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Python

Run

python3 ./Python/test.pl.py

Variables

n, m = 0, "abc"
# none is null
n = 4
n = None # reassign value

Input & output

print('Hello world!')
print("Hello world!")
print("""Hello world!""")
print("Program print 'Hello world!'")

# print
name = "User"
print("Hello,", name, ".") # Hello, User .
print("Hello,", name, ".", sep="") # Hello,User.
print(f"Hello, {name}.") # Hello, User.

# adding zeroes
print(f"{123:0>9}") # 000000123
print(f"{123:0<9}") # 123000000
print(f"{123:0^9}") # 000123000

# string operations
print("-" * 10) # ----------

# string to int (see also float(), str())
n_1 = "1"
n_2 = "2"
print(n_1 + n_2) # 12
n_1 = int(n_1)
n_2 = int(n_2)
print(n_1 + n_2) # 3

# operations
print(f"{2 ** 0.5:.2f}") # 1.41
print(10 // 3) # 3 Floor division
print(10 % 3) # 1 Modulus
last_digit = 1234 % 10 # 4
hour = 9
min = 5
print(f"{hour:02d}:{min:02d}") # 09:05

Logical operators

# if-elif-else
yesterday_temp = int(input())
today_temp = int(input())
if today_temp > yesterday_temp:
    print("Today is warmer than yesterday.")
elif today_temp < yesterday_temp:
    print("Today is colder than yesterday.")
else:
    print("Today is the same temperature as yesterday.")

# or, and
first_letter = input()
last_letter = input()
if (first_letter == "а" or first_letter == "А") and (
        last_letter == "я" or last_letter == "Я"):
    print("Right")
else:
    print("Wrong")

# if in string contains string
text = input()
if "if" in text:
    print("Right")
else:
    print("Wrong")

# match
color = input()
match color:
    case 'red' | 'yellow':
        print('Stop.')
    case 'green':
        print('drive')
    case _:
        print('wrong value')

# short if
print("A") if a > b else print("B")

String functions

  • len(str) - string length
# string immutable
s = "123"
print(s[1:2])

# valid numeric strings can be converted
print(int("100") + int("100")) # 200

# and numbers can be converted to strings
print(str(100) + str(100)) # 100100

# ASCII value
print(ord("a")) # 97

# combine with a delimiter
strings = ["1", "2", "3"]
print(",".join(strings)) # 1,2,3

Arithmetic functions

  • min("y", "c", "z") # 1
  • max(1, 2) # 2
  • abs(-10) # 10
  • 5 / 2 # 2.5
  • 5 // 2 # 2
  • 3 // -2 # -2 round down, so negative round to -2
  • int(3 / -2) # -1
  • 10 % 3 # 1
  • -10 % 3 # 2
  • math.fmod(-10, 3) # -1
  • math.floor(3 / 2) # 1
  • math.ceil(3 / 2) # 2
  • math.sqrt(4) # 2.0
  • math.pow(2, 3) # 8.0
  • float("inf") # max Int
  • float("-inf") # min Int

Loops

while

n = 0
while n < 5:
    print(n)
    n += 1

for

for i in range(5):
    print(i)

for i in range(2, 5):
    print(i)

for i in range(5, 1, -1):
    print(i)

arr = [1, 2, 3]
for i in range(len(arr)):
    print(arr[i])

for n in arr:
    print(n)

# index and value
for i, n in enumerate(arr):
    print(f"{i} {n}")

# loop through two arrays simultaneously
nums1 = [1, 2, 3]
nums2 = [3, 4, 5]

for n1, n2 in zip(nums1, nums2):
    print(f"{n1} {n2}")

Arrays

like list

arr = [1, 2, 3]
arr[-1] # 3 last value
arr[-2] # 2 last second value
arr.append(4)
arr.append(5)
arr.pop()
arr.insert(1, 7)
arr[0] = 0

arr[1] * 5 # init array of size 5
len(arr)

arr = [1, 2, 3, 4]
arr[1:3] # [2, 3] sublist aka slicing
arr.reverse()
arr.sort() # ascending order
arr.sort(reverse=True) # descending order

arr = ["fff", "aa", "erere"]
arr.sort(key=lambda x: len(x))

arr = [i for i in range(5)] # [0, 1, 2, 3, 4]
arr = [i+i for i in range(5)] # [0, 2, 4, 6, 8]

