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Python Learning Aid (MACHINE TRANSLATED)

General Information

  • Code lines are executed from top to bottom.
  • Everything after # is ignored by Python. You can use this to write comments for humans.

Data Types

There are different data types in Python:

  • Integer (Whole number)

    5

  • String (Text)

    "Hello World!"
"123" # This is also a string because it is in "", even if it only contains numbers
# You cannot do math with strings.

  • Float (Decimal number)

    3.14

  • Boolean (True or False)

    True
False

Converting Variables

Note

Everything after a # is ignored by Python. It is only a note for humans.

Examples:

int(x)   # Convert to integer (whole number)
str(x)   # Convert to string (text)
float(x) # Convert to float (decimal number)

age = "17" # String (text)
age_number = int(age) # Convert to integer (whole number)

Assigning Variables

You can create variables and assign values to them. You can name them as you like, but it's good if the name relates to their content.

a = 5 # Integer (whole number)
b = 10
name = "Alex" # String (text), must have "" or ''

Calculating with Variables

Operators:

  • + Addition
  • - Subtraction
  • * Multiplication
  • / Division
  • % Modulo (remainder of division)
c = a + b # c will be 15 because 5 + 10 = 15
c = a * b # c will be 50 because 5 * 10 = 50
a = a + 1 # a is increased by 1, so a = 6

Modulo operator returns the remainder of a division:

10 % 2 # Returns 0. When you divide 10 by 2, the remainder is 0.
23 % 2 # Returns 1. 23 / 2 is 11 with remainder 1.

Console Output/Input

print("Hello World!") # Prints "Hello World!" to the console

User input

print("What is your name?: ")
name = input() # The user must enter something in the console, which is then stored in the variable 'name'
# Shortcut:
name = input("What is your name?: ")

if-else (Conditions)

With if, elif, and else, Python can make decisions. Python reads the conditions from top to bottom and only runs the first matching block.

A block is the indented lines below a condition. So you press Tab to show Python: "This code belongs to this condition."

if = "If this condition is true, do this."
elif (optional) = "Otherwise, if this other condition is true, do this."
else (optional) = "Otherwise, if none of the conditions above were true, do this."

Note

Everything after a # is ignored by Python. It is only a note for humans and does not need to be copied.

if a < b: # If a is less than b
    print("a is less than b")
elif a == b: # If a is equal to b
    print("a is equal to b")
else: # If none of the previous conditions apply
    print("a is greater than b")

Loops

You need loops when Python should do something multiple times. There are 2 important types of loops: for and while.

for loop

A for loop goes through a collection step by step. On each loop run, Python takes the next value from the list and stores it temporarily in a variable.

fruits = ["Apple", "Banana", "Cherry", "Lemon"]
for fruit in fruits: # fruit is always the current fruit
    print(fruit) # prints the current fruit

Python roughly does this:

  1. fruit is "Apple", then "Apple" is printed.
  2. fruit is "Banana", then "Banana" is printed.
  3. fruit is "Cherry", then "Cherry" is printed.
  4. fruit is "Lemon", then "Lemon" is printed.

Output:

Apple
Banana
Cherry
Lemon

The name fruit can be chosen freely. The important part is that you use the same name again in the indented block.

If you want to do something a specific number of times, you can use range():

for i in range(1, 10):
    print("Loop run number " + str(i))

range(1, 10) means: Start at 1 and stop before 10. So 10 is not included.

Output:

Loop run number 1
Loop run number 2
Loop run number 3
Loop run number 4
Loop run number 5
Loop run number 6
Loop run number 7
Loop run number 8
Loop run number 9

while loop

A while loop runs as long as a condition is true. Python checks the condition again before every loop run.

a = 0
b = 5
while a < b: # As long as a is less than b
    print(a)
    a = a + 1 # a gets bigger so the loop eventually ends

Output:

0
1
2
3
4

When a reaches the value 5, a < b is no longer true. Then the loop stops.

To make an infinite loop, you can write while True:.

Getting Input from the User

print("Please enter your name:")
name = input()

Important: input() always returns a string (text), so you cannot do calculations with it directly. You can convert the result to a whole number with int(...):

number = int(input("Enter a number: "))

Tip

Shortcut: name = input("Please enter your name:")

String Interpolation (Insert variables into strings/text)

There are 2 variants:

print("Hello " + name + ", welcome!") # '+' joins strings

or

print(f"Hello {name}, welcome!") # You must add 'f' at the beginning to insert variables with {}

If the variable is a number, you must first convert it to a string:

age = 17
print("I am " + str(age) + " years old.")
print(f"I am {age} years old.") # with the f-string variant, Python automatically converts the number to text

Arrays / Lists

Instead of just one variable with one value, you can define an array with a list of values:

fruits = ["Apple", "Banana", "Cherry"]
print(fruits[0]) # prints the first value of the array (Counting starts at 0) (Apple)
print(fruits[1]) # prints the second value of the array (Banana)

new_fruit = "Orange"
fruits.append(new_fruit) # Adds 'Orange' to the array 'fruits'
# ["Apple", "Banana", "Cherry", "Orange"]

Convert a string to an array (list):

text = "Apple"
text_array = list(text)
# text_array -> ['A', 'p', 'p', 'l', 'e']

To do something, for example, 10 times:

for i in range(1, 10):
    print(i) # Prints the numbers from 1 to 9 (1, 2, 3, 4, 5, 6, 7, 8, 9)
# 'range(1, 10)' would create an array with the values 1 to 9 ([1, 2, 3, 4, 5, 6, 7, 8, 9]).

Count entries in an array:

fruits = ["Apple", "Banana", "Cherry"]
number_of_fruits = len(fruits) # Returns the number of entries in the array 'fruits' (3 in this case)

Modules

You can import modules to use additional functions.

Examples:

import random
random_number = random.randint(1, 10) # Returns a random number between 1 and 10

fruits = ["Apple", "Banana", "Cherry"]
random_fruit = random.choice(fruits) # Returns a random fruit from the array 'fruits'
print(random_fruit) # "Cherry" or "Banana" or "Apple"

Functions

You can create your own functions to encapsulate repeating logic and make the code clearer.

def greeting(name): # Function with one parameter 'name'
    print(f"Hello {name}, welcome!") # Prints a greeting

name = input("What is your name?: ")
greeting(name) # Calls the function 'greeting' with the parameter 'name'

def sum_numbers(number_one, number_two): # Function with 2 parameters 'number_one' and 'number_two'
    return number_one + number_two # Returns the sum of number_one and number_two

a = 5
b = 10
result = sum_numbers(a, b) # a fills the parameter 'number_one' and b fills the parameter 'number_two'
print(f"The sum of {a} and {b} is {result}.") # Prints the sum of a and b

My example solutions to some of the more difficult tasks