Scala: Iterate over mutable map of maps

Question

I have defined a mutable map of maps

import scala.collection.mutable.Map

val default = Map.empty[String, Int].withDefaultValue(0)
val count = Map.empty[Any, Map[String, Int]].withDefaultValue(default)

which I populate/update as in

count("furniture")("table") += 1
count("furniture")("chair") = 6
count("appliance")("dishwasher") = 1

How can I iterate over all items in count? And why does count.keys return an empty Set()?

You might want to check my answer regarding withDefaultValue here stackoverflow.com/questions/34774362/… — Łukasz
– Łukasz, Commented Feb 10, 2016 at 9:54

mavarazy · Accepted Answer · 2016-02-10 10:13:18Z

1

With default, does not create new Map when no value exists in collection, it just returns default value on such requests, and other changes are done on this default value.

count("furniture")("table") += 1
count("furniture")("chair") = 6
count("appliance")("dishwasher") = 1

count("banana") // will return Map with "table", "chair" & "dishwasher"

is equivalent

default("table") += 1
default("chair") = 6
default("dishwasher") = 1

And since you return this default value on any key, this default map will be returned on every call.

Your code will work like this.

count("furniture") = Map.empty[String, Int].withDefaultValue(0)
count("appliance") = Map.empty[String, Int].withDefaultValue(0)

count("furniture")("table") += 1
count("furniture")("chair") = 6
count("appliance")("dishwasher") = 1

edited Feb 10, 2016 at 10:13

answered Feb 10, 2016 at 9:41

mavarazy

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4 Comments

sam Over a year ago

So would you know how a map that is instantiated with a new copy of the default map for any key can be created?

sam Over a year ago

Also, why does count.keys return an empty Set(), while count("furniture").keys returns Set(chair, table) with the code I've posted in my question above?

mavarazy Over a year ago

In my code, key set returns list of all keys, and it works. In your case there are no keys, but a default value, which you modify, and count(Any) for anything will return this original default value, which you modified

sam Over a year ago

You're right, count("furniture") returns the updated default map with all its entries. Would you think it would make sense to just check for the existence of a key before updating/inserting entries, and insert a copy (default.clone()) for any non-existing key?

Martin Ring · Accepted Answer · 2016-02-10 09:41:18Z

0

There are several problems with your approach:

Issue #1:

val default = Map.empty[String,Int].withDefaultValue(0)

defines a value default. There is only one instance of this value and it can not be changed, since you defined as a val.

That means that your count map has a default value which is always the same instance of an empty map. Since count is empty, count("furniture") or count("appliance") is exactly the same as just default.

Issue #2:

withDefaultValue does not add entries to a map it just returns a default for undefined keys.

See @mavarazys answer

answered Feb 10, 2016 at 9:41

Martin Ring

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3 Comments

sam Over a year ago

I don't think that «count("furniture") or count("applicance") is exactly the same as just default». The code in my question above actually returns Map(chair -> 6, table -> 1) for count("furniture")...

Martin Ring Over a year ago

no it doesn't, i executed it and it returns Map(dishwasher -> 1, chair -> 6, table -> 1)

sam Over a year ago

You're totally right, sorry for confusing things – and thanks for your help.

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