Python_Basic

Data Types

Numbers

• If we use 1e9, it will be Floats. Thus, we convert it into an integer with the function int(1e9).
• If we produce the result in float, we recommend using the function round().
  • Round(0.789, 2) = 0.79
• 2**3: $2^3$, The exponentiation operator (**)
• 7%2: 1, The modulo operator (%) returns the remainder of dividing two numbers.
• 7//2 : 3, The Floor division operator (//)

Strings

• Multiline string: Use triple double quotes """ or triple single quotes '''

a = '''
"Life" is short
you need python
'''
print(a)

#Output: Life is "short"
#        you need python

• Escape \n: a newline (In Python strings, the backslash \ is a special character, also called the “escape” character. )
• Join Two or More Strings

greet = "Hello, "
name = "Jack"

result = greet + name
print(result)
# Output: Hello, Jack

• Multiply a string

print("=" * 10)
print("My Program")
print("=" * 10)
# Output: 
#==========
#My Program
#==========

• Braces: In order to make a brace appear in your string, you must use double braces .

Functions

Methods	Description
‘abc’.upper()	converts the string to uppercase
‘ABC’.lower()	converts the string to lowercase
‘hobby’.count(‘b’)	Counts the specific character
‘hobby’.find(‘b’)	returns the index of the first occurrence of a substring. If there does not exist a substring, it returns ‘-1’.
‘hobby’.index(‘b’)	returns the index of the first occurrence of a substring. If there does not exist a substring, it returns an error message.
” hi. “.rstrip()	removes right spaces.
” hi. “.lstrip()	removes left spaces.
” hi. “.strip()	removes all spaces.
d.split(), e.split(‘:’)	splits string from left, d = “Life is too short”, e = “ab:c:d”
”,”.join(‘abcd’)	#’a,b,c,d’
c.replace(“Life”, “Your leg”)	c = “Life is too short.”

String Formatting (f-Strings)

Python f-Strings make it really easy to print values and variables. The f in f-strings may as well stand for “fast.” f-strings are faster than both %-formatting and str.format().

name = 'Hailey'
country = 'the States'

print(f'{name} is from {country}')
#Hailey is from the States

# Dictionary
d = {'name': 'Hailey', 'country':'the States'}
print(f'{d["name"]} is from {d["country"]}')

#

If you want to spread strings over multiple lines, you also have the option of escaping a return with a \:

message = f"Hi {name}. " \
...       f"You are a {profession}. " \
...       f"You were in {affiliation}."
...
#message
#'Hi Eric. You are a comedian. You were in Monty Python.'

Methods	Description
f’{“hi”:<10}’	‘hi ‘
f’{“hi”:>10}’	’ hi’
f’{“hi”:^10}’	’ hi ‘
f’{“hi”:=^10}’	’====hi====’
f’{“hi”:!<10}’	‘hi!!!!!!!!’
f’{y:0.4f}’	‘3.4213’
f’{y:10.4f}’	’ 3.4213’

Lists

Functions

Remove items

Methods	Description
del a[0]	Remove items by index or slice l = [0, 10, 20, 30, 40, 50] del l[0] # [10, 20, 30, 40, 50]
a.remove(): O(N)	Remove an item by value l = [‘Alice’, ‘Bob’, ‘Charlie’, ‘Bob’, ‘Dave’] l.remove(‘Alice’) # [‘Bob’, ‘Charlie’, ‘Bob’, ‘Dave’]
a.pop()	Remove an item by index and get its value a = [0, 10, 20, 30, 40, 50] print(l.pop(0)) #0 # [10, 20, 30, 40, 50]
a.clear()	Remove all items

Add an item

Methods	Description
a.append(‘abc’) : O(1)	Add an item to a list l.append([3, 4, 5]), # [0, 1, 2, 100, [3, 4, 5]]
a.extend([10, 11]), +, +=	Combine lists a = [0, 1, 2], # [0, 1, 2, 10, 11] a +=[10, 11] # [0, 1, 2, 10, 11]
a.insert(1, 100): O(N)	Insert an item into a list a= [‘a’, ‘b’, ‘c’] a.insert(1, 100) # [‘a’, 100, ‘b’, ‘c’]

