Idiomatic Scala Map upsert

Question

I'm working with a map in Scala and doing the usual "if there's no value associated with a key, create it, put it in the map and return it":

def alphaMemory(key : AlphaMemoryKey) = {
    var am = map.getOrElse(key, null)
    if(am == null) {
        am = new AlphaMemory(key)
        map.put(key, am)
    }
    am
}

To me, this does not feel like idiomatic Scala code. It feels like Java. Is there a more succinct way of writing this? It looked like maybe I could override Map.default() to insert the new value and return it. Not sure though.

Thanks!

score 40 · Accepted Answer · edited Jun 12 '17 at 17:20

40

mutable.Map has getOrElseUpdate which does exactly what you want, no idiom necessary.

edited Jun 12 '17 at 17:20

Hosam Aly

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answered Jan 03 '09 at 04:46

sblundy

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Excellent! I didn't realize that the default parameter was "by-name" so I had convinced myself it would be evaluated each time it was called. Thanks! – Dave Ray Jan 03 '09 at 04:51
I'd be interested in seeing how this would be done with immutable maps? – Marcus Downing May 22 '09 at 19:20
1

@Marcus Downing Have a look at [this question](http://stackoverflow.com/q/4385976/460387) for solutions for `immutable.Map` – Frank S. Thomas Apr 30 '11 at 06:50

Idiomatic Scala Map upsert

1 Answers1