Arduino For Loop

Introduction

For loops are one of the most important control structures in Arduino programming. They allow you to execute a block of code repeatedly a specific number of times, making your code more efficient and compact. Instead of writing the same code multiple times, you can use a for loop to automate repetitive tasks.

In this tutorial, you'll learn how for loops work in Arduino, their syntax, and how to use them effectively in your projects.

Basic Syntax

The basic syntax of a for loop in Arduino (and C/C++) is:

cpp

for (initialization; condition; increment) {
  // Code to be repeated
}

Let's break down these components:

1. Initialization: Sets up a counter variable before the loop starts (executed once)
2. Condition: Evaluated before each loop iteration; the loop continues as long as this is true
3. Increment: Updates the counter after each iteration

Simple For Loop Example

Let's start with a basic example that counts from 0 to 9:

cpp

void setup() {
  Serial.begin(9600);
  
  for (int i = 0; i < 10; i++) {
    Serial.print("Count: ");
    Serial.println(i);
  }
}

void loop() {
  // Nothing here
}

Output:

Count: 0
Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
Count: 6
Count: 7
Count: 8
Count: 9

How It Works

1. int i = 0: We create a variable i and set it to 0
2. i < 10: The loop continues as long as i is less than 10
3. i++: After each iteration, we increment i by 1
4. The code inside the curly braces executes 10 times (with i values from 0 to 9)

Controlling LED Blinks

For loops are perfect for controlling hardware. Here's an example that blinks an LED five times:

cpp

const int ledPin = 13;  // Built-in LED on most Arduino boards

void setup() {
  pinMode(ledPin, OUTPUT);
  
  // Blink LED 5 times
  for (int i = 0; i < 5; i++) {
    digitalWrite(ledPin, HIGH);  // Turn LED on
    delay(500);                 // Wait 500ms
    digitalWrite(ledPin, LOW);   // Turn LED off
    delay(500);                 // Wait 500ms
  }
}

void loop() {
  // Nothing here
}

Backwards Counting

You can also count backwards by using a different increment expression:

cpp

void setup() {
  Serial.begin(9600);
  
  for (int i = 10; i > 0; i--) {
    Serial.print("Countdown: ");
    Serial.println(i);
  }
  
  Serial.println("Blast off!");
}

void loop() {
  // Nothing here
}

Output:

Countdown: 10
Countdown: 9
Countdown: 8
Countdown: 7
Countdown: 6
Countdown: 5
Countdown: 4
Countdown: 3
Countdown: 2
Countdown: 1
Blast off!

Custom Increments

You're not limited to counting by 1. You can increment by any value:

cpp

void setup() {
  Serial.begin(9600);
  
  Serial.println("Counting by 2s:");
  for (int i = 0; i <= 10; i += 2) {
    Serial.println(i);
  }
}

void loop() {
  // Nothing here
}

Output:

Counting by 2s:
0
2
4
6
8
10

Nested For Loops

You can place one for loop inside another to perform more complex repetition patterns:

cpp

void setup() {
  Serial.begin(9600);
  
  for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 3; col++) {
      Serial.print("Row ");
      Serial.print(row);
      Serial.print(", Column ");
      Serial.println(col);
    }
    Serial.println("---");
  }
}

void loop() {
  // Nothing here
}

Output:

Row 1, Column 1
Row 1, Column 2
Row 1, Column 3
---
Row 2, Column 1
Row 2, Column 2
Row 2, Column 3
---
Row 3, Column 1
Row 3, Column 2
Row 3, Column 3
---

Flow Diagram of a For Loop

Here's a visual representation of how a for loop works:

