Why my list is empty?

Question

I have this code:

 val products = List() def loadProducts(report: (Asset, Party, AssetModel, Location, VendingMachineReading)) = { report match { case (asset, party, assetModel, location, reading) => EvadtsParser.parseEvadts(reading.evadts, result) (result.toMap).map(product => ReportData( customer = party.name, location = location.description, asset = asset.`type`, category = "", product = product._1, counter = product._2, usage = 0, period = "to be defined")).toList } } results.foreach(result => products ::: loadProducts(result)) println(products)

Can you please tell me what I am doing wrong because products list is empty? If I println products inside loadProducts method, products is not empty. Is the concatenation I am doing wrong?

PS: I am a scala beginner.

::: yields a new list instead of mutating the one you already have in place. — om-nom-nom
– om-nom-nom, Commented Jan 30, 2013 at 19:47
@om-nom-nom I thought that it ads elements to the end of my list... — Cristian Boariu
– Cristian Boariu, Commented Jan 30, 2013 at 19:48

om-nom-nom · Accepted Answer · 2013-01-30 20:17:56Z

As I've already said, ::: yields a new list instead of mutating the one you already have in place.

http://take.ms/WDB http://take.ms/WDB

You have two options to go: immutable and mutable

Here is what you can do in immutable and idiomatic style:

def loadProducts(report: (...)): List[...] = { ... } val products = result.flatMap(result => loadProducs(result)) println(products)

But also, you can tie with mutability and use ListBuffer to do the things you've wanted:

def loadProducts(report: (...)): List[T] = { ... } val buffer = scala.collection.mutable.ListBuffer[T]() result.foreach(result => buffer ++ = loadProducs(result)) val products = buffer.toList println(products)

P.S. flatMap( ...) is an analog to map(...).flatten, so don't be confused that I and Tomasz written it so differently.

Tomasz Nurkiewicz · Accepted Answer · 2013-01-30 20:05:13Z

List type is immutable, val implies a reference that never changes. Thus you can't really change the contents of products reference. I suggest building a "list of lists" first and then flattening:

val products = results.map(loadProducts).flatten println(products)

Notice that map(loadProducts) is just a shorthand for map(loadProducts(_)), which is a shorthand for map(result => loadProducts(result)).

If you become more experienced, try foldLeft() approach, which continuously builds products list just like you wanted to do this:

results.foldLeft(List[Int]())((agg, result) => agg ++ loadProducts(result))

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