The problem this code solves

A ticket kiosk prints boarding passes numbered 1 through N, a countdown timer ticks from 10 down to 0, and a scoreboard totals points across a fixed number of rounds. Each is the same idea: repeat an action a known number of times. That is exactly what a for loop is built for. When the count is known up front, the for loop puts the start, the stop, and the step in one place so the counting cannot drift.

For loops: counting made into one line

A for loop packs the three parts of counting into one header: where to start, when to stop, and how to step. Read it left to right, then watch it run: initialize once, then test, body, update, test, body, update, until the test fails.

The anatomy of a for loop

for (int i = 0; i < 3; i++) {
    System.out.println(i);
}

The update i++ runs after the body each pass, and then the condition is retested. When the condition is false the loop ends and the counter is discarded.

The four steps, in order

• 1. Initialize once: int i = 0
• 2. Test the condition; if it is false, stop
• 3. Body runs
• 4. Update (i++), then back to step 2

A for loop is a while loop in disguise

// for: the count lives in the header
for (int i = 0; i < n; i++) {
    total += i;
}

// exactly the same work
int i = 0;
while (i < n) {
    total += i;
    i++;
}

Three counting patterns

• Count up: for (int i = 0; i < n; i++)
• Count down: for (int i = n; i > 0; i--)
• Step by k: for (int i = 0; i < n; i += 2)

Vocabulary

Predict first, then check

See the loop run

This is the accumulator from Problem 1, traced. Each pass adds i to sum. When i reaches 6, the condition i <= 5 is false and the loop stops. Press Play to watch it build.

int sum = 0;
for (int i = 1; i <= 5; i++) {
    sum += i;
}

Live code editor

Eight problems, six scaffolded and two open-ended. Write Java, press Run to execute it, and Check to test against the expected output. Hints and the solution are here if you need them, but using them is tracked, so try first.

For Loop Arcade

Six quick rounds. Read the loop, then type the value it produces or the number of times the body runs. This builds the trace-in-your-head speed the timed exam rewards.

Multiple choice

Predict your answer before reading the options. Watch for the qualifier words. After you answer, the rationale explains every distractor.

FRQ 1 connection: Methods and Control Structures

FRQ 1 always asks you to implement a method with a loop and a conditional. Here it is in miniature: a for loop over an array with an if inside. Write it, run it, and check it.

Choose your lane