Loops and repetition

Learn

What you’ll learn

• for loops: repeat a set number of times
• while loops: repeat while something is true
• How the main loop() fits with these, and how to avoid freezing your robot

for loops

for (int i = 0; i < 5; i++) {
  // runs 5 times, with i = 0, 1, 2, 3, 4
}

The three parts: start (int i = 0), keep going while (i < 5), change each time (i++, add one).

while loops

while (digitalRead(2) == HIGH) {
  // keep waiting until the button is pressed
}

A while loop runs as long as its condition is true. If the condition never becomes false, it runs forever.

The forever loop

loop() itself is a forever loop: the Arduino calls it again and again. In a robot, loop() should usually run quickly so the robot keeps re-reading its sensors.

Don't trap your robot

A long while or a long delay() inside loop() stops the robot from checking its sensors. while (true) {} freezes it completely. In the robot simulator, a loop() that takes over a second is stopped automatically.

Examples

Example: blink 3 times at start-up

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  for (int i = 0; i < 3; i++) {
    digitalWrite(LED_BUILTIN, HIGH); delay(200);
    digitalWrite(LED_BUILTIN, LOW);  delay(200);
  }
}
void loop() {}

Example: wait for a start button

void setup() {
  pinMode(2, INPUT_PULLUP);
  while (digitalRead(2) == HIGH) {}   // wait here until pressed
  // ...then the robot starts
}

Example: averaging readings

long total = 0;
for (int i = 0; i < 10; i++) total += analogRead(A0);
int average = total / 10;

Practice

Try it

loop() in the robot simulator already repeats 50 times a second. Here’s a counter that uses it:

Counting with the forever loop

▶ Run↺ Reset

Your code (JavaScript)let ticks = 0; function loop() { ticks = ticks + 1; if (ticks % 50 === 0) print("seconds: " + ticks / 50); motors(0, 0); }Load starter code

Robot commands

• motors(left, right): wheel power, −100 to 100
• lineLeft(), lineRight(): true over the black line
• distance(): cm to the nearest thing ahead (max 200)
• distanceLeft(), distanceRight(): cm to the sides
• linePosition(): where the line is, −1 (left) to 1 (right), or null
• millis(): milliseconds since Run
• print(value): show a message below
• setup() runs once; loop() runs 50 times a second

Count to 10Easy

Write a for loop that prints 1 to 10. for (int i = 1; i <= 10; i++) Serial.println(i);

Times tableMedium

Print the 7 times table up to 7 × 12. for (int i = 1; i <= 12; i++) { Serial.print(i); Serial.print(" x 7 = "); Serial.println(i * 7); }

Stop after 5 secondsHard

In the simulator, drive forward and stop after 5 seconds without using millis(). loop() runs 50 times per second. Count the calls.

Show an answer

let ticks = 0;
function loop() {
  ticks++;
  if (ticks < 250) motors(50, 50);   // 250 calls = 5 s
  else motors(0, 0);
}

Quick check

1. How many times does for (int i = 0; i < 4; i++) run?

345Forever

2. When does a while loop stop?

After 10 runsWhen its condition becomes falseNeverAfter 1 second

3. Why avoid long delays inside a robot’s loop()?

They use more batteryThe robot stops checking its sensorsThey cause syntax errorsThey are fine

Check my answers