The others

Methods	Description
a.sort(reverse=False): O(NlogN)	Sort the list ascending by default.
a.reverse(): O(N)	Reverses the sorting order of the elements.
a.index(3)	Returns the position at the first occurrence of the specified value. a = [1, 2, 3] #2
a.count(1): O(N)	a = [1, 2, 3, 1] #2

List Comprehension

array = [i for i in A if __]

array = [i for i in range(4)]
print(array) #[0, 1, 2, 3]

fruits = ["apple", "banana", "cherry", "mango"]
newlist = [i for i in fruits if "a" in i]
print(newlist) #['apple', 'banana', 'mango']

a=[1,2,3,4]
result = [num * 3 for num in a if num %2 == 0] #[6, 12]
print(result)

Initialize a list with the given size

# N * M
n = 4
m = 3
array = [ [0]*m for _ in range(n) ] # Not [ [0]*m ] * n 
#Underscore : as a variable in looping

How to zip two lists

1. Using the built-in zip() function

list_a = [1, 3, 4]
list_b = [5, 7, 11]

list_zip = list(zip(list_a, list_b))
print(list_zip) 
#[(1, 5), (3, 7), (4, 11)]

2. Using map() + __add__

map() function is another in-built python method similar to zip() function above. It enables you to zip the elements of the iterable by mapping the elements of the first iterable with the elements of the second iterable. By using the map() function along with the addition operator, you can merge two lists in python as shown in the below example:

list_1 = [[2, 3], [4, 5], [7, 6]]
list_2 = [[4, 9], [4, 2], [11, 10]]
  
print ("The given list 1 is : " + str(list_1))
print ("The given list 2 is : " + str(list_2))
#The given list 1 is : [[2, 3], [4, 5], [7, 6]]
#The given list 2 is : [[4, 9], [4, 2], [11, 10]]


res = list(map(list.__add__, list_1, list_2))
      
print ("The zipped list is : " +  str(res))

#The zipped list is : [[2, 3, 4, 9], [4, 5, 4, 2], [7, 6, 11, 10]]

Tuples

• Tuples that contain immutable elements can be used as a key for a dictionary. With lists, this is not possible.
• If you have data that doesn’t change, implementing it as a tuple will guarantee that it remains write-protected.

# Empty Tuples
my_tuple = ()

# nested tuple
my_tuple = ("mouse", [8, 4, 6], (1, 2, 3))

#We can also create tuples without using parentheses:
my_tuple = 1, "Hello", 3.4

Creating a Tuple with one Element

In Python, creating a tuple with one element is a bit tricky. Having one element within parentheses is not enough.

var1 = ("hello")
print(type(var1))  # <class 'str'>

# Creating a tuple having one element
var2 = ("hello",)
print(type(var2))  # <class 'tuple'>

# Parentheses is optional
var3 = "hello",
print(type(var3))  # <class 'tuple'>

Check if an Item Exists

languages = ('Python', 'Swift', 'C++')

print('C' in languages)    # False
print('Python' in languages)    # True

Dictionaries

# Empty Dictionaries
my_dictionary = {}

#Add Elements to a Python Dictionary
capital_city = {"Nepal": "Kathmandu", "England": "London"}
capital_city["Japan"] = "Tokyo"
print("Updated Dictionary: ",capital_city)

#Removing elements
student_id = {111: "Eric", 112: "Kyle", 113: "Butters"}
print("Initial Dictionary: ", student_id)
del student_id[111]

Methods	Description
a.keys()	Returns the list of keys.
a.values()	Returns the list of values.
a.items()	Returns the key-value pairs of the dictionary as tuples in a list.
a.get(key)	Returns the value of the “key”

• value if the key is not found and value is specified.

person = {'name': 'Phill', 'age': 22}

print('Name: ', person.get('name'))

# value is provided
print('Salary: ', person.get('salary', 0))

Sets

# Create an empty set
empty_set = set()

# Create an empty dictionary
empty_dictionary = { }

s1 = set([1, 2, 3, 4, 5, 6])
s2 = set([4, 5, 6, 7, 8, 9])
print(s1 & s2)
print(s1 | s2)
print(s1 - s2)

Methods	Description
a.add(32)	Add an item to a set Initial Set: {34, 12, 21, 54} Updated Set: {32, 34, 12, 21, 54}
a.update(tech_companies)	Update the set with items of other collection types (lists, tuples, sets, etc). companies = {‘Lacoste’, ‘Ralph Lauren’} tech_companies = [‘apple’, ‘google’, ‘apple’] # Output: {‘google’, ‘apple’, ‘Lacoste’, ‘Ralph Lauren’}
a.items()	Returns the key-value pairs of the dictionary as tuples in a list.

