Loops

Hook

A teammate writes code to auto-align the robot. It works perfectly — the arm moves to exactly 90° on the first try. But the moment you enable, the robot dashboard freezes. The joysticks stop responding. The entire robot locks up for two seconds, then continues.

The code wrote a while loop that ran inside periodic():

void periodic() {
    while (arm.getAngle() < 90) {
        arm.setMotor(0.5);
    }
}

The WPILib scheduler calls periodic() every 20ms. The while loop inside it never returns until the arm reaches 90°. While it’s running, nothing else in the robot’s program can execute — no joystick input, no safety checks, nothing.

Core concept

Walk-through

A while loop:

int i = 0;
while (i < 4) {
    i = i + 1;
}
// i is 4 here

A for loop (equivalent, more compact):

for (int i = 0; i < 4; i++) {
    // body
}

Use for when you know how many iterations you want. Use while when you’re waiting for a condition.

The FRC robot loop

Your robot’s main loop already exists — it’s the WPILib scheduler. It calls periodic() on every subsystem and runs any active commands, 50 times per second. You don’t write that loop; you write what goes inside it.

The correct way to handle the arm alignment above:

void periodic() {
    if (arm.getAngle() < 90) {
        arm.setMotor(0.5);
    } else {
        arm.setMotor(0.0);
    }
}

This checks the condition once per call and returns immediately. The arm moves a little each cycle. Over many cycles, it reaches 90°.

Interactive demo

Trace a while loop that counts up. Watch how the condition is checked before each iteration:

Try it yourself

int i = 1;
int product = 1;
while (i <= 4) {
    product = product * i;
    i = i + 1;
}

Key takeaways

• A while loop checks its condition before each iteration. When the condition is false, the loop exits immediately.
• The final value of the loop variable is the first value that failed the condition.
• periodic() is the robot’s main loop — it must return within ~20ms or the robot misbehaves.
• Replace blocking while loops in periodic() with stateful conditionals that check once and return.

Common confusions

“I wrote while (true) — now the robot doesn’t respond.” You’ve blocked periodic(). Kill the code (Ctrl+C or disable), remove the blocking loop, and use a state machine or command instead.

“My loop runs one too many or one too few times.” Check whether the condition uses < or <=. Off-by-one errors are extremely common.

Challenge

What’s next

In Lesson 05, we’ll look at functions — how to name and reuse blocks of code so you’re not copying the same logic into five different places.