Question

How to use lambda? and other abstract list functions such as map, filter, etc?

Answer 1

LAMBDA

The lambda feature was added to Python due to the demand from Lisp programmers.

A lambda function is a small anonymous function.

A lambda function can take any number of arguments, but can only have one expression.

The general syntax of a lambda function is quite simple:

lambda argument_list: expression

Sample Program 0n Lambda:

sum = lambda x, y : x + y
sum(3,4)

Output:

7

MAP:

map() function returns a map object(which is an iterator) of the results after applying the given function to each item of a given iterable (list, tuple etc.)

produces a new iterator, applying function to each element in list.

Requires: datatype consumed by function must match type in list

syntax of map:

map(function, list)

Sample Program On MAP:

def pull_to_passing(mark):
if mark < 50 and mark > 46:
print('50')
else:
print(mark)
list(map(pull_to_passing, [34, 89, 46, 49, 52]))

output:

34 89 46 50 52

Example2:

# Add two lists using map and lambda

numbers1 = [1, 2, 3]
numbers2 = [4, 5, 6]

result = map(lambda x, y: x + y, numbers1, numbers2)
print(list(result))

Output:

[5,7,9]

Example3:

# List of strings
l = ['sat', 'bat', 'cat', 'mat']

# map() can listify the list of strings individually
test = list(map(list, l))
print(test)

Output:

[['s', 'a', 't'], ['b', 'a', 't'], ['c', 'a', 't'], ['m', 'a', 't']]

FILTER:

produces an iterator matching the elements in list for which function produces True.

Requires: type consumed by function must match type in list.

syntax of filter:

filter(function, list)

Sample Program On Filter:

def fun(variable):

    letters = ['a', 'e', 'i', 'o', 'u']

    if (variable in letters):

        return True

    else:

        return False

  sequence = ['g', 'e', 'e', 'j', 'k', 's', 'p', 'r']

  filtered = filter(fun, sequence)

print('The filtered letters are:')

for s in filtered:

    print(s)

Output

e e

Application

It is normally used with Lambda functions to separate list, tuple, or sets.

Sample Example of application

# a list contains both even and odd numbers.

seq = [0, 1, 2, 3, 5, 8, 13]

  

# result contains odd numbers of the list

result = filter(lambda x: x % 2==1, seq)

print(list(result))

  

# result contains even numbers of the list

result = filter(lambda x: x % 2 == 0, seq)

print(list(result))