Scala foldLeft on Maps

Question

How do you use Map.foldLeft? According to the docs it looks like

foldLeft [B] (z: B)(op: (B, (A, B)) ⇒ B) : B

But I'm having difficulty:

Map("first"->1,"second"->2).foldLeft(0)((a,(k,v)) => a+v )
error: not a legal formal parameter

The error points to the open bracket in front of k.

score 93 · Accepted Answer · answered Nov 14 '10 at 18:10

93

If you want to use the (a, (k, v)) syntax, you need to advise the compiler to use pattern matching.

Map("first"->1, "second"->2).foldLeft(0){ case (a, (k, v)) => a+v }

Note that a case statement requires curly braces.

answered Nov 14 '10 at 18:10

Debilski

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Why is pattern matching required? Are there any other patterns which can be matched for in a foldLeft ? – ron_ron May 26 '20 at 05:02

score 19 · Answer 2 · answered Nov 14 '10 at 17:47

19

I think, you can't do the pattern match on tuples as you expect:

Map("first"->1,"second"->2).foldLeft(0)((a, t) => a + t._2)

Actually, using values and sum is simpler.

Map("first"->1,"second"->2).values.sum

answered Nov 14 '10 at 17:47

Thomas Jung

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He _can_ pattern match on tuples. To pattern match, though, one needs to use `case`. – Daniel C. Sobral Nov 14 '10 at 21:28
@Daniel That's the advantage of my lawyer-like sentence: it's correct (he cannot pattern-match as he expected) but I've forgotten the case syntax. – Thomas Jung Nov 15 '10 at 08:56
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For problems like this though, using `values` or `mapValues` is absolutely the clearest solution (which will almost always make it the right choice) – Kevin Wright Nov 15 '10 at 09:22

score 8 · Answer 3 · answered Nov 14 '10 at 20:52

8

The trick is to use a partial function as the code block, in other words you add a case statement that matches on the arguments:

Map("first" -> 1, "second" -> 2).foldLeft(0) { case (a, (k, v)) => a + v }

answered Nov 14 '10 at 20:52

Theo

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Thank you sooo much Theo! This is what I've been looking for a while...but how can `case` be used without `match` keyword and how come is it being used as destructing action? – NikolaS May 30 '22 at 15:18

score 5 · Answer 4 · answered Nov 14 '10 at 19:55

This is not really an answer to your question but I found it useful when starting out with folds, so I'll say it anyway! Note that the /: method "alias" for foldLeft can be clearer for two reasons:

xs.foldLeft(y) { (yy, x) => /* ... */ }

(y /: xs) { (yy, x) => /* ... */ }

Note that in the second line:

it's more clear that the value y is being folded into the collection xs
you can easily remember the ordering of the Tuple2 argument is the same as the ordering of the method "call"

Scala foldLeft on Maps

4 Answers4