Conditionals

What is it?

A conditional evaluates a Boolean expression and runs a block only if it is true: if, else if, else, and in some languages switch or match. Combined with the logical operators and, or, and not, it lets a program choose among paths. Most languages short-circuit: in A and B, B is skipped when A is already false.

Why does it matter?

Branches are where programs differ from calculators — and where most bugs and most tests live. Each branch is a path you must reason about and test, and the number of paths grows quickly, so clear conditions are worth more than clever ones.

Mental model

A fork in the road with signs checked in order. The first sign that matches sends you down that road; the rest are never read.

How it works

• Conditions are evaluated top to bottom. The first true branch wins and the rest are skipped.
• Short-circuit evaluation: 'x is not None and x.ready' is safe because the second half only runs when the first is true.
• Compare like with like: = assigns and == compares. In JavaScript prefer === to avoid type coercion.
• Guard clauses — return early on the bad case — keep the main path flat and readable.
• Every condition has a boundary. Test the value just below it, at it, and just above it.

Steps

• Evaluate the condition
• If true, run this block
• Otherwise test the next condition
• Fall through to else
• Continue after the block

Code example: Guard clause, then ordered branches

def grade(score):
    if score < 0 or score > 100:
        raise ValueError("score out of range")   # guard clause
    if score >= 90:
        return "A"
    elif score >= 80:
        return "B"
    return "C or below"

print(grade(90), grade(89))   # A B

Real-world usage

• Validating input before using it
• Choosing behavior by state: logged in or out, empty or full
• Feature flags and configuration switches

Common mistakes

• Using = where == was meant (a syntax error in Python, a silent bug in C).
• Ordering branches so a general case hides a specific one, such as score >= 50 before score >= 90.
• Testing the middle of a range and forgetting its boundaries.

Interview questions

Prerequisites

• Data Types

Related concepts

• Functions
• Error Handling

Continue learning

• Loops