Conditions and Loops

Conditions let a program make decisions, while loops let it repeat actions. Together, they allow your code to react to driver input, sensor readings, and changing robot states.

Conditions

If you have used an if-else block in Scratch or another block-based language, Java's version works in basically the same way:

if (condition) {
  // Run this code when the condition is true
} else if (anotherCondition) {
  // Run this code when the first condition is false
  // and this condition is true
} else {
  // Run this code when none of the conditions above are true
}

The condition inside the parentheses must evaluate to a boolean: either true or false. An if statement is required, but the else if and else sections are optional. You can also use more than one else if.

Notice that if and else if each need a condition. else does not need one because it handles everything that did not match an earlier condition.

Example: Checking a Grade

Create a new Java file and try the example below. Replace 87.5 with your own mark.

double grade = 87.5;

if (grade < 0 || grade > 100) {
  System.out.println("Invalid number.");
} else if (grade >= 90) {
  System.out.println("You did an excellent job!");
} else if (grade >= 75) {
  System.out.println("You are really smart.");
} else {
  System.out.println("Keep working hard!");
}

Only the first matching branch runs. This is why the invalid range is checked first and the remaining checks go from highest grade to lowest grade.

For example, a grade of 95 satisfies both grade >= 90 and grade >= 75, but Java prints only "You did an excellent job!" because that is the first matching branch.

Conditional Operators

Comparison and logical operators let you build conditions from values:

Operator	Explanation	Example
==	Checks whether two values are equal	speed == 0
!=	Checks whether two values are not equal	speed != 0
>	Checks whether the left value is greater	temperature > 80
<	Checks whether the left value is smaller	distance < 1.5
>=	Checks whether the left value is greater than or equal	grade >= 90
<=	Checks whether the left value is smaller than or equal	speed <= 1.0
&&	Is true when both conditions are true	grade >= 0 && grade <= 100
||	Is true when at least one condition is true	buttonA || buttonB
!	Reverses a condition	!isRobotOn

Assignment vs. comparison

Use one equals sign (=) to assign a value and two equals signs (==) to compare values.

int speed = 5;          // Assigns 5 to speed
boolean stopped = speed == 0; // Checks whether speed equals 0

You can also use a boolean variable directly as a condition:

boolean hasGamePiece = true;

if (hasGamePiece) {
  System.out.println("Ready to score!");
}

if (!hasGamePiece) {
  System.out.println("Run the intake.");
}

Loops

Suppose you want to print every number from 1 to 100. You could write 100 print statements, but copying and editing nearly identical code is slow and creates opportunities for mistakes.

A loop performs the repeated work for you.

For Loops

A for loop is useful when you know how many times the code should repeat.

for (int i = 1; i <= 100; i++) {
  System.out.println(i);
}

The loop header has three parts:

for (startingValue; condition; update) {
  // Code to repeat
}

In the number example:

• int i = 1 creates a counter named i and starts it at 1.
• i <= 100 keeps the loop running while i is less than or equal to 100.
• i++ increases i by one after each repetition.

Three lines instead of 100. Beautiful.

The counter can start at a different value or change by a different amount. This loop prints the even numbers from 2 to 10:

for (int number = 2; number <= 10; number += 2) {
  System.out.println(number);
}

Enhanced For Loops

An enhanced for loop, sometimes called a for-each loop, visits every item in an array or collection. Use it when you need each value but do not need its position.

String[] motorNames = {"frontLeft", "frontRight", "backLeft", "backRight"};

for (String motorName : motorNames) {
  System.out.println(motorName);
}

On each repetition, motorName contains the next item in motorNames. You will learn more about arrays and collections in a later lesson.

While Loops

A while loop repeats code for as long as its condition is true. It is useful when you do not know exactly how many times the code needs to repeat.

int number = 1;

while (number <= 100) {
  System.out.println(number);
  number++;
}

The condition is checked before every repetition. The number++ line eventually makes the condition false and stops the loop.

Infinite loops

If a loop's condition never becomes false, the loop runs forever.

int number = 1;

while (number <= 100) {
  System.out.println(number);
  // number never changes!
}

Be especially careful with while loops in FRC robot code. A long or infinite loop can prevent the rest of the robot program from running. WPILib already calls the robot's periodic methods repeatedly, so an if statement is usually the right choice inside those methods.

Do-While Loops

A do-while loop is similar to a while loop, but it checks its condition after running the code. This means its body always runs at least once.

int countdown = 3;

do {
  System.out.println(countdown);
  countdown--;
} while (countdown > 0);

System.out.println("Go!");

do-while loops are less common, but they are helpful when an action must happen once before the program can decide whether to repeat it.

Break and Continue

The break keyword exits a loop immediately:

for (int number = 1; number <= 10; number++) {
  if (number == 5) {
    break;
  }

  System.out.println(number);
}

This prints 1 through 4. When number becomes 5, the loop ends.

The continue keyword skips the rest of the current repetition and moves to the next one:

for (int number = 1; number <= 10; number++) {
  if (number % 2 != 0) {
    continue;
  }

  System.out.println(number);
}

This prints only the even numbers. The % operator returns the remainder after division, so an odd number has a remainder that is not 0 when divided by 2.

Try It Yourself

1. Write a condition that prints whether a number is positive, negative, or zero.
2. Write a for loop that prints the numbers from 10 down to 1.
3. Write a loop that prints every multiple of 5 from 5 to 50.
4. Create two boolean variables named buttonPressed and robotEnabled. Print "Motor running" only when both are true.

Show possible answers

// 1. Positive, negative, or zero
int number = -4;

if (number > 0) {
  System.out.println("Positive");
} else if (number < 0) {
  System.out.println("Negative");
} else {
  System.out.println("Zero");
}

// 2. Countdown
for (int i = 10; i >= 1; i--) {
  System.out.println(i);
}

// 3. Multiples of 5
for (int i = 5; i <= 50; i += 5) {
  System.out.println(i);
}

// 4. Two requirements
boolean buttonPressed = true;
boolean robotEnabled = true;

if (buttonPressed && robotEnabled) {
  System.out.println("Motor running");
}

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Now you can make your program choose what to do and repeat work without duplicating code. Next, you will learn how to organize instructions into reusable methods.