Question

I need to map through a List[(A,B,C)] to produce an html report. Specifically, a

List[(Schedule,GameResult,Team)]

Schedule contains a gameDate property that I need to group by on to get a

Map[JodaTime, List(Schedule,GameResult,Team)]

which I use to display gameDate table row headers. Easy enough:

val data = repo.games.findAllByDate(fooDate).groupBy(_._1.gameDate)

Now the tricky bit (for me) is, how to further refine the grouping in order to enable mapping through the game results as pairs? To clarify, each GameResult consists of a team’s “version” of the game (i.e. score, location, etc.), sharing a common Schedule gameID with the opponent team.

Basically, I need to display a game result outcome on one row as:

3 London Dragons vs. Paris Frogs 2

Grouping on gameDate let’s me do something like:

data.map{case(date,games) =>
  // game date row headers
  <tr><td>{date.toString("MMMM dd, yyyy")}</td></tr>

  // print out game result data rows
  games.map{case(schedule,result, team)=>
    ...
    // BUT (result,team) slice is ungrouped, need grouped by Schedule gameID
  }
}

In the old version of the existing application (PHP) I used to

for($x = 0; $x < $this->gameCnt; $x = $x + 2) {...}

but I’d prefer to refer to variable names and not the come-back-later-wtf-is-that-inducing:

games._._2(rowCnt).total games._._3(rowCnt).name games._._1(rowCnt).location games._._2(rowCnt+1).total games._._3(rowCnt+1).name

maybe zip or double up for(t1 <- data; t2 <- data) yield(?) or something else entirely will do the trick. Regardless, there’s a concise solution, just not coming to me right now…

Answer 1

Maybe I’m misunderstanding your requirements, but it seems to me that all you need is an additional groupBy:

repo.games.findAllByDate(fooDate).groupBy(_._1.gameDate).mapValues(_.groupBy(_._1.gameID))

The result will be of type:

Map[JodaTime, Map[GameId, List[(Schedule,GameResult,Team)]]]

(where GameId is the type of the return type of Schedule.gameId)

Update: if you want the results as pairs, then pattern matching is your friend, as shown by Arjan. This would give us:

val byDate = repo.games.findAllByDate(fooDate).groupBy(_._1.gameDate)
val data = byDate.mapValues(_.groupBy(_._1.gameID).mapValues{ case List((sa, ra, ta), (sb, rb, tb)) => (sa, (ta, ra), (tb, rb)))

This time the result is of type:

Map[JodaTime, Iterable[ (Schedule,(Team,GameResult),(Team,GameResult))]]

Note that this will throw a MatchError if there are not exactly 2 entries with the same gameId. In real code you will definitely want to check for this case.