Input

How to take integer input

n = int(input())

data = list(map(int, input().split())) #From string to integer

import sys
data = sys.stdin.readline().rstrip()
print(data)

#used for the addition of any string at the end of the output of the python print statement.
print(7, end = " ")

Conditions

Pass statement

When the pass statement is executed, nothing happens, but you avoid getting an error when empty code is not allowed.

def myfunction():
  pass

class Person:
  pass

if b > a:
  pass

Conditional Expression

score = 85
result = "Success" if score >= 80 else "Fail" #Success

For & While Loops

# While loop
i = 1
result = 0

while i <= 9:
  if i % 2 == 1:
    result += i
  i += 1
  
print(result)

# For loop
result = 0
for i in range(1, 10):
  result += i
  print(result) #Sum of 1- 9: 45
  

Break statement in For Loop

Breakpoint is a unique function in For Loop that allows you to break or terminate the execution of the for loop. We declared the numbers from 10-20, but we want that our for loop to terminate at number 15 and stop executing further.

for x in range (10,20):
			if (x == 15): break
			print(x)

Continue statement in For Loop

Continue function, as the name indicates, will terminate the current iteration of the for loop BUT will continue execution of the remaining iterations. In our example, we have declared values 1-9, but between these numbers, we only want those numbers that are NOT divisible by 2.

#Sum of odd integers from 1 to 9
result = 0
for i in range(1, 10):
  if i % 2 == 0: continue
  result += i
print(result) # 25

Enumerate() in For Loop

enumerate() is a built-in function used for assigning an index to each item of the iterable object. It adds a loop on the iterable objects while keeping track of the current item and returns the object in an enumerable form. This object can be used in a for loop to convert it into a list by using list() method.

#use a for loop over a collection
Months = ["Jan","Feb","Mar","April","May","June"]
for i, m in enumerate (Months):
		print(i,m)
    
0 Jan
1 Feb
2 Mar
3 April
4 May
5 June

Function

Global variables

When you create a variable inside a function, that variable is local and can only be used inside that function. If you use the global keyword, the variable belongs to the global scope:

def myfunc():
  global x
  x = "fantastic"

myfunc()
print("Python is " + x) # Python is fantastic

Multiple returns

As you already know, a function can return a single variable, but it can also return multiple variables.

def getPerson():
    name = "Leona"
    age = 35
    country = "UK"
    return name,age,country

name,age,country = getPerson()

Lambda function

In Python, a lambda function is a special type of function without the function name.

lambda argument(s) : expression 

greet = lambda : print('Hello World')
greet() #Call the lambda

• argument(s) - any value passed to the lambda function
• expression - expression is executed and returned

print( (lambda a, b: a + b)(3, 7) ) #10

Built-in Functions

sorted()

sorted( [9, 1, 8, 5, 4], reverse=True) : Return a new sorted list from the items in iterable. #[9, 8, 5, 4, 1]

array = [('Hailey', 34), ('James', 30), ('Mike', 29)]
result = sorted(array, key = lambda x: x[1], reverse=True)
print(result)

counter()

from collections import Counter
counter = Counter(['red', 'blue', 'red', 'green', 'blue', 'blue'])
print(counter['blue'])
print(counter['green'])
print(dict(counter))

Permutations and Combination

from itertools import permutations
data = ['A','B','C']
result = list(permutations(data, 3))
print(result)

from itertools import combinations
result = list(combinations(data, 2))
print(result)

from itertools import combinations_with_replacement
result = list(combinations_with_replacement(data, 2))
print(result)

GCD and LCM

import math

def lcm(a, b)
		return a * b // math.gcd(a, b)

a = 21
b = 14
print(math.gcd(21, 14))
print(math.lcm (21, 14))