[Tutor] A couple of somewhat esoteric questions

[Clayton Kirkwood]

As I've contemplated the usage of dictionaries, I face the question of
efficiency. Going back before most of you were probably born;<)) if I
remember correctly dictionaries(assoc. arrays), having hashes, are efficient
for storing sparse arrays with the added benefit of hiding the traversal of
the dictionary looking for the proper key, and being much less efficient if
looking for the value and trying to retrieve its key. But it also depends on
the hash key and algorithm and how many spaces are "available" for filling.
If a hash is created for a small array, which is known ahead of time with
some possible hints, the array is efficient but has the distinct difficulty
of use when more and more pairs are created, because either the new pair
matches an already used hash point and a trail must be descended off of that
particular hash point or a mechanism must be used to determine how the new
pair should be stored elsewhere in the array like at the end and matching
becomes time consuming. And/or, a new hash key and perhaps a different
algorithm must be used for creating the larger array. Also, since the array
allows for multiple keys pointing to possibly identical values, the array is
not necessarily 1 to 1 (could be 1 to multiple although we use replacement
of the value normally) and definitely cannot be onto: there is no rule that
says the mapping from value back to key is singular and unique.

 

I've not had much use of dictionaries in the past so maybe I am missing
something. As I contemplated dictionary use, it seems changing the order of
entries is best left to double single arrays - which can be modified
together when changing order. Dicts don't seem to support moving pairs
around and in fact would be broken if changes were attempted. Kind of like
using a real dictionary, you can have a word (key) have multiple meanings
(values), and you can have multiple words (keys) having the same meaning
(value). Again, with the difference from a real dictionary, our dicts can't
have keys pointing to different values. But there are definitely uses for
dicts.

 

Seem right???

 

Clayton

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