Logical Operators.

The logical operators for AND (&&) and OR (||) are used to combine simple relational statements into more complex expressions. The NOT (!) operator is used to negate a boolean statement.

int RED = 0;  // red LED on digital pin 0
int BLUE = 1;  // blue LED on digital pin 0
boolean op = false;

void setup() {
  pinMode(RED, OUTPUT);
  pinMode(BLUE, OUTPUT);
}

void loop() {
  for(int i=5; i<=195; i+=5) {
    // Logical AND
    if((i > 35) && (i < 100)) {
      op = false;
    }

    // Logical OR
    if((i <= 35) || (i >= 100)) {
      op = true;
    }

    // Testing if a boolean value is "true"
    // The expression "if(op)" is equivalent to "if(op == true)"
    if(op) {
      digitalWrite(RED, LOW);
      digitalWrite(BLUE, HIGH);
      delay(200);  // delay to visualize the LED change
    }

    // Testing if a boolean value is "false"
    // The expression "if(!op)" is equivalent to "if(op == false)"
    if(!op) {
      digitalWrite(RED, HIGH);
      digitalWrite(BLUE, LOW);
      delay(200);  // delay to visualize the LED change
    }
  }
}