Python – Theory 2 — Conditional statements

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Python Programming: Module 1 – Conditional Statements (if, else, elif). Goal: Make Decisions in Python.
Boolean Type
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match Statement

Python Programming: Module 1 – Conditional Statements

if, else, elif  |  Make Decisions in Python

🔷 Boolean Type (bool)

Booleans represent truth values: True or False. They are often produced by comparison operators:

  • == (equal), != (not equal)
  • > (greater), < (less)
  • >= (greater or equal), <= (less or equal)

Logical operators: and, or, not.

True is equivalent to 1, False to 0 (can be used in arithmetic).

Пример 1: Boolean Values and Arithmetic
💡 Open example 
print(True + True)   # 2
print(True * 10)     # 10
print(False + 5)     # 5

Interesting fact: bool is a subclass of int — True and False are actually 1 and 0.

⚖️ Comparison Operators

Python supports the following comparison operators:

OperatorDescriptionExample
==Equal tox == 5
!=Not equal tox != 5
>Greater thanx > 3
<Less thanx < 3
>=Greater than or equalx >= 5
<=Less than or equalx <= 5
Пример 2: Assignment vs Comparison
💡 Open example 
# ASSIGNMENT – gives a value
age = 25
 
# COMPARISON – checks equality
if age == 25:
    print("You are 25")
 
# WRONG! This assigns, not compares
# if age = 25:  # SyntaxError!
#     print("This won't work")

Key takeaways: = is for assignment, == is for comparison. Mixing them up causes errors.

Interesting fact: In Python, == compares values, while is compares identity (whether two variables point to the same object).

🧠 Logical Operators

📌 Operator and

A and B is True if both A and B are True. In all other cases — False.

ABA and B
FalseFalseFalse
FalseTrueFalse
TrueFalseFalse
TrueTrueTrue
Пример 3: Using and
💡 Open example 
age = int(input("age? "))
grade = int(input("grade? "))
print("Access granted: ", age >= 12 and grade >= 7)

Key takeaway: and = "both must be true". Useful for checking multiple requirements.

📌 Operator or

A or B is False if both A and B are False. In all other cases — True.

ABA or B
FalseFalseFalse
FalseTrueTrue
TrueFalseTrue
TrueTrueTrue
Пример 4: Using or
💡 Open example 
city = "Moscow"
print("Access granted:", city == "Moscow" or city == "Ufa")

Key takeaway: or = "at least one must be true". Useful for checking multiple valid options.

📌 Operator not

not A has the opposite value of A.

Anot A
FalseTrue
TrueFalse
Пример 5: Using not
💡 Open example 
age = 8
print("Access granted:", not (age < 12))

Key takeaway: not = "opposite of". Use carefully to avoid confusion.

📌 Combining Logical Operators

Precedence: not (highest), and, or (lowest). Use parentheses for clarity.

Пример 6: Combined Conditions
💡 Open example 
age = int(input())
grade = int(input())
city = input()
 
if age >= 12 and grade >= 7 and (city == "Moscow" or city == "Ufa"):
    print("Access granted.")
else:
    print("Access denied.")

📌 Common Mistakes with Logical Operators

Пример 7: Mistake 1 – Confusing and and or
💡 Open example 
# WRONG – always True
if x > 1 or x < 100:
    print("In range")
 
# CORRECT
if x > 1 and x < 100:
    print("In range")
Пример 8: Mistake 2 – Incomplete Conditions
💡 Open example 
# WRONG
if age >= 7 and <= 9:
    print("Child")
 
# CORRECT
if age >= 7 and age <= 9:
    print("Child")
 
# BETTER – chained comparison
if 7 <= age <= 9:
    print("Child")

Interesting fact: Chained comparisons like a < b < c are a unique Python feature — many languages don't support this!

🔀 The if Statement

Programs must make decisions based on data — this is called branching. The if statement executes a block of code only if the condition is True.

Syntax:

if condition:
    # code block – executed if condition is True
Пример 9: Basic if Statement
💡 Open example 
number = 5
if number > 0:
    print("The number is positive")

Key points:

  • Colon : after the condition.
  • Indented block (4 spaces).
  • Block executes only if condition is True.
Пример 10: if with User Input
💡 Open example 
print("What programming language are we learning?")
answer = input()
 
if answer == "Python":
    print("Correct! We are learning Python =)")
    print("Python is a great language!")

Example run:

What programming language are we learning?
Python
Correct! We are learning Python =)
Python is a great language!

The indented block executes only when the condition is true.



↔️ The else Statement

else provides an alternative path when the condition is False.

Syntax:

if condition:
    # code if True
else:
    # code if False
Пример 11: if/else
💡 Open example 
number = -3
if number > 0:
    print("Positive")
else:
    print("Negative or zero")

Key takeaway: else is the "otherwise" of programming.

