Loops

Groundhog day

The naive way of writing code to blink a LED 50 times is really tiresome to write:

digitalWrite(led, HIGH);
delay(100);
digitalWrite(led, LOW);
delay(100);
digitalWrite(led, HIGH);
delay(100);
digitalWrite(led, LOW);
//...

Programmers are lazy, so they invented a really cool solution to this problem: Loops. A loop is a special block which executes the code in the block until a condition is met.

Structure of loops

Loop expressions contain two parts. The head of the loop contains the condition which controls for how often the code in the body of the loop should be repeated. The body usually starts and ends with curly braces ({ }).

There are several types of loops each tailored for different needs. The following sections will show you two of the most common loop types.

for loops

for loops are usually used for when you exactly want to specify how often the code in the body should be executed. In this example, a LED will be switched on and off 50 times. The head of the for loop contains three parts delimited by a semicolon (;).

1. Variable initialization. Allows to initialize a variable which holds the count of loops
2. A Condition. The condition allows you to explicitly say when the loop should run. If the expression evaluates truthy, the loop will continue. How to write a condition is explained in the chapter about if-else
3. An expression to run after each loop of the code in the body. Usually it is used to increment the counter variable.

for (int counter = 1; counter < 50; counter = counter + 1) {
  // blink LED
}

In this example the counter variable is named counter. The condition is “while counter is below 50After each loop iteration, increasecounter` by one.

Hint:

• The code counter = counter + 1 is the longer form of counter++. Both forms do the same. Take the number in the variable counter, increment by one and store the result in the counter variable.
• You can use any name for the counter variable you like. Often the name i for “index” is used.

Exercise 1

Think of a loop body which prints the current value of the counter variable to the serial monitor.

Hint: The chapter Serial Monitor explains how to communicate over the serial interface.

• a) Observe the output if you exchange counter = counter+ 1 with counter = counter*2 or counter--.
• b) Observe the output if you exchange int counter= 0 with int counter = 25.

The while loop

Sometimes you don’t know how often the code in the block should be executed. For this, you can use the while loop. The while loop only contains a condition and a block. The condition is evaluated before the code in the block runs. If it evaluates truthy, the code in the body is executed.

while (condition) {
  // blink LED
}

For example, this allows you to evaluate the state of a button attached to the Arduino and only execute the loop if the button is pressed.

Attention: A common error is to write a condition which is always true. (For example 1 > 0). In this case, your program will hang indefinitely in the loop and will not continue. This is called an endless loop.

Exercise 2

• a) Write the code for a program that prints Statement true! in the serial monitor if a variable a is above 0.
• b) Write the code for a program that blinks a LED if a button is pressed.
• c) Every for loop can be expressed as a while loop. Rewrite this for loop so that it uses a while loop:

   for (int i = 10; i > 0; i--) {
     Serial.print("Countdown: ");
     Serial.println(i);
   }