python-tricks.md

Python Tricks

String

ord(char) - ord('a')  
>>> ord('a') = 97
>>> ord('A') = 65
>>> ord('z') = 97+25 = 122 # total 26 letters calculations
>>> ord('Z') = 65+25 = 90  # total 26 letters calculations

Array

array = [1, 1, 1, 3]

array.count(1) # 3
array.find(1) # 0 
array.rfind(1) # 2
array.find(2) # -1 

Dict

d ={}
d.get('cat', 0) # 0 - setting default if key doesn't exist

from collections import defaultdict

d = defaultdict(int) # (dict, but if key doesn't exist then create one with some default value)
d['cat'] # 0

Set

val in set1           # O(1) operation  
set([1,2]) - set([1]) = set([2])  # substraction
[set()] * 9  ## DON'T DO THIS - all 9 will be ref to same set object

Tuple

ordered + immutable collection of items

dict[tuple(1,2,3)] = 2      # hashable: can be used as dict key

Float

float('inf')
float('-inf')

Math

import math

math.ceil(2.2) # 3
math.floor(2.8) # 2

Libraries

• defaultdict (dict, but if key doesn't exist then create one with some default value)
• dequeu (push and pop from both ends with O(1))
• Counter (makes counter dict of the iterrable)

Methods

• sorted - O(n log n) worst, O(n) best