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LESSON 273 OF 677 · TUPLES, SORTING & BUILT-INS

Min, Max and Sum in Scala: Built-in Collection Methods

Lesson Read the full explanation

TL ; DR:

buf.min gives the smallest value of a collection.

buf.max gives the largest value.

buf.sum adds all the values up.


val prices = scala.collection.mutable.ArrayBuffer(4, 9, 2, 7)

println(prices.min)    // 2
println(prices.max)    // 9
println(prices.sum)    // 22

Full lesson:

In Maximum Number In Array, a loop found the largest value. In Sum of Numbers, a loop added numbers up. These jobs come up all the time. Scala collections have them built in.

  • buf.min gives the smallest value of buf.
  • buf.max gives the largest value.
  • buf.sum adds all the values up.

They have no parentheses. They work the same on an ArrayBuffer, an Array or a List.

math.max(a, b) from Maximum Number In Array compares two values. buf.max looks at a whole collection.

Watch out for an empty collection. min and max throw an UnsupportedOperationException on it. An empty collection has no smallest value. minOption and maxOption give an Option instead, as in the Option lesson. So buf.minOption.getOrElse(0) gives 0 for an empty one. sum of an empty collection is 0.

Your task

minutes holds how many minutes you practiced coding on each day of a week.

Store the shortest day in shortest and the longest day in longest. Store the week's total in total.

Use the built-ins from this lesson. The code at the bottom prints all three.

Expected output:

shortest = 15
longest = 60
total = 235
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    SCALA · TUPLES, SORTING & BUILT-INS

    Min Max Sum in Scala, explained

    Built-ins for common jobs

    Finding the largest value and adding numbers up are jobs that come up all the time, and you have written loops for both. Scala collections have them built in:

    • buf.min gives the smallest value of buf.
    • buf.max gives the largest value.
    • buf.sum adds all the values up.
    val prices = scala.collection.mutable.ArrayBuffer(4, 9, 2, 7)
println(prices.min)
println(prices.max)
println(prices.sum)

    2
9
22

    There are no parentheses after the names. They work the same way on an ArrayBuffer, an Array or a List. Each call looks at the whole collection and returns one value.

    These are different from math.max(a, b), which compares just two values. buf.max looks at all the values in a collection.

    Built-in versus loop

    A loop that adds numbers needs a starting var total = 0, a for loop and a += line, and the loop for the largest value needs a starting candidate. Each of those is a chance for a slip, such as starting a maximum at 0 when every number is negative. The built-ins have the starting values right already, and they say in one word what you want. Use a loop when the job is more than a plain minimum, maximum or sum, for example when you also need to skip some values.

    Other types

    min and max work on any items that Scala knows how to order, not only numbers. List("pear", "fig", "apple").max is pear, because strings compare alphabetically. sum needs numbers.

    Using the results together

    The three values combine in ordinary arithmetic. With the week of minutes ArrayBuffer(25, 40, 15, 60, 30, 45, 20), minutes.max - minutes.min is 45, the gap between the longest and the shortest day. The average takes a little care. minutes.sum / minutes.length divides two Int values, so the fraction is cut off and the result is 33. Converting the sum first, with minutes.sum.toDouble / minutes.length, gives 33.57142857142857.

    The sum of Int values is an Int, which has 32 bits and wraps around silently if the total is too large. A week of minutes is nowhere near that, but a very long list of big numbers could be.

    Watch out for an empty collection

    An empty collection has no smallest value. Calling min or max on it throws an UnsupportedOperationException, and the program stops with an error. Two safer methods, minOption and maxOption, return an Option instead: Some(value) when there is one, and None when the collection is empty.

    val empty = scala.collection.mutable.ArrayBuffer[Int]()
println(empty.minOption.getOrElse(0))
println(empty.maxOption)
println(empty.sum)

    0
None
0

    getOrElse(0) picks a default for the empty case. sum of an empty collection is 0, so it needs no such care. For a non-empty buffer, maxOption gives, for example, Some(60).

    The exercise

    minutes holds how many minutes you practiced coding on each day of a week. Store the shortest day in shortest, the longest in longest and the week's total in total, using the built-ins. The code below prints all three.

    val minutes = scala.collection.mutable.ArrayBuffer(25, 40, 15, 60, 30, 45, 20)
val shortest = minutes.min
val longest = minutes.max
val total = minutes.sum
println(s"shortest = $shortest")
println(s"longest = $longest")
println(s"total = $total")

    The output is:

    shortest = 15
longest = 60
total = 235

    Common mistakes

    • Using the loop from the earlier lessons when a built-in is enough. The built-in is shorter and has less room for mistakes, like starting a running maximum at the wrong value.
    • Calling min or max on a collection that might be empty, without minOption or maxOption.
    • Mixing up the index and the value. max gives the largest value, not its position.

    Key takeaways

    • min, max and sum replace the usual loops and need no parentheses.
    • They fail on an empty collection for min and max; use minOption and maxOption.
    • sum of an empty collection is 0.
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