# Topic covered
* Built-in Functions
    * Anonymous/lambda Functions
    * map() function
    * reduce() function
    * filter() function
    * eval() function

10.1 Anonymous/lambda Functions

Sometimes we can declare a function without any name, such type of nameless functions are called anonymous/lambda functions

The main purpose of anonymous function is just for instant use(i.e. for one time usage)

AKA lambda functions or shorthand function

# Syntax
lambda argument_list : expression

print((lambda x: x + 1)(2)) # 3

s = lambda n:n*n
print(s(4)) # 64

s = lambda a,b:a+b
print(s(2, 3)) # 5

s = lambda a,b:a if a>b else b
print(s(10,20)) # 20
print(s(100,200)) # 200

print((lambda x, y, z=3: x + y + z)(1, 2))

Note

Lambda Function internally returns expression value and we are not required to write return statement explicitly

Sometimes we can pass function as argument to another function. In such cases lambda functions are the best choice.

We can use lambda functions very commonly with filter(), map() and reduce() functions, because these functions expect function as argument.

10.2 map() function

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.)

# str to int
num = ['1', '2', '3']
res = list(map(int, num))
print(res)

# Square 
def square(number):
	return number ** 2

numbers = [1, 2, 3, 4, 5]
print(list(map(square, numbers)))

# Square using map and lambda
print(list(map(lambda x:x*x, numbers)))


first_it = [1, 2, 3]
second_it = [4, 5, 6, 7]
print(list(map(pow, first_it, second_it)))

# x- y
print(list(map(lambda x, y: x - y, [2, 4, 6], [1, 3, 5])))
# [1, 1, 1]

print(list(map(lambda x, y, z: x + y + z, [2, 4], [1, 3], [7, 8])))
# [10, 15]


# Eg 5
string_it = ["processing", "strings", "with", "map"]
print(list(map(str.upper, string_it)))
# ['PROCESSING', 'STRINGS', 'WITH', 'MAP']


with_spaces = ["processing ", "  strings", "with   ", " map   "]
print(list(map(str.strip, with_spaces)))
# ['processing', 'strings', 'with', 'map']

with_dots = ["processing..", "...strings", "with....", "..map.."]
print(list(map(lambda s: s.strip("."), with_dots)))
# ['processing', 'strings', 'with', 'map']


# Eg 6
import re
def remove_punctuation(word):
	return re.sub(r'[!?.:;,"()-]', "", word)

text = """Some people, when ,,confronted... with a problem, think"""
words = text.split()
print(list(map(remove_punctuation, words)))

10.3 reduce() function

reduce() function reduces sequence of elements into a single element by applying the specified function. reduce() function present in functools module and hence we should write import statement.

reduce(function, sequence)

from functools import *
l = [10,20,30,40,50]
result=reduce(lambda x,y:x+y,l)
print(result) # 150

l = [1,2,3,4,5,4]

# Does not work on 3 variable
reduce(lambda x , y , z : x+y+z , l) # TypeError

reduce(lambda x , y : x+y , [1])
# 1

# Biggest emement in list
reduce(lambda x,y : x if x> y else y , l)
# 5

10.4 filter() function

We can use filter() function to filter values from the given sequence based on some condition.

filter(function, sequence) - where function argument is responsible to perform conditional check sequence can be list or tuple or string.

#  Working of filter()
def check_even(number):
    if number % 2 == 0:
          return True  
    return False

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
list(filter(check_even, numbers))
# Output: [2, 4, 6, 8, 10]

# Using Lambda Function Inside filter()

list(filter(lambda x: (x%2 == 0), numbers))
list(filter(lambda x: True if x%2==0 else False, numbers))
# [2, 4, 6, 8, 10]


l2 = ["sudh" , "pwskills" , "kumar" , "bengalore" , "krish"]
print(list(filter(lambda x : len(x) < 6 , l2)))

10.5 eval() function

• The eval function evaluates the String like a python expression and returns the result as an integer.
• eval(expression, globals=None, locals=None)
  • The globals must be represented as a dictionary and the locals as a mapped object.

x = 1
print(eval('x + 1'))
print(eval("1024 + 1024"))

code = compile("5 + 4", "<string>", "eval")
print(eval(code))
print()

x = 100
print(eval("x + 10", {"x": x}))

print(eval("x + y", {"x": x, "y": x}))