Machine Learning Udemy Bootcamp 02 - Python basics

String format

x = 'hello'
y = 'world'

print('{} is {}'.format(x, y))
print('{a} is {b}'.format(a=x, b=y))
# Python3
print(f'{x} is {b}')

Slice notation

x = 'abdefghijk'

# takes everything after index 0
print(s[0:]) => abdefghijk
# takes everything before index 3
print(s[3:]) => abc
# take range 0 to 3
print[s[0:3]] => abc
print(s[3:6]) => def

Lists

list = ['a', 'b', 'c']
list.append('d')
list[0] => 'a'
list[1:3] => ['b', 'c']

Dictionaries

d = { 'key1': 'value', 'key2': [1,2,3] }
d['key1'] => 'value'
d['key1'][1] => 2

Tuples

t = (1,2,3)
t[0] => 1

!! isn't possible to reassign tuples

Sets

Collection of unique elements
set([1,1,1,1,1,2,2,2,2,6,6,6,6]) => {1, 2, 6}

s = {1, 2, 3}
s.add(5) => {1, 2, 3, 5}

Utilities

range

r = range(0, 5)
# by default starts from 0
r2 = range(10)

list(r) => [0, 1, 2, 3, 4]
list(r2) => [0, 1, 2, 4, 5, 6, 7, 8, 9]

for i in r:
  print(i)

map

# normal function
def doublize(n): return n * 2
# callback function
t = lambda n: n * 2
seq = [1,2,3,4,5]
list(map(t, seq))

filter

# filter odd numbers
list(filter(lambda n: n%2, seq))

Short-hands

List for-loop generation

out = []

for num in x:
  out.append(num**2)

# is writable as
out = [num**2 for num in x]