Пример 12: if/else with User Input
💡 Open example 
print("What programming language are we learning?")
answer = input()
 
if answer == "Python":
    print("Correct! We are learning Python =)")
else:
    print("Not quite! We are learning Python.")

Example runs:

What programming language are we learning?
Python
Correct! We are learning Python =)

What programming language are we learning?
Java
Not quite! We are learning Python.

Key takeaway: if/else handles two cases. Every input leads to one of the two paths.



🔗 The elif Statement

elif (else if) allows checking multiple conditions in sequence. The first True condition wins.

Syntax:

if condition1:
    # code if condition1 is True
elif condition2:
    # code if condition2 is True
elif condition3:
    # code if condition3 is True
else:
    # code if none are True
Пример 13: elif Chain
💡 Open example 
number = int(input())
 
if number > 0:
    print("Positive")
elif number < 0:
    print("Negative")
else:
    print("Zero")

Key takeaways:

  • elif handles multiple specific cases.
  • Order matters — first True wins.
Пример 14: Grading with elif
💡 Open example 
grade = int(input("Enter score: "))
 
if grade >= 90:
    print("5 - Excellent")
elif grade >= 80:
    print("4 - Good")
elif grade >= 70:
    print("3 - Satisfactory")
elif grade >= 60:
    print("2 - Poor")
else:
    print("1 - Fail")

Example run:

Enter score: 85
4 - Good

Key takeaways:

  • elif chains are great for grading, categorisation, etc.
  • Order conditions from most specific to most general.

📌 Cascaded vs Nested Conditionals

Cascaded (better):

if grade >= 90:
    print("5")
elif grade >= 80:
    print("4")
elif grade >= 70:
    print("3")
else:
    print("1")

Nested (worse):

if grade >= 90:
    print("5")
else:
    if grade >= 80:
        print("4")
    else:
        if grade >= 70:
            print("3")
        else:
            print("1")

Key takeaway: Use elif instead of nested if/else. Keep your code flat and readable.



⛓️ Chained Comparisons

Comparison operators in Python can be chained together.

Пример 15: Chained Comparison
💡 Open example 
age = int(input())
if 3 <= age <= 6:
    print('You are a child')

The code checks whether the value of the variable age is within the range of 3 to 6.

if a == b == c:
    print('equal numbers')
else:
    print('the numbers are not equal')

The code checks whether all three variables a, b, and c are equal to each other.



❓ Ternary Operator

Python has a conditional expression that is an analogue of the ternary operator.

Пример 16: Ternary Operator
💡 Open example 
student_1 = 5
student_2 = 2
 
excellent = "first student" if student_1 > student_2 else "second student"
print(excellent)  # first student



🔍 The in Operator

The in operator checks if a single character is in a string.

Пример 17: Using in
💡 Open example 
s = input()
if s == 's' or s == 't' or s == 'u' or s == 'd' or s == 'y':
    print('YES')
 
# With 'in' - clean and concise
if len(s) == 1 and s in 'study':
    print('YES')

Key takeaway: in is great for checking against multiple values. We'll explore it more with lists.



📊 Minimum and Maximum

Using if/else, you can find the maximum and minimum among variables.

Пример 18: Maximum of Two Variables
💡 Open example 
x, y = int(input()), int(input())
max_ = float('-inf')
if x > y:
    max_ = x
else:
    max_ = y
print(max_)

Пример 19: Maximum and Minimum of Two Variables
💡 Open example 
x, y = int(input()), int(input())
max_ = float('-inf')
min_ = float('inf')
 
if x > y:
    max_ = x
    min_ = y
else:
    max_ = y
    min_ = x
 
print(f"max = {max_} min = {min_}")



🎯 The match Statement (Python 3.10+)

match is an alternative to long if/elif chains. The underscore _ is the default case.

Пример 20: Simple match
💡 Open example 
command = "start"
 
match command:
    case "start":
        print("Starting work")
    case "stop":
        print("Stopping")
    case _:
        print("Unknown command")

Пример 21: Calculator with match
💡 Open example 
match op:
    case "ADD":
        print("Sum = 2")
    case "MUL":
        print("Multiplication")
    case _:
        print("Unknown operation")



📐 If Statement and Equations

Integers x, y are given. Calculate the value of the function:

f(x,y) =
| -2 * x * y |,   if (x > 0, y < 0) or (x < 0, y > 0)
(x + y)²,         if (x > 0, y > 0) or (x < 0, y < 0)
x,                if y = 0
y,                if x = 0
Пример 22: Solving a Piecewise Function
💡 Open example 
x = int(input("x = "))
y = int(input("y = "))
 
if (x > 0 and y < 0) or (x < 0 and y > 0):
    f = abs(-2 * x * y)
elif (x > 0 and y > 0) or (x < 0 and y < 0):
    f = (x + y) ** 2
elif y == 0:
    f = x
elif x == 0:
    f = y
 
print(f"f({x}, {y}) = {f}")

Example run:

Input integers x, y:
>>> 7
>>> -5
f(7, -5) = 70

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