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python_for_algorithms.md

Python Algorithm Cheat Sheet

Variables

s = "name"
n = 0
n1, n2 = 1, 2.0
none = None

# casting
i = int("123") # 123
s = str(123) # "123"

Math and numbers

## division
max(10, 11) # 11
min(10, 11) # 10
abs(-10) # 10

# int() rounds float down
int(2.7) # 2 
int(-2.7) # -2 
5/2 # float 2.5

# // rounds down
5//2 # int 2.5 -> 2
-5//2 # int -2.5 -> -3 # HEADSUP

# % rounds down CAREFUL with negative numbers
10%3 # int 1
-10%3 # int 2 better use math.fmod(-10, 3)

# exponent
10**3 # 10*10*10 1000

# python numbers are infinite and never overflow
import math
math.fmod(10, 3) # float 1.0
math.fmod(-10, 3) # float -1.0
math.floor(10/3) # int 3.3333333333333335 -> 3
math.ceil(10/3) # int 3.3333333333333335 -> 4
math.sqrt(25) # float 5.0
math.pow(2, 3) # float 8.0

import random
random.randrange(10) # Integer from 0 to 9 inclusive
random.randint(0, 10) # Integer from 0 to 9 inclusive
random.randrange(0, 101, 2) # Even integer from 0 to 100 inclusive
random.choice(['win', 'lose', 'draw']) # any
myArray = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0]
random.shuffle(myArray)
myArray # [6, 0, 7, 4, 3, 5, 9, 8, 2, 1]

Conditionals

if True and False or True: 
    # true
elif "12" == 12: 
    # False
elif 12.0 == 12:
    # True
elif 12.0 is 12: 
    # False
elif 12.0 is 12.0: 
    # True
elif None is None:
    # True
else:
    print("else")

Arrays and Lists

myArray = [1, 2]
len(myArray) # 2

myArray = [1] * 5 # [1, 1, 1, 1, 1]
myArray = [i for i in range(5)] # [0, 1, 2, 3, 4]
myArray = [[0] * 4] * 3 # [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
myArray = [[0] * 4 for i in range(3)] # [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
myArray = [[0 for j in range(4)] for i in range(3)] # [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]

myArray = [1, 2]
myArray.append(4) # [1, 2, 4]
myArray.pop() # 4 

myArray.insert(1, 7) # [1, 7, 2, 4]
myArray[0] = 0 # [0, 7, 2, 4]

# get last index
myArray[-1] # 4

# HEADSUP 
myArray = [1, 2, 3]
myArray[4] = 1 # IndexError: list assignment index out of range
myArray[4] # IndexError: list index out of range
myArray.index(4) # ValueError: '4' is not in list
myArray.remove(4) # ValueError: '4' is not in list

# sublisting / slicing
myArray = [1, 2, 3, 4, 5, 6]
myArray[1:3] # [2, 3, 4]
myArray[2:9999] # [3, 4, 5, 6] # no IndexError

myArray = [1, 2, 3]
myArray.extend([4, 5, 6]) # [1, 2, 3, 4, 5, 6]

# sorting default to ascending (min to max)
myArray = [2, 3, 1]
myArray.sort() # [1, 2, 3]
myArray.sort(reverse=True) # [3, 2, 1]
myArray.reverse() # [3, 2, 1]

myArray = ["egg", "banana", "apple"]
myArray.sort() # ["apple", "banana", "egg"] by default sort alphabeticaly
myArray.sort(key=lambda x: len(x)) # ["egg", "apple", "banana"]

# unpacking
a, b, c = [1, 2, 3]

Loops

myArray = [10, 20, 30, 40]
for i in myArray: # without index
    print(i) # 10, 20, 30, 40

for i in range(len(myArray)): # with index
    print(myArray[i]) # # 10, 20, 30, 40

for i, v in enumerate(myArray): # with index and value
    print(i, v) # (0, 10), (1, 20), (2, 30), (3, 40)

for i in range(5): # from 0 to 4
    print(i) # 0, 1, 2, 3, 4

for i in range(1, 5): # from 1 to 4
    print(i) # 1, 2, 3, 4

for i in range(4, -1, -1): # from 4 to 0
    print(i) # 4, 3, 2, 1, 0

arr = [1, 2, 3, 4, 5]
for i in range(len(arr)): # from 0 to length of arr
    for j in range(len(arr)-1, i, -1): # from length of arr to i+1
        print((i, j))

for i in range(8, -1, -2): # from 8 to 0 jump -2
    print(i) # 8, 6, 4, 2, 0

n = 0
while n < 5:
    print(n) # 0, 1, 2, 3, 4
    n += 1

while True:
    print("infinite")

Strings

myStr = "abc"
myStr[0:2] # "ab"

# strings are inmutable
myStr += "def" # "abcdef" # this takes O(n)

# casting
int("123") + int("123") # 246
str(123) + str(123) # "123123"

# get ascii value
ord("a") # 97

# transform
",".join(["ab", "cd", "ef"]) # "ab,cd,ef"
"ab cd ef".split(" ") # ["ab", "cd", "ef"]
"  asdf\n  ".strip() # "asdf"
"apple".upper() # APPLE
"apple".lower() # apple
"apple".capitalize() # Apple
"apple".endswith("le") # True
"apple".startswith("goog") # False

# formatting
"hello {name}".format(name="carlos") # "hello carlos"
"${price:.2f}".format(price=49.356) # "$49.36" # HEADSUP rounds

