Conditional (if/else)

Running different code depending on a condition.

Also known as: if else, if statement, conditional statement, branching, if/else

A conditional runs different code depending on whether a condition is true. It’s how programs make decisions.

if age >= 18:
    print("adult")
elif age >= 13:
    print("teen")
else:
    print("child")

if (age >= 18) { ... } else if (age >= 13) { ... } else { ... }

The conditions are checked top to bottom, and only the first branch whose condition is true runs.

Things to know

Conditions are booleans, or values the language treats as true or false (truthy and falsy).
Order matters: put the most specific condition first. if age >= 13 before if age >= 18 would catch adults too.
= vs ==: = assigns, == compares (and in JavaScript, prefer ===, see == vs ===).
Combine conditions with and/or (&&/||), and use parentheses when mixing them.
Avoid deep nesting. Use early returns (guard clauses).
Cover the else: what happens when none match? Unhandled cases cause bugs.

For	Use
Choosing between many fixed values	a switch statement or a dictionary lookup
A simple value choice	a ternary operator: `x = a if cond else b`
Replacing repeated `if` chains on type	polymorphism or a lookup table

A function full of nested conditions is a sign to split it up. See control flow.