Control Flow

Making decisions and repeating actions

Overview

Programs rarely run straight from top to bottom. Control flow statements let your code make decisions, repeat actions, and respond to changing robot state — all essential skills for writing useful robot behavior.

See table of contents for a breakdown of this section.

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if / else (conditional statements)

An if statement runs a block of code only when a condition is true.

Speed clamp

        if (speed > 1.0) {
            speed = 1.0;  // clamp to maximum
        }

Add else to handle the false case:

/// tab | SparkMax

Limit switch: stop on trigger

        if (limitSwitch.get()) {
            motor.set(0);       // stop if limit switch is triggered
        } else {
            motor.set(0.5);     // otherwise keep moving
        }

/// /// tab | TalonFX

Limit switch: stop on trigger

        if (limitSwitch.get()) {
            motor.setControl(new DutyCycleOut(0));      // stop if limit switch is triggered
        } else {
            motor.setControl(new DutyCycleOut(0.5));    // otherwise keep moving
        }

///

Chain multiple conditions with else if:

/// tab | SparkMax

Elevator position control

        if (encoderPosition < LOW_SETPOINT) {
            elevator.set(0.3);
        } else if (encoderPosition > HIGH_SETPOINT) {
            elevator.set(-0.3);
        } else {
            elevator.set(0);    // within range — hold position
        }

/// /// tab | TalonFX

Elevator position control

        if (encoderPosition < LOW_SETPOINT) {
            elevator.setControl(new DutyCycleOut(0.3));
        } else if (encoderPosition > HIGH_SETPOINT) {
            elevator.setControl(new DutyCycleOut(-0.3));
        } else {
            elevator.setControl(new DutyCycleOut(0));    // within range — hold position
        }

///

Tip

Conditions must evaluate to a boolean (true or false). See Variables and Data Types for comparison and logical operators.

FRC Example: soft limit

/// tab | SparkMax

Soft limit with if/else

    public void moveUp(double speed) {
        if (getHeight() < MAX_HEIGHT) {
            motor.set(speed);
        } else {
            motor.set(0);
        }
    }

/// /// tab | TalonFX

Soft limit with if/else

    public void moveUp(double speed) {
        if (getHeight() < MAX_HEIGHT) {
            motor.setControl(new DutyCycleOut(speed));
        } else {
            motor.setControl(new DutyCycleOut(0));
        }
    }

///

Warning

If you are setting a value based on a condition, always include an else case to handle the false condition. In FRC code, this is a critical safety practice — for example, if you only set motor power in the if block, then when the condition is false the motor will continue doing whatever it was doing before, which could lead to dangerous runaway behavior.

---

while Loop

A while loop repeats its body as long as its condition remains true.

/// tab | SparkMax

While loop example

        while (!atTarget()) {
            motor.set(0.5);
        }
        motor.set(0);

/// /// tab | TalonFX

While loop example

        while (!atTarget()) {
            motor.setControl(new DutyCycleOut(0.5));
        }
        motor.setControl(new DutyCycleOut(0));

///

Do not use while loops inside robot commands

In FRC robot code, do not put while loops inside execute(). The robot scheduler calls execute() repeatedly on its own. An inner while loop would block the scheduler and freeze the robot. Use isFinished() to signal when a command is done instead:

isFinished() pattern

    @Override
    public boolean isFinished() {
        return m_subsystem.atTarget();
    }

while loops are appropriate in:

• Standalone Java programs (learning exercises, GitHub Classroom assignments)
• Utility code that runs outside the robot framework

---

for Loop

A for loop runs a fixed number of times, or once for each item in a collection. It is the most common loop for counting and iterating.

Counting loop

Counting loop

        for (int i = 0; i < 5; i++) {
            System.out.println("Iteration: " + i);
        }

The three parts of the for header:

Part	Example	Purpose
Initializer	int i = 0	Runs once before the loop starts
Condition	i < 5	Checked before each iteration; loop stops when false
Update	i++	Runs after each iteration — i++ is shorthand for i = i + 1 (see Arithmetic Operators)

Enhanced for-each loop

When you want every item in a collection and don't need the index, the for-each form is cleaner:

/// tab | SparkMax

For-each loop

        SparkMax[] motors = {leftLeader, leftFollower, rightLeader, rightFollower};

        for (SparkMax motor : motors) {
            motor.setSmartCurrentLimit(40);
        }

/// /// tab | TalonFX

For-each loop

        TalonFX[] motors = {leftLeader, leftFollower, rightLeader, rightFollower};
        TalonFXConfiguration config = new TalonFXConfiguration();
        config.CurrentLimits.SupplyCurrentLimit = 40;   // cap current draw to 40 amps

        for (TalonFX motor : motors) {
            motor.getConfigurator().apply(config);
        }

