Loops

Oftentimes we want to do the same action multiple times in a row. For instance, if you know your grades are about to be released, you might repeatedly check them until they come out. In Java we can use loops to do this.

We will be going over two types of loops which are:

• while
• for

Note

There is another type of loop called do while and because it’s not commonly used in FRC we will not be covering it. However, if you want to learn more about do while loops, you can learn more on w3school’s page on do while loops

Conditionals, and loops may seem similar but they have different purposes. Conditionals are used to make decisions based on whether the condition is true or false. Loops are used to repeat code until the condition is met. This can be thought of conditionals make decisions, and loops are for repeating.

while

while loops are the most simple type of loops. If you have used block coding before, this is similar to the repeat block. If you have taken an CS class, you might already be familiar with a while loop.

The syntax of a while loops is as shown:

while (condition) {
// code to run when condition is true
}

You might notice that a while loop has a similar syntax to conditionals, which was discussed in the previous section. A while loop’s syntax includes:

• while is a keyword
• ( ) holds the condition that is either true or false. Example: (5 > 1) is a condition and it is true because 5 is greater than 1
• { an open bracket is used to denote the opening of the loop
• } a closed bracket is used to denote the end of the loop

Another way of understanding how a while loop works is with the flow chart below

Similar to conditionals, a while loop will run the code block inside the while loop when the condition is true. After the code block is run, it goes back to the condition. If the condition is true, the code block will run again. This will repeat until the condition is false. When it’s false, the loop will be skipped. To understand how this works, let’s look at an example.

int i = 0;
while (i < 6) {
    System.out.println(i); // prints 0, 1, 2, 3, 4, 5
    i++;
}

In the example, there is an int called i which holds the value 0. The while loop has the condition i < 6. First, the compiler checks if the condition is true. We know that 0 is less than 6 so the while code block runs and the value of i, which is currently 0, is printed to the terminal. The value of i is also updated because of i++ and it changes to 1. Now the condition checks if 1 is less than 6. We know that it is true, so the value of i gets printed and the value of i is increased by 1. This repeats and eventually i’s value becomes 6. 6 is not less than 6, therefore the loop is done and the while code block does not run.

Tip

Does the i++ look familiar? This was covered in the operator section. If you need a refresher on what an operator is and how it works, check out the operator page.

Lets look at an another example! In the example below we have:

int autoTime = 0;
while (autoTime <= 15){
    System.out.println("AutoMode is happening");
    autoTime++;
}

Without running the code, what do you think happens?

autoTime is set to 0. The while loop’s condition is autoTime <= 15. 0 is less than 15 so the code enters the while code block, and prints outs Auto is happening. autoTimer++ is run which increases autoTimer’s value by one. The code goes back to the condition, 1 is less than 15, so the code enters the while code block again, printing out Auto is happening. Like before, autoTimer++ runs which increases autoTimer by one again making the value be 2. This repeats until autoTime is 15. When that happens the condition is false and the while loop is skipped.

for

Sometimes, we might want code that loops an exact number of times. This can be done with a for loop. The syntax of a for is as follows:

 rli:ignore
for (statement1; statement2; statement3) {
    // code to run when statement 2 is true
}

In the syntax:

• for is a keyword
• () holds the three statements
• { an open bracket is used to denote the opening of the loop
• } a closed bracket is used to denote the end of the loop

The three statements are

• initialization: creates a variable that sets the starting point of the loop.
• condition: a condition, which uses the variable, that is checked before each iteration. If the condition is true, the code inside the loop runs
• update: updates the variable created in initialization after each iteration

The best way to explain how for loops work is with an example.

for (int i = 0; i < 5; i++){
    System.out.println(i); // prints 0, 1, 2, 3, 4
}

In this example, we create a variable i that is set to 0. When i is less than 5, the for code block runs, printing the value of i which is 0. Then it goes back to the update statement which is i++, i has increased by 1 and i’s value is now 1. We check the condition, 1 is less than 5, so for code block runs, the new value of i is printed, i is updated and this repeats until i is no longer less than 5. When i is no longer less than 5, the for loop is skipped.

Another way of understanding how a for loop works is with the flow chart below

Be Careful about Infinite Loops

It is important to make sure that the condition can become false. If the condition is always true, the loop will always run causing an infinite loop. Infinite loops can be prevented by ensuring that the value inside the condition changes by incrementing or decrementing.

int timer = 0;
while (timer < 7){
    drivetrain.setThrottle(1); // sets drive motors to full speed
}

In this example, we want our drive train to run for 7 seconds. We have our variable called timer which is set to 0. 0 is less than 7 so the code inside runs and the robot drives forward at full speed. However, timer’s value never changes. This means timer will always be 0. 0 is less than 7 so the loop will continuously run and the robot never stops driving!

To fix this, we would make sure to increment the timer value as shown below.

int timer = 0;
while (timer < 7){
    drivetrain.setThrottle(1); // sets drive motors to full speed
    timer++; // increments timer by 1
}

timer now increments by 1 each time the loop is run, and the robot will stop driving after 7 seconds.

Loops Exercise

Exercise

WIP