python-cheat-sheet.md

Cheat Sheet for Algorithm - Python

TOC

• I/O
• Assignment
• Calculations
• Condition
• Loop
• Function
• String
• List
• Stack
• Heap
• Dictionary
• String indexing
• Type Check
• Type Conversion
• OS
• Pipenv
• Trivial Python Tips

I/O

⬆ Top

print('hello world')
input_str = input()

Assignment

⬆ Top

test_str = "0 1 2"

# map : a=0, b=1, c=2
a, b, c = map(int, test_str.split())

# arr = [0,1,2]
arr = [int(n) for n in test_str.split()]

Calculations

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x**y   # Exponentiation(x^y)

x%y    # Remainder(x%y)

x // y # Division(x/y)

abs(x) # Absolute Value(|x|)

• No Prefix/Postfix Operator

i += 1 # i++ or ++i

Condition

⬆ Top

name = input()

if name == 'godori':
  print("Hi, godori")
elif name == 'irodog':
  print("Hi, irodog")
else:
  print("Who are you?")

• and (&&) or(||)

a = true
b = false
if a == true and b == true:
  print('can not reach here')

if a == true or b == true:
  print('you are here!')

Loop

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members = ['godori', 'irodog', 'roodig']

# for item in items
for member in members:
  print(member)

# for idx in range(number)
for i in range(len(members)):
  print(members[i])

# enumerate()
for idx, val in enumerate(arr):
  print(idx, val)

Function

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• Function call with parameter & return

  def func(num):
      return 'godori' * num
  
  print(func(3))

• Function string & number parameters

  def func(str, num):
      return str * num
  
  print(func('hey', 3))

String

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• loop through

  str = 'hello'
  for s in str:
    print(s)

• Reverse String

  str = "GODORI"
  str[::-1]       # "IRODOG"

List

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• Length

  arr = [1, 2, 3]
  len(arr)

• Append

  arr = [1, 2, 3]
  arr.append(4)

• Sort

  arr = [3, 1, 2]
  arr.sort()

• Initialize

  arr = [0] * 3    # [0,0,0]

Stack

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• using list like stack
• push

  stack = []
  stack.append(1) # add item into the last index
  stack.append(2)
  # stack = [1,2]

• pop

  stack = [1,2,3] # remove tail item
  stack.pop()
  # stack = [1,2]

Heap

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• Using list and heapq (built-in)

  myheap = []
  
  heapq.heappush(myheap, 2)
  heapq.heappush(myheap, 1)
  heapq.heappush(myheap, 3)
  

Dictionary

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  dic = dict()              # {}
  dic = {}
  
  dic['name'] = 'godori'    # {'name': 'godori'}
  dic['job'] = 'developer'  # {'job':'developer', 'name': 'godori'} (no order)
  print('name' in dic)      # true
  len(dic)                  # 2

• Count Frequency of characters in string

   dic = {}
   for c in str:
      if c in dic:
        dic[c] += 1
      else:
        dic[c] = 0

String Indexing

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• str[first:last(not inclding)]

  s = "Life is too short, You need Python"
  s[0]     # L
  s[0:3]   # Lif

Type Check

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• isalpha(): Check if alphabet
• isdigit(): Check if number

  s = 'str'
  k = '!@#'
  n = 123
  
  if s.isalpha():
    print('is string')
  
  if k.isalpha():
    print('Are you an alphabet?')
  else:
    print('no i am not an alphabet')
    
  if n.isdigit():
    print('is number')

Type Conversion

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• string to int

  int('13')

• int to string

  str(13)

OS

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• Access environment from python

  $ export OUTPUT_PATH="filename.txt"
  $ echo $OUTPUT_PATH  # filename.txt

  import os
  
  if __name__ == '__main__':
    f = open(os.environ['OUTPUT_PATH'], 'w')
    f.write('write something...')
    f.close()

Pipenv

⬆ Top Run pipenv shell

$ pipenv shell

Trivial Python Tips

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• Booleans represent True/False (not true/false)
• Using and, || instead of &&, ||
• Python has no switch/case
• Check indentation