Comparison Operators

Comparison operators compare two values and return a Boolean result — true (1) or false (0). They are the building blocks of decision-making in Arduino, used inside if, while, and for conditions to control program flow based on sensor readings, timing, or user input.

== (equal to)

The == operator checks whether two values are equal. It returns true if they are identical and false otherwise. Do not confuse this with = (single equals), which is the assignment operator.

void setup() {
  Serial.begin(9600);
  int a = 10;
  int b = 10;

  if (a == b) {
    Serial.println("a equals b");  // this prints
  }

  // Common mistake — using = instead of ==
  if (a = 5) {  // assigns 5 to a, always true!
    Serial.println("This always runs!");
  }
}

void loop() {
  // nothing to do here
}

In this example, a == b correctly compares the two values. The second if uses = instead of ==, which assigns 5 to a and then evaluates to true (non-zero). This is a common bug — always double-check your equality comparisons.

!= (not equal to)

The != operator checks whether two values are different. It returns true if they are not equal and false if they are equal.

void setup() {
  Serial.begin(9600);
  pinMode(2, INPUT_PULLUP);
}

void loop() {
  if (digitalRead(2) != HIGH) {
    Serial.println("Button is pressed");  // pin is LOW (pressed)
  }
  delay(100);
}

This sketch checks if a button is pressed by testing whether pin 2 is not HIGH. When the button is pressed, the pin reads LOW, so digitalRead(2) != HIGH is true.

< (less than)

The < operator returns true if the left value is smaller than the right value.

void setup() {
  Serial.begin(9600);
}

void loop() {
  int sensor = analogRead(A0);
  if (sensor < 100) {
    Serial.println("Too dark");
  }
  delay(500);
}

This example prints a warning when the light sensor reading drops below 100. The < operator is commonly used for threshold detection, loop bounds, and range checking.

> (greater than)

The > operator returns true if the left value is larger than the right value.

void setup() {
  Serial.begin(9600);
}

void loop() {
  int sensor = analogRead(A0);
  if (sensor > 800) {
    Serial.println("Very bright");
  }
  delay(500);
}

This sketch prints a message when the sensor reading exceeds 800. Paired with <, you can check if a value falls within a specific range using &&: if (value > 100 && value < 200).

<= (less than or equal to)

The <= operator returns true if the left value is less than or equal to the right value.

void setup() {
  Serial.begin(9600);
}

void loop() {
  for (int i = 0; i <= 5; i++) {
    Serial.println(i);  // prints 0, 1, 2, 3, 4, 5
  }
  delay(3000);
}

In this example, the loop runs while i <= 5, so it prints 0 through 5 inclusive. Note the difference from i < 5, which would print only 0 through 4. The <= operator is useful when you want to include the upper bound.

>= (greater than or equal to)

The >= operator returns true if the left value is greater than or equal to the right value.

void setup() {
  Serial.begin(9600);
  unsigned long start = millis();
}

void loop() {
  unsigned long now = millis();
  if (now - start >= 5000) {
    Serial.println("5 seconds elapsed!");
    start = now;  // reset timer
  }
}

This sketch prints a message every 5 seconds using millis() and the >= operator. Using >= for timing is more reliable than checking for exact equality (==), since millis() might increment past the exact target value before you check.