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Story-on-maps-vs-sets.md

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Set and Map similarities and differences

Set and Map are closely related data structures. Map has keySet, Map[V,V] is sometimes used as backing store for Set, many more similarities probably could be found. But also there are some differences

  • why Set.add(e) and Map.add(e, v) for existing entry - for set is no-op and for map is update?
  • this leads to unexplainable (other than by specific implementation) behavior of Set.from(it) and Map.from(it) producing different results (set contains first occurrence, while map last one).

This all somehow works, but does not fit as nicely as it could.

Lookups

Anyone wrote/seen code like this: val lookup = data.map(e => e.id -> e).toMap

Always wondered why there is no ready-made Map.index[K,V](it: Iterable[V])(index: V => K)

Features

Index[A](identity) === Set[A]
Index[(K,V)](_._1) === Map[K,V]

Problems

How to handle removal?

  • look up key
  • check if element actually removed is the one being removed