Conditionals in Arduino with if / else

Until now, our programs have been like a shopping list: “Do this, then wait, then do that.” The Arduino obeyed in order, line by line.

We don’t always want the LED to blink. Maybe we want it to turn on ONLY IF we press a button, or to turn off IF there is too much light.

Today we are going to learn the most fundamental structure in programming: the conditional statement if / else (this is the moment your Arduino learns to make decisions).

The Concept: A Fork in the Road

Imagine you are driving and you come to a fork in the road. You have to make a decision based on a condition:

In programming, this works exactly the same way. We give the Arduino a condition (a Yes or No question). If the answer is Yes, it executes a block of code. If it’s No, it ignores it (or does something else).

The Syntax

The structure in C++ (Arduino’s language) is quite intuitive:

if (condition) {
   // Code that executes if the condition is TRUE
}
else {
   // Code that executes if the condition is FALSE
}

Comparison Operators

For the if to work, we need to ask questions whose answer is True or False. For this we use comparison operators.

Practical Example: The Smart Button

Let’s revisit the setup from the previous article: a button connected to Pin 2 and the built-in LED on Pin 13.

The code would look something like this:

void setup() {
  pinMode(13, OUTPUT);
  pinMode(2, INPUT_PULLUP);  // we'll see this in due time
}

void loop() {
  // 1. Read the button
  int state = digitalRead(2);
  
  // 2. Make the decision 
  if (state == LOW) {
    // If the condition is true (button is pressed)
    digitalWrite(13, HIGH); // Turn LED ON
  }
  else {
    // If the condition is false (button is not pressed)
    digitalWrite(13, LOW);  // Turn LED OFF
  }
}

What if I want it to work the other way around?

Imagine you want a “dead man’s switch” light (turns off if you press, turns on if you release). You just need to change the logic inside the curly braces, or change the question:

if (state == HIGH) { // If the button is RELEASED
   digitalWrite(13, HIGH);
} else {
   digitalWrite(13, LOW);
}

Nesting Decisions

What if we want more complex conditions? We can put an if inside another if.

If I press the button… and also the LED was off… then turn it on.

This would allow us to make a toggle-type switch (a push button that turns on and off alternately), although that requires managing state variables which we’ll see later.

const int buttonPin = 2;
const int ledPin = 13;

void setup() {
  pinMode(ledPin, OUTPUT);
  pinMode(buttonPin, INPUT);
}

void loop() {
  // First question: Is the button pressed?
  if (digitalRead(buttonPin) == HIGH) {
      
      // If we get here, the button IS pressed.
      // Now we ask the second question (nested):
      
      if (digitalRead(ledPin) == LOW) {
          // If we get here, BOTH conditions are met:
          // 1. Button pressed
          // 2. The LED was off
          
          digitalWrite(ledPin, HIGH); // Turn the LED ON!
      }
  }
  
  delay(100); // A small pause for stability
}

If this is too complicated for now, don’t worry. For now, just keep in mind that inside the curly braces { ... } you can write any valid code: turn LEDs on, wait with delay, or ask another if question.