map, filter, reduce, zip

lambda
map()
filter() - filter (check by True, False)
reduce()
zip()

`lambda`

# Syntax: lambda arguments: expression

# EXAMPLE 1
add = lambda a, b, c: a + b + c
print(add(4, 5, 1))  # 10

# EXAMPLE 2
double = lambda x: x * 2
print(double(5)) # 10

`map()`

map() returns map object (which is iterator), it is the result after applying the given function to each item in iterable (list, tuple)

# Syntax: map(fun, iter)

# EXAMPLE 1
numbers = (1, 2, 3, 4)
result = map(lambda x: x + x, numbers)
print(list(result))  # [2, 4, 6, 8]

# EXAMPLE 2
numbers1 = [1, 2, 3]
numbers2 = [4, 5, 6]
result = map(lambda x, y: x + y, numbers1, numbers2)
print(list(result))  # [5, 7, 9]

# EXAMPLE 3
l = ['sat', 'bat', 'cat', 'mat']
test = list(map(list, l))  # map() can listify the list of strings individually
print(test)  # [['s', 'a', 't'], ['b', 'a', 't'], ['c', 'a', 't'], ['m', 'a', 't']]

# EXAMPLE 4: Convert all elements of list in uppercase
people = ["lokesh", "bob", "tom", "developer"]
up = list(map(lambda x: x.upper(), people))
print(up) # ['LOKESH', 'BOB', 'TOM', 'DEVELOPER']

# EXAMPLE 5: Working with dictionary 
names = [
    {'first': 'lokesh', 'last': 'sharma'},
    {'first': 'Astha', 'last': 'verma'},
    {'first': 'jiu', 'last': 'rai'}
]
first_names = list(map(lambda x: x['first'], names))
print(first_names)  # ['lokesh', 'Astha', 'jiu']

`filter()` - filter (check by True, False)

# Syntax: filter(fun, Iter)
# fun: function that tests if each element of a sequence true or not.
# Iter: Iterable which needs to be filtered

# EXAMPLE 1: Filter out even and odd numbers from list
seq = [0, 1, 2, 3, 4, 5]
odd_seq = filter(lambda x: x % 2 == 1, seq)
even_seq = filter(lambda x: x % 2 == 0, seq)
print(list(odd_seq)) # [1, 3, 5]
print(list(even_seq)) # [0, 2, 4]

# EXAMPLE 2: Given a twitter database, check which users don't have tweets
users = [
    {"username": 'samuel', "tweets": ["i love cake", "i am good"]},
    {"username": 'andy', "tweets": []},
    {"username": 'kumal', "tweets": ["India", "Python"]},
    {"username": 'sam', "tweets": []},
    {"username": 'lokesh', "tweets": ["i am good"]},
]
inactive_users = list(filter(lambda a: not a['tweets'], users))
print(inactive_users)
Output: [{'username': 'andy', 'tweets': []}, 
         {'username': 'sam', 'tweets': []}]

# EXAMPLE 3: Filter inactive users with just username in uppercase.
inactive_users = list(map(lambda x: x["username"].upper(),
                      filter(lambda a: not a['tweets'], users)))
print(inactive_users)
Output:['ANDY', 'SAM']

# EXAMPLE 4: Return a new list with the string “your name is” + name ,but only if length of name is bigger than 4
names = ['lokesh','lassie','bob','to']
new = list(map(lambda name: f"your name is {name}",
           filter(lambda x: len(x) > 4, names)))
print(new)  # ['your name is lokesh', 'your name is lassie']

`reduce()`

reduce() accepts a function and a sequence and returns a single value calculated as follows:

Initially, the function is called with the first two items from the sequence and the result is returned.
The function is then called again with the result obtained in step 1 and the next value in the sequence. This process keeps repeating until there are items in the sequence.

from functools import reduce

# EXAMPLE 1: Find Multiply of all elements in list
seq = [2,3,4,5,6]
multiply = reduce(lambda a, b: a * b, seq)
print(multiply)
"""
Output: 720
First:  It takes 2,3 and    return 6
Second: It takes 6 and 4,   retuen 24
Third:  It takes 24 and 5,  return 220
Fourth: It takes 220 and 6, return 720
All elements in sequence are over so it returns 720 as output.
"""

`zip()`

zip() - The zip() function take iterables (can be zero or more), makes iterator that aggregates elements based on the iterables passed, and returns an iterator of tuples.

If no arg → zip() returns empty iterator
If 1 arg → it returns each item bounded in tuple
If 2 and more args → it returns aggregation till shortest one

# Syantax: zip(*iterables)

# EXAMPLE 1: Create list of tuples
name = ["Manjeet", "Nikhil", "Shambhavi"]
roll_no = [4, 1, 3]
marks = [40, 50, 60]
mapped = zip(name, roll_no, marks)
print(list(mapped))  # [('Manjeet', 4, 40), ('Nikhil', 1, 50), ('Shambhavi', 3, 60)]

# EXAMPLE 2: Create dictionary from Two iterables
name = ["Manjeet", "Nikhil", "Shambhavi"]
marks = [40, 50, 60]
mapped = zip(name, marks)
print(dict(mapped))  # {'Manjeet': 40, 'Nikhil': 50, 'Shambhavi': 60}

lambda

map()

filter() - filter (check by True, False)

reduce()

zip()

`lambda`

`map()`

`filter()` - filter (check by True, False)

`reduce()`

`zip()`