, seq)
print(list(res))

Output:

[5, 7, 9]
[0, 6, 8]

Approaches

For filtering list, we have three approaches:

1. A for loop approach
2. A list comprehension
3. filter () method function.

A For Loop Approach

• A for loop will be repeated until its condition fails.
• A for loop goes through every number of a list.
• Based upon the condition numbers are added.

Example:

ages = [15,18,20,10,30]
young_ages = []
for age in ages:
    if age >= 18:
young_ages.append(age)
print(young_ages)

Output:

[18, 20, 30]

List Comprehension Approach

In short, list comprehension is a shorthand for looping through a list with one line of code. It follows this syntax:

[number for number in numbers if condition]

Example:

ages = [15, 18, 20, 10, 30]
young_ages = [age for age in ages if age >= 18]
print(young_ages)

Output:

[18, 20, 30]

Filter () Function

The filter () function returns a filter object with the filtered elements. You can convert this filter object into a list using the list() function.

Syntax:

Filter (function, numbers)

Example:

Filtering function that tests ages.

def age_test(age):
    return age >= 18
// passing age_test () function
ages = [15, 18, 20, 10, 30]
young_ages = filter(age_test, ages)
print(list(young_ages))

Output:

[18, 20, 30]