Topic 2.3 — if Statements

Goal: write and trace if statements (with and without else), and connect boolean expressions to decision-making in code.

The big idea

An if statement lets your program choose what to do based on a boolean condition. If the condition is true, the “then” part runs. If it’s false, it’s skipped (or the else part runs if you have one).

Basic structure

if (condition) {
  // runs when condition is true
}

The condition must be a boolean expression (true/false).

`if` with `else`

Use else when you want exactly one of two paths to run.

if (score >= 70) {
  grade = "pass";
} else {
  grade = "retry";
}

Tracing example (what happens?)

Always plug in values and evaluate the condition first.

int x = 5;

if (x > 10) {
  x = x + 1;
} else {
  x = x * 2;
}

// x becomes 10

Common comparison patterns

Goal	Typical condition
Check if a value is in a range	`(x >= low) && (x <= high)`
Check if two values are different	`x != y`
Check if at least one thing is true	`a \|\| b`

Classic mistakes

Using = instead of == in a condition.
Forgetting braces and accidentally controlling only one line.
Writing a condition that is always true/false by accident.

// Only the first line is controlled by the if:
if (x > 0)
  x++;
  y++;  // this runs no matter what (oops)

Exam mindset

Evaluate the boolean condition first.
Only one branch runs in if/else.
Be careful with braces: they define the controlled block.
Use && and || for compound conditions.

Quick self-check

What must the expression inside if ( ... ) evaluate to?
When does the else block run?
Trace: if x = 3, what happens in if (x > 3)?
Write a condition for “x is between 10 and 20 inclusive.”
Why are braces helpful even for one line?

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