Booleans

if Expressions demonstrated the “not” operator !, which negates a Boolean value. This atom introduces more Boolean Algebra.

We start with the operators “and” and “or”:

• && (and): Produces true only if the Boolean expression on the left of the operator and the one on the right are both true.
• || (or): Produces true if either the expression on the left or right of the operator is true, or if both are true.

In this example, we determine whether a business is open or closed, based on the hour:

// Booleans/Open1.kt

fun isOpen1(hour: Int) {
  val open = 9
  val closed = 20
  println("Operating hours: $open - $closed")
  val status =
    if (hour >= open && hour < closed) // [1]
      true
    else
      false
  println("Open: $status")
}

fun main() = isOpen1(6)
/* Output:
Operating hours: 9 - 20
Open: false
*/

main() is a single function call, so we can use an expression body as described in Functions.

The if expression in [1] Checks whether hour is between the opening time and closing time, so we combine the expressions with the Boolean && (and).

The if expression can be simplified. The result of the expression if(cond) true else false is just cond:

// Booleans/Open2.kt

fun isOpen2(hour: Int) {
  val open = 9
  val closed = 20
  println("Operating hours: $open - $closed")
  val status = hour >= open && hour < closed
  println("Open: $status")
}

fun main() = isOpen2(6)
/* Output:
Operating hours: 9 - 20
Open: false
*/

Let’s reverse the logic and check whether the business is currently closed. The “or” operator || produces true if at least one of the conditions is satisfied:

// Booleans/Closed.kt

fun isClosed(hour: Int) {
  val open = 9
  val closed = 20
  println("Operating hours: $open - $closed")
  val status = hour < open || hour >= closed
  println("Closed: $status")
}

fun main() = isClosed(6)
/* Output:
Operating hours: 9 - 20
Closed: true
*/

Boolean operators enable complicated logic in compact expressions. However, things can easily become confusing. Strive for readability and specify your intentions explicitly.

Here’s an example of a complicated Boolean expression where different evaluation order produces different results:

// Booleans/EvaluationOrder.kt

fun main() {
  val sunny = true
  val hoursSleep = 6
  val exercise = false
  val temp = 55

  // [1]:
  val happy1 = sunny && temp > 50 ||
    exercise && hoursSleep > 7
  println(happy1)

  // [2]:
  val sameHappy1 = (sunny && temp > 50) ||
    (exercise && hoursSleep > 7)
  println(sameHappy1)

  // [3]:
  val notSame =
    (sunny && temp > 50 || exercise) &&
      hoursSleep > 7
  println(notSame)
}
/* Output:
true
true
false
*/

The Boolean expressions are sunny, temp > 50, exercise, and hoursSleep > 7. We read happy1 as “It’s sunny and the temperature is greater than 50 or I’ve exercised and had more than 7 hours of sleep.” But does && have precedence over ||, or the opposite?

The expression in [1] uses Kotlin’s default evaluation order. This produces the same result as the expression in [2] because, without parentheses, the “ands” are evaluated first, then the “or”. The expression in [3] uses parentheses to produce a different result. In [3], we’re only happy if we get at least 7 hours of sleep.

Exercises and solutions can be found at www.AtomicKotlin.com.