///

FRC Example: configuring multiple motors

Instead of calling the configuration method four separate times, apply config to all motors in a loop:

/// tab | SparkMax

Motor configuration loop

        SparkMax[] motors = {leftLeader, leftFollower, rightLeader, rightFollower};

        for (SparkMax motor : motors) {
            motor.setSmartCurrentLimit(40);       // REV API: cap current draw to 40 amps
            motor.setIdleMode(IdleMode.kBrake);   // REV API: hold position when not powered
        }

/// /// tab | TalonFX

Motor configuration loop

        TalonFX[] motors = {leftLeader, leftFollower, rightLeader, rightFollower};
        TalonFXConfiguration config = new TalonFXConfiguration();
        config.CurrentLimits.SupplyCurrentLimit = 40;           // CTRE: cap current draw to 40 amps
        config.CurrentLimits.SupplyCurrentLimitEnable = true;
        config.MotorOutput.NeutralMode = NeutralModeValue.Brake; // CTRE: hold position when not powered

        for (TalonFX motor : motors) {
            motor.getConfigurator().apply(config);
        }

///

---

Putting It Together

A realistic example combining if/else and a field to implement elevator soft limits:

/// tab | SparkMax

Elevator subsystem

public class ElevatorSubsystem extends SubsystemBase {

    private final SparkMax motor;
    private static final double MAX_HEIGHT = 50.0;  // encoder rotations
    private static final double MIN_HEIGHT = 0.0;

    public ElevatorSubsystem() {
        motor = new SparkMax(5, MotorType.kBrushless);
    }

    public void moveUp(double speed) {
        if (getHeight() < MAX_HEIGHT) {
            motor.set(speed);
        } else {
            motor.set(0);
        }
    }

    public void moveDown(double speed) {
        if (getHeight() > MIN_HEIGHT) {
            motor.set(-speed);
        } else {
            motor.set(0);
        }
    }

    public double getHeight() {
        return motor.getEncoder().getPosition();
    }

    @Override
    public void periodic() {
        // Could log getHeight() to the dashboard here
    }
}

/// /// tab | TalonFX

Elevator subsystem

public class ElevatorSubsystemTalonFX extends SubsystemBase {

    private final TalonFX motor;
    private static final double MAX_HEIGHT = 50.0;  // rotations
    private static final double MIN_HEIGHT = 0.0;

    public ElevatorSubsystemTalonFX() {
        motor = new TalonFX(5);
        TalonFXConfiguration config = new TalonFXConfiguration();
        config.MotorOutput.NeutralMode = NeutralModeValue.Brake;
        motor.getConfigurator().apply(config);
    }

    public void moveUp(double speed) {
        if (getHeight() < MAX_HEIGHT) {
            motor.setControl(new DutyCycleOut(speed));
        } else {
            motor.setControl(new DutyCycleOut(0));
        }
    }

    public void moveDown(double speed) {
        if (getHeight() > MIN_HEIGHT) {
            motor.setControl(new DutyCycleOut(-speed));
        } else {
            motor.setControl(new DutyCycleOut(0));
        }
    }

    public double getHeight() {
        return motor.getPosition().getValueAsDouble();
    }

    @Override
    public void periodic() {
        // Could log getHeight() to the dashboard here
    }
}

///

Note

Notice that moveUp and moveDown each contain an if/else — the motor only runs if the elevator is within safe bounds. This pattern of checking limits before applying power is a fundamental safety practice in FRC.

---

Knowledge Check

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#

Why should you not put a while loop inside a command's execute() method in FRC robot code?

while loops are slower than for loops

while loops require a return value

It blocks the command scheduler, preventing other commands from running and potentially freezing the robot

while loops cannot use boolean conditions inside robot code

The WPILib command scheduler calls execute() repeatedly — that repetition IS the loop. Adding your own while loop inside execute() traps the scheduler and stops the rest of the robot program from running.

#

In a for loop header (for (int i = 0; i < 5; i++)), which part runs exactly once before the loop begins?

The initializer (int i = 0)

The condition (i < 5)

The update statement (i++)

The loop body

The initializer runs once to set up the counter variable. The condition is checked before every iteration, and the update runs after every iteration.

#

Which loop form is the cleanest choice when you need to visit every element in an array and do not need the index?

while loop

Traditional for loop with an index variable

Enhanced for-each loop (for (Type item : collection))

do-while loop

The for-each loop (for (SparkMax motor : motors)) removes the index variable entirely, reducing the chance of off-by-one errors and making the intent clearer.

#

What must always be true about the condition inside an if statement?

It must use the == operator

It must compare two int values

It must call a method

It must evaluate to a boolean (true or false)

Java if statements require a boolean condition. Comparison operators (<, >, ==) and logical operators (&&, ||, !) all produce boolean results and are the typical building blocks of conditions.

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