Foundations

Variables

Values store data that later expressions can reuse.

val Use `val` for a local value that is assigned once.

Variables

unitPrice
Variables.scala
Replay: real traced execution (multi-file project)
object Main {
  def main(args: Array[String]): Unit = {
    val unitPrice = 12
    val quantity = 3
    val total = unitPrice * quantity

    println("unit=" + unitPrice)
    println("total=" + total)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val unitPrice = 8
    val quantity = 3
    val total = unitPrice * quantity

    println("unit=" + unitPrice)
    println("total=" + total)
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    val unitPrice = 20
    val quantity = 3
    val total = unitPrice * quantity

    println("unit=" + unitPrice)
    println("total=" + total)
  }
}

  1. unitPrice ← 12, quantity ← 3, total ← 36

    1object Main {2  def main(args: Array[String]): Unit = {3    val unitPrice→ 12 = 12 //@unitPrice=8, 204    val quantity→ 3 = 35    val total→ 36 = unitPrice12 * quantity367    println("unit=" + unitPrice12)8    println("total=" + total36)9  }10}
    outputunit=12
total=36

  1. unitPrice ← 8, quantity ← 3, total ← 24

    1object Main {2  def main(args: Array[String]): Unit = {3    val unitPrice→ 8 = 84    val quantity→ 3 = 35    val total→ 24 = unitPrice8 * quantity367    println("unit=" + unitPrice8)8    println("total=" + total24)9  }10}
    outputunit=8
total=24

  1. unitPrice ← 20, quantity ← 3, total ← 60

    1object Main {2  def main(args: Array[String]): Unit = {3    val unitPrice→ 20 = 204    val quantity→ 3 = 35    val total→ 60 = unitPrice20 * quantity367    println("unit=" + unitPrice20)8    println("total=" + total60)9  }10}
    outputunit=20
total=60

What Happens

  1. unitPrice starts at 12.
  2. quantity starts at 3.
  3. total = unitPrice * quantity multiplies 12 * 3.
  4. total becomes 36.
  5. The program prints unit=12 and total=36.

Follow the Values

| unitPrice | quantity | total | | --- | --- | --- | | 8 | 3 | 24 | | 12 | 3 | 36 | | 20 | 3 | 60 |

Exercise: Variables.scala

Reproduce unit=12 and total=36, then use the pinned unitPrice values 8 and 20 to predict each total.