Practical Example: LED Sequence

Let's create a sequence of 8 LEDs that light up in a pattern using a for loop:

cpp

// Define the LED pins
const int ledPins[] = {2, 3, 4, 5, 6, 7, 8, 9};
const int numLeds = 8;

void setup() {
  // Set all LED pins as outputs
  for (int i = 0; i < numLeds; i++) {
    pinMode(ledPins[i], OUTPUT);
  }
}

void loop() {
  // Light up LEDs one after another
  for (int i = 0; i < numLeds; i++) {
    digitalWrite(ledPins[i], HIGH);  // Turn current LED on
    delay(100);                     // Wait
    digitalWrite(ledPins[i], LOW);   // Turn current LED off
  }
  
  // Light up LEDs in reverse order
  for (int i = numLeds - 1; i >= 0; i--) {
    digitalWrite(ledPins[i], HIGH);  // Turn current LED on
    delay(100);                     // Wait
    digitalWrite(ledPins[i], LOW);   // Turn current LED off
  }
}

This example creates a "Knight Rider" or "Cylon" effect with 8 LEDs connected to pins 2-9.

Advanced: Breaking Out of a For Loop

Sometimes you need to exit a loop before it completes all iterations. You can use the break statement for this:

cpp

void setup() {
  Serial.begin(9600);
  
  for (int i = 0; i < 10; i++) {
    Serial.print("Count: ");
    Serial.println(i);
    
    if (i == 5) {
      Serial.println("Reached 5, breaking out of loop!");
      break;  // Exit the loop when i equals 5
    }
  }
  
  Serial.println("Loop finished");
}

void loop() {
  // Nothing here
}

Output:

Count: 0
Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
Reached 5, breaking out of loop!
Loop finished

Advanced: Skipping Iterations

The continue statement allows you to skip to the next iteration of a loop:

cpp

void setup() {
  Serial.begin(9600);
  
  for (int i = 0; i < 10; i++) {
    // Skip printing odd numbers
    if (i % 2 != 0) {
      continue;  // Skip to the next iteration
    }
    
    Serial.print("Even number: ");
    Serial.println(i);
  }
}

void loop() {
  // Nothing here
}

Output:

Even number: 0
Even number: 2
Even number: 4
Even number: 6
Even number: 8

Common Pitfalls

1. Infinite Loops

If the condition in your for loop never becomes false, you'll create an infinite loop that can freeze your Arduino:

cpp

// WARNING: Infinite loop!
for (int i = 0; i < 10; i--) {  // i will never reach 10
  // Code here will run forever
}

2. Off-by-One Errors

Be careful with the boundary conditions in your loops:

cpp

// Array with 5 elements (indexes 0-4)
int myArray[5] = {10, 20, 30, 40, 50};

// INCORRECT: This will cause an error by accessing index 5, which doesn't exist
for (int i = 0; i <= 5; i++) {
  Serial.println(myArray[i]);  // Error when i = 5
}

// CORRECT: Use < instead of <=
for (int i = 0; i < 5; i++) {
  Serial.println(myArray[i]);
}

Summary

For loops are powerful tools in Arduino programming that allow you to:

• Repeat code a specific number of times
• Iterate through sequences like arrays
• Create patterns and sequences for outputs
• Process data efficiently
• Control multiple components with minimal code

By mastering for loops, you'll write more efficient, cleaner code and unlock the ability to create more complex behaviors in your Arduino projects.

Exercises

1. LED Counter: Create a program that uses a for loop to blink an LED a number of times equal to a button press count.

2. Pattern Generator: Use a for loop to create a specific pattern of LED flashes (e.g., three short, three long, three short).

3. Tone Sequence: Use a for loop with the tone() function to play an ascending scale on a piezo buzzer.

4. Multi-LED Binary Counter: Create a binary counter with 4 LEDs that counts from 0 to 15 using a for loop.

5. Servo Sweep: Use a for loop to make a servo motor sweep back and forth smoothly between 0 and 180 degrees.

Next Steps

Now that you understand for loops, you can combine them with other control structures like if statements and while loops to create even more sophisticated Arduino programs. In the next tutorials, we'll explore these combinations and build more complex projects.