[Introduction to Data Scientists] Basics of Python ♬ Functions and anonymous functions, etc.

Continued from last night. 【Caution】 ["Data Scientist Training Course at the University of Tokyo"](https://www.amazon.co.jp/%E6%9D%B1%E4%BA%AC%E5%A4%A7%E5%AD%A6%E3 % 81% AE% E3% 83% 87% E3% 83% BC% E3% 82% BF% E3% 82% B5% E3% 82% A4% E3% 82% A8% E3% 83% B3% E3% 83 % 86% E3% 82% A3% E3% 82% B9% E3% 83% 88% E8% 82% B2% E6% 88% 90% E8% AC% 9B% E5% BA% A7-Python% E3% 81 % A7% E6% 89% 8B% E3% 82% 92% E5% 8B% 95% E3% 81% 8B% E3% 81% 97% E3% 81% A6% E5% AD% A6% E3% 81% B6 % E3% 83% 87% E2% 80% 95% E3% 82% BF% E5% 88% 86% E6% 9E% 90-% E5% A1% 9A% E6% 9C% AC% E9% 82% A6% I will read E5% B0% 8A / dp / 4839965250 / ref = tmm_pap_swatch_0? _ Encoding = UTF8 & qid = & sr =) and summarize the parts that I have some doubts or find useful. Therefore, I think the synopsis will be straightforward, but please read it, thinking that the content has nothing to do with this book.

Chapter1-2 Python Basics

1-2-5 Function

1-2-5-1 Function basics

A function has at least one of the following properties ⓪ Follow the instructions and perform the prescribed processing ① Collect and standardize a series of processes (2) Pass an argument and get the return value

Simple function

③ The basic format is as follows

Function that does nothing

def do_nothing():
    pass
do_nothing()

result do nothing

Output hello world

def hello_world():
    print('hello world')
hello_world()

result

hello world

Output changes depending on the argument

def hello_world(arg='hello_world'):
    print(arg)
    
hello_world('hello_world')
hello_world('good morning')

result

hello_world
good morning

* args Output argument as tuple

def hello_world(*args):
    print(args)
    
hello_world('hello_world','good morning')

result In-function and output will be tuple

('hello_world', 'good morning')

** Output the kwargs argument as a dictionary

def hello_world(**kwargs):
    print(kwargs)
    
hello_world(day='hello_world',morning='good morning', evening='good evening')

result The inside and output of the function will be a dictionary, the argument name will be the key of the dictionary, and the value of the argument will be the value of the dictionary.

{'day': 'hello_world', 'morning': 'good morning', 'evening': 'good evening'}

Function with a return value

def calc_multi(a, b):
    return a * b

print(calc_multi(3,5))

result

fibonacci

def calc_fib(n):
    if n ==1 or n== 2:
        return 1
    else:
        return calc_fib(n-1) + calc_fib(n-2)
    
for n in range(1,21):
    print(calc_fib(n),end=' ')

result

1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181 6765

1-2-5-2 Anonymous functions lambda and map functions

As shown below, a function without a function name is called an anonymous function lambda.

print((lambda a, b:a*b)(3, 5))

result

map function

def calc_double(x):
    return x * 2

for num in [1, 2, 3, 4]:
    print(calc_double(num))

result

Calculate [1, 2, 3, 4] at once with map function and output with list

print(list(map(calc_double, [1, 2, 3, 4])))

result

[2, 4, 6, 8]

map function and lambda

You can replace a normal function with lambda and output it in one line

print(list(map(lambda x : x*2, [1, 2, 3, 4])))

result

[2, 4, 6, 8]

Exercise 1-2

print((lambda x : sum(x))(i for i in range(1,51)))
print((lambda x : sum(x))(range(1,51)))
print( sum(i for i in range(1,51)))
print( sum(range(1,51)))
print(reduce(lambda x, y: x + y, range(1,51)))

result

filter and reduce functions

a = [-1, 3, -5, 7, -9]
print(list(filter(lambda x: abs(x) > 5, a)))

result

[7, -9]

The reduce function is the result of sequential addition of elements

from functools import reduce
print(reduce(lambda x, y: x + y, a))

result

-5

【reference】 Basic Python functions (map () filter () reduce ())

Summary

・ Function basics ・ How to use * args and ** kwargs ・ I tried to arrange how to use anonymous functions lambda, map function, filter function, and reduce function.

It is easy to understand if you go back to the basics and arrange them side by side.