Conditional Statements in Python

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

<h3 style="margin:12px 0 6px; font-size:1.05rem;">2) Conditional expression (one-liner)</h3>
<pre style="background:#f6f8fa; padding:12px; border-radius:8px; overflow:auto;"><code class="language-python">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.

# 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")