Programming
Coding in Python
Conditional Statements in Python

Conditional Statements in Python

Theory

Until now, our programs ran line by line. A conditional changes that flow: depending on a condition, the program chooses one of several paths.

Motivating example: absolute value

Given an integer x, print its absolute value. If x > 0, print x; otherwise print -x.

x = int(input())
if x > 0:
    print(x)
else:
    print(-x)

How it works:

  • if is followed by a condition and a colon :.
  • The indented block under if runs when the condition is True.
  • else: introduces the alternative block, which runs when the condition is False.

Syntax you’ll use

if CONDITION:
    # Block 1
else:
    # Block 2

Block 1 runs if CONDITION is true. Otherwise, Block 2 runs.

Single-branch (“incomplete”) if

You can omit else. Only the if block runs when the condition is true; the rest of the program continues regardless.

x = int(input())
if x < 0:
    x = -x  # change only when needed
print(x)    # always runs

Multiple choices: if–elif–else

Use elif (“else if”) for more than two branches.

score = int(input("Score: "))

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(grade)

Indentation = block structure

  • All lines belonging to the same block must have the same indentation.
  • Recommended: 4 spaces per indentation level.
  • Do not mix tabs and spaces (it can cause errors). Configure your editor to insert spaces on Tab.

Python vs other languages: Python uses indentation to mark blocks. Languages like C, C++, Java, and JavaScript use braces { ... }, while Pascal uses begin ... end and Ruby uses end.

Conditions & truthiness

  • Common comparison operators: ==, !=, <, <=, >, >=.
  • Logical operators: and, or, not.
  • “Falsy” values: 0, 0.0, "", [], {}, set(), None. Everything else is “truthy”.
name = input("Name (optional): ")
if name:  # empty string is False
    print("Hello,", name)
else:
    print("Hello!")

Clean patterns

1) Guard clause (early exit)

def abs_val(x):
    if x >= 0:
        return x
    return -x

2) Conditional expression (one-liner)

x = int(input())
abs_x = x if x >= 0 else -x
print(abs_x)

Common mistakes (and fixes)

  • Forgetting the colon after if/elif/else.
  • Inconsistent indentation (mixing tabs/spaces or misaligned blocks).
  • Assignment vs comparison: use == to compare, not =.
  • Unreachable code: placing code after a return in the same block.

Quick practice

  1. Read an integer and print even if it’s even, otherwise odd.
  2. Given two integers, print the larger one (no built-ins like max).
  3. Classify a number: positive, negative, or zero.
Show sample solutions 
# 1) even / odd
n = int(input())
if n % 2 == 0:
    print("even")
else:
    print("odd")

# 2) larger of two
a = int(input())
b = int(input())
if a >= b:
    print(a)
else:
    print(b)

# 3) sign
x = int(input())
if x > 0:
    print("positive")
elif x < 0:
    print("negative")
else:
    print("zero")

Key takeaways: end your conditions with :, use consistent 4‑space indentation, and choose among if, if/else, and if/elif/else based on how many paths you need.

Problems

Maximum of Two Numbers

Full Problem

Leap Year

Determine whether the given year is a leap year. (A leap year is divisible by 4 but not by 100, unless it is also divisible by 400.)

Full Problem

Rook

Determine whether a rook on a square with given coordinates (row and column) attacks a piece on another specified square.

Full Problem

Bishop

Determine whether a bishop on a square with given coordinates (row and column) attacks a piece on another specified square.

Full Problem

Queen

Determine whether a queen on a square with given coordinates (row and column) attacks a piece on another specified square.

Full Problem

Knight

Determine whether a knight on a square with given coordinates (row and column) attacks a piece on another specified square.

Full Problem

Chocolate Bar

Given a chocolate bar of size n × m squares, determine if it is possible to break off exactly k squares with a single straight-line break between squares (splitting into two rectangles).

Full Problem

Counters

Each edge square of a square board is given one counter. Could there be exactly k counters placed? (For example, a 2×2 board has 4 counters, a 6×6 board has 20.)

Full Problem

Equation

Solve the linear equation ax + b = 0 in integers.

Full Problem

Complex Equation

Solve the rational equation (ax + b) / (cx + d) = 0 in integers.

Full Problem

Change

An item costs a roubles and b kopeks. It was paid with c roubles and d kopeks. How much change should be returned?

Full Problem

Ice Cream

Ice cream is sold in packs of 3 and 5 scoops. Can exactly k scoops be bought?

Full Problem

Cutlets

You can fry k cutlets at once in a pan. Each cutlet must be fried m minutes per side without interruption. What is the minimum time required to fry n cutlets on both sides?

Full Problem

Coordinate Quadrants

Given coordinates of two points on a plane, determine if they lie in the same quadrant (all coordinates are non-zero).

Full Problem

Which Number Is Greater?

-

Full Problem

Maximum of Three

-

Full Problem

Is the Triangle Possible?

-

Full Problem

How Many Are Equal? (Among Three Numbers)

-

Full Problem

King’s Move

A square on a chessboard is identified by two numbers (a, b) from 1 to 8. The first is the column, the second is the row. Given two squares, determine if a king can move from the first to the second in one move.

Full Problem

Quadratic Equation

Given real numbers a, b, c, find all solutions of the quadratic equation ax² + bx + c = 0.

Full Problem

Triangle Type

Determine the type of triangle (acute, obtuse, or right-angled) based on its side lengths.

Full Problem

Cows

Given a number n, complete the phrase “On the meadow grazes...” with the correct form of the word “cow” in Russian.

Full Problem

Tube Tickets

A tube ticket for 1 trip costs 15 roubles, for 10 trips – 125 roubles, and for 60 trips – 440 roubles. A passenger plans to take n trips. Determine how many of each ticket type should be purchased to cover at least n trips at the lowest cost.

Full Problem

Tube Tickets – 2

A tube ticket for 1 trip costs 15 roubles, 5 trips – 70 roubles, 10 trips – 125 roubles, 20 trips – 230 roubles, and 60 trips – 440 roubles. A passenger plans to take n trips. Determine the optimal combination of tickets to cover at least n trips for the lowest total price.

Full Problem

Neighbour Coordinates

For a cell at coordinates (x, y) in a table of size M × N, output the coordinates of its neighbours. Neighbours share a side with the given cell.

Full Problem

Even and Odd Numbers

Given three integers A, B, and C, determine if there is at least one even and one odd number among them.

Full Problem

Roman Numerals

Given a number X, convert it to Roman numerals.

Full Problem

Sign of a Number

The mathematical function sign(x) is defined as: sign(x) = 1 if x > 0, sign(x) = -1 if x < 0, sign(x) = 0 if x = 0. Output the value of sign(x) for the given number.

Full Problem

Test System

In one of the problems on the site, the condition says: “If the given four-digit number is symmetric, output 1, otherwise output any other integer.” A pupil believes they solved the problem, but the system rejects it. They think it’s because they returned a different “any other number” than the expected one. Write a program that compares the system’s expected output and the student’s answer, and checks if the solution is correct.

Full Problem

Sort Three Numbers

You are given three numbers in separate lines. Sort them in non-decreasing order. The program must read a, b, c, reorder their values to ensure a ≤ b ≤ c, and output them.

Full Problem
Intro to Python
Data Types
Type Conversion
Data Input: input() function
Data Output: print() function
Our First Program
Turtle: Pong Game
Arithmetic Operations
Conditional Statements in Python
Nested Conditions
Boolean in Python
Logical Operators
Cascading Conditional Statements in Python

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