Queues and Stacks

from collections import deque

myStack = deque() # []
myStack.append(1) # [1]
myStack.append(2) # [1, 2]
myStack.append(3) # [1, 2, 3]
top = myStack.pop() # 3

myQueue = deque()
myQueue.append(1) # [1]
myQueue.append(2) # [1, 2]
myQueue.append(3) # [1, 2, 3]
dequeued = myQueue.popleft() # [1]

myQueue.appendleft(1) # [1, 2, 3]

HashSet

mySet = set()
mySet.add(1) # {1}
mySet.add(2) # {1, 2}
len(mySet) # 2
print(1 in mySet) # True
print(3 not in mySet) # True
mySet.remove(2) # {1}

mySet = set([1, 2]) # {1, 2}
mySet = set({i for i in range(3)}) # {0, 1, 2}

HashMap

myMap = {} # {}
myMap["key1"] = "one" # {"key1": "one"}
myMap["key2"] = 2 # {"key1": "one", "key2": 2}
len(myMap) # 2
print("key1" in myMap) # True
myMap.pop("key1") 

# HEADSUP 
myMap = {}
myMap["key3"] # KeyError: 'key3'
myMap.get("key3") # None

# Disc comprehencion
myMap = { i: i*2 for i in range(3)} # {0: 0, 1: 2, 2: 4}

# To list of keys
keys = list(myMap) # ["key1", "key2"]

# Iterate over HashMaps
myMap = {"key1": 1, "key2": 2}
for key in myMap:
    print(key, myMap[key])

for val in myMap.values():
    print(val)

for key, val in myMap.items():
    print(key, val)

OrderedDict

from collections import OrderedDict

myOrderedMap = OrderedDict()
myOrderedMap["key1"] = 1 # {"key1": 1}
myOrderedMap["key2"] = 2 # {"key1": 1, "key2": 2}
myOrderedMap["key3"] = 3 # {"key1": 1, "key2": 2, "key3": 3}

# Stack/LIFO
myOrderedMap.move_to_end("key2", last=True) # {"key1": 1, "key3": 3, "key2": 2}
myOrderedMap.popitem(last=True) # {"key1": 1, "key3": 3}
# Queue/FIFO
myOrderedMap.move_to_end("key2", last=False) # {"key2": 2, "key1": 1, "key3": 3}
myOrderedMap.popitem(last=False) # {"key1": 1, "key3": 3}

Tuples

# Tuple are like arrays but inmutables
myTuple = (1, 2, 3)
myTuple[1] # 2

# can be used as keys in maps and sets
myMap = { (1, 2): 3}
myMap[(1, 2)] # 3

mySet = set() # []
mySet.add((1, 2)) # [(1, 2)]
print((1, 2) in mySet) # True

Heaps

import heapq

minHeap = []
heapq.heappush(minHeap, 3) # [3]
heapq.heappush(minHeap, 2) # [2, 3]
heapq.heappush(minHeap, 4) # [2, 3, 4]
minHeap[0] # 2 min
minHeap[-1] # 4 max
# minHeap[-2] # 3 max

# # build a heap from initial values
myArray = [2, 1, 8, 4, 5]
heapq.heapify(myArray) # [1, 2, 4, 5, 8]

# iterate over heaps
while len(minHeap):
    print(heapq.heappop(minHeap)) 

# myArray = [2, 1, 8, 4, 5]

# heapq.heapify(myArray) # [1, 2, 4, 5, 8]
# while len(myArray):
#     print(heapq.heappop(myArray))

# heaps accepts tuples  
# orders them by the first value
tupleHeap = []
heapq.heappush(tupleHeap, (3, "task1")) # [(3, "task1")]
heapq.heappush(tupleHeap, (2, "task2")) # [(2, "task2"), (3, "task1")]
heapq.heappush(tupleHeap, (4, "task3")) # [(2, "task2"), (3, "task1"), (4, "task3")]
print(minHeap[0]) # (2, "task2") min
print(minHeap[-1]) # (4, "task3") max

Functions

def myFunc(n: int, m: int) ->  int:
    return n*m

myFunc(2, 3) # 6

Classes

from typing import List

class MyClass:
    def __init__(self, nums: List[int]) -> None:
        self.nums = nums
        self.size = len(nums)
    def getLength(self) -> int:
        return self.size
    
    def getDoubleLenght(self) -> int:
        return 2 * self.getLength()
    
myClass = MyClass([1, 2, 3, 4, 5, 6, 7, 8, 9])
myClass.getLength() # 9

stdin

# single line
i = input()

# multiline
import sys
lines = sys.stdin.readlines()

lines = [line for line in sys.stdin]

# from file
with open("myfile.txt") as f:
    lines = f.readlines()

requests

import requests

x = requests.get('https://w3schools.com/python/demopage.htm')

print(x.text)
print(x.content)
print(x.headers)

Try Catch

try: 
    pass
except ValueError: # IndexError: KeyError:
    pass

Exceptions

Exception Name Possible Causes
SyntaxError Syntax errors in your code, e.g., missing colons or quotes.
IndentationError Issues with code indentation, like mixing spaces and tabs.
NameError Accessing an undefined variable or function.
TypeError Inappropriate operation or function on an object.
ValueError Function receives a correct type but inappropriate value.
KeyError Attempting to access a non-existent dictionary key.
IndexError Trying to access an index out of range in a sequence.
FileNotFoundError Opening or manipulating a file that doesn't exist.
ModuleNotFoundError Module you're trying to import can't be found.
ZeroDivisionError Division by zero.
AttributeError Accessing an attribute or method that doesn't exist.
ImportError Issues with importing, e.g., circular imports or missing dependencies.
IOError Various I/O-related issues, like file reading or writing problems.
PermissionError Attempting an operation without necessary permissions, e.g., writing to a read-only file.
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