# 2D List
arr = [[0] * 4  for i in range(4)] # 2d array of length 4 filled 0

Unpacking

a, b, c = [1, 2, 4]

Deque

Queues (double ended queue)

from collections import deque

queue = deque()
queue.append(1)
queue.append(2)
print(queue) # deque([1, 2])

queue.popleft()
print(queue) # deque([2])

queue.appendleft(3) # deque([3, 2])
queue.pop() # deque([3])

HashSet

mySet = set()
mySet.add(1)
mySet.add(2)
mySet.add(2)
print(mySet) # {1, 2}
print(len(mySet)) # 2
print(1 in mySet) # True
mySet.remove(2) # {1}
# list to set
pritn(set([1, 2, 3]))

mySet = { i in i in range(5) }

HashMap

aka dict

myMap = {}
myMap["alice"] = 88
myMap["bob"] = 77
print("alice" in myMap)
myMap.pop("alice")

myMap = { "alice": 80, "bob": 90 }

myMap = { i: i*3 for i in range(3) }

val = myMap["xx"] # KeyError if key is absent
print(map.get("xx")) # None
print(map.get("xx", "default")) # Default value

# looping through map
for key in myMap:
    print(key, myMap[key])
    
for val in myMap.values():
    print(val)
    
for key, value in myMap.items():
    print(key, value)

if key in dic.keys()

list (array) can't be used as key in hashmap. A list can be converted to a tuple, which can be used as a dict key: e.g.

d = {tuple([1,2,3]): 'value'}

Tuples

like array but immutable

tup = (1, 2, 3)

can be used as key's for hash map/set !! list can't be used as keys

myMap = { (1,2) : 3 }

mySet = set()
mySet.add((1,2))
print((1,2) in mySet)

DefaultDict

Return a new dictionary-like object. defaultdict is a subclass of the built-in dict class. Provides default value for missing key.

Using list as the default_factory, it is easy to group a sequence of key-value pairs into a dictionary of lists:

s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
d = defaultdict(list)
for k, v in s:
    d[k].append(v)

sorted(d.items())
# [('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

Setting the default_factory to int makes the defaultdict useful for counting (like a bag or multiset in other languages):

s = 'mississippi'
d = defaultdict(int)
for k in s:
    d[k] += 1

sorted(d.items())
# [('i', 4), ('m', 1), ('p', 2), ('s', 4)]

Heap

under the hood are arrays

import heapq

minHeap = []
heapq.heappush(minHeap, 3)
heapq.heappush(minHeap, 2)
heapq.heappush(minHeap, 4)

print(minHeap[0]) # 2 - min value

while len(minHeap):
    print(heapq.heappop(minHeap))

no max heap by default, work around is to use min heap and multiply by -1 when push and pop

maxHeap = []
heapq.heappush(maxHeap, -1 * 3)
heapq.heappush(maxHeap, -1 * 2)
heapq.heappush(maxHeap, -1 * 4)

print(-1 * maxHeap[0]) # 2 - min value

while len(maxHeap):
    print(-1 * heapq.heappop(maxHeap))

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

while arr:
    print(heapq.heappop(arr))

Functions

def myFunc(m, n):
    return m * n

print(myFunc(2, 3))

Nested functions have access to outer variables

def outer(a, b):
    c = "c"
    
    def inner():
        return a + b + c
    return inner()

print(outer("a", "b"))

Can modify objects but not to reassign unless using nonlocal word

def double(val):
    val = val * 2

val = 5
double(val)
print(val)

Class

class MyClass:
    # Constructor
    def __init__(self, nums):
        # Create member variables
        self.nums = nums
        self.size = len(nums)

    def getLength(self):
        return self.size

Csv to Sql

# https://konbert.com/convert/excel/to/csv
import uuid;

f = open("data.csv", "r")
fr = open("result.sql", "w", encoding = "utf-8")
for str in f:

    stringData = str.split(';')
    stringData = ["'{0}'".format(element) for element in stringData]
    stringData = [element.replace('\n', '') for element in stringData]
    formattedString = ", ".join(stringData)
    formattedString.replace('\n', ' ').replace('\r', '')

    fr.write(f"('{uuid.uuid4()}', {formattedString}),\n")

f.close()