In Scala, how do I keep track of running totals without using var?

Question

For example, suppose I wish to read in fat, carbs and protein and wish to print the running total of each variable. An imperative style would look like the following:

var totalFat = 0.0
var totalCarbs = 0.0
var totalProtein = 0.0
var lineNumber = 0

for (lineData <- allData) {
    totalFat += lineData...
    totalCarbs += lineData...
    totalProtein += lineData...
    lineNumber += 1

    printCSV(lineNumber, totalFat, totalCarbs, totalProtein)

}

How would I write the above using only vals?

missingfaktor · Accepted Answer · 2011-07-23 09:31:34Z

13

Use scanLeft.

val zs = allData.scanLeft((0, 0.0, 0.0, 0.0)) {  case(r, c) =>
  val lineNr = r._1 + 1
  val fat = r._2 + c...
  val carbs = r._3 + c...
  val protein = r._4 + c...
  (lineNr, fat, carbs, protein)
}

zs foreach Function.tupled(printCSV)

answered Jul 23, 2011 at 9:31

missingfaktor

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Mchl · Accepted Answer · 2011-07-23 09:23:13Z

3

Recursion. Pass the sums from previous row to a function that will add them to values from current row, print them to CSV and pass them to itself...

answered Jul 23, 2011 at 9:23

Mchl

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Comments

kiritsuku · Accepted Answer · 2011-07-23 09:34:06Z

2

You can transform your data with map and get the total result with sum:

val total = allData map { ... } sum

With scanLeft you get the particular sums of each step:

val steps = allData.scanLeft(0) { case (sum,lineData) => sum+lineData}
val result = steps.last

If you want to create several new values in one iteration step I would prefer a class which hold the values:

case class X(i: Int, str: String)
object X {
  def empty = X(0, "")
}
(1 to 10).scanLeft(X.empty) { case (sum, data) => X(sum.i+data, sum.str+data) }

answered Jul 23, 2011 at 9:34

kiritsuku

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Comments

user unknown · Accepted Answer · 2011-07-23 15:57:48Z

0

It's just a jump to the left,
and then a fold to the right /:

class Data (val a: Int, val b: Int, val c: Int) 
val list = List (new Data (3, 4, 5), new Data (4, 2, 3), 
                 new Data (0, 6, 2), new Data (2, 4, 8)) 
val res = (new Data (0, 0, 0) /: list) 
  ((acc, x) => new Data (acc.a + x.a, acc.b + x.b, acc.c + x.c))

answered Jul 23, 2011 at 15:57

user unknown

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