if Statements: Revision Notes (Computer Science A 2.3)

Recap

An if statement is a selection statement: it changes the normal top-to-bottom flow by running a segment of code only when a Boolean expression is true.

Form	Syntax	What runs
One-way (if)	if (cond) { A }	A runs only if cond is true; otherwise nothing
Two-way (if-else)	if (cond) { A } else { B }	A if cond is true, B if it is false: exactly one
Two separate ifs	if (c1) { A } if (c2) { B }	A, B, both or neither, tested one after the other

Key relationships

Idea	Rule
Condition	A Boolean expression inside parentheses; no semicolon after it
Else	No condition; runs when the matching if condition was false
Braces	Body is the block; without braces only one statement belongs to the if
Trace	Write variable values, evaluate the condition, follow one path
Rejoin	After the chosen body, execution continues with the next statement
Boolean variable	if (isReady) is the same as if (isReady == true)

Assumptions

• The condition is exactly true or false.
• Variables used in the condition have been assigned before the if.
• For if-else, the two bodies cover the two possible outcomes of one decision.

Mistakes to avoid

• Putting a semicolon straight after if (...).
• Leaving off braces and trusting the indentation.
• Using two ifs when you need an if-else (or the reverse).
• Writing = instead of == in the condition.
• Forgetting that the second of two ifs sees the values changed by the first.
• Printing or returning inside only one branch and expecting a value in the other.

Worked mini-example

int stock = 3;
int order = 5;
if (order > stock) {
    order = stock;
}
stock = stock - order;

Trace: order > stock is 5 > 3, true, so the body runs and order becomes 3. Then stock = 3 - 3 = 0. After the code, stock is 0 and order is 3. With order = 2 instead, the condition is false, the body is skipped and stock becomes 1. The if here adjusts a value before the next statement uses it, which is the most common job for a one-way selection.

Quick self-check

1. How many bodies run in an if-else? Answer: exactly one.
2. With x = 8, what does this print? if (x > 5) { System.out.print("A"); } if (x > 7) { System.out.print("B"); } Answer: AB, because both conditions are true and the ifs are independent.
3. What does int k = 1; if (k > 3); { k = 9; } leave k as? Answer: 9. The semicolon is the whole body, so the block always runs.
4. Rewrite if (n < 0) { s = "neg"; } if (n >= 0) { s = "pos"; } as one statement. Answer: if (n < 0) { s = "neg"; } else { s = "pos"; }.

Full explanations and diagrams are in the Topic 2.3 study guide.