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188Re: [python-iter] Digest Number 21

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  • Magnus Lie Hetland
    Mar 15, 2001
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      > > May it really be meaningless?
      >
      > Yes. An easier example: a set. Elements of the set are neither keys
      > nor values.
      >
      > If you claim that they are keys or values, please define keys or
      > values of a collection.

      Well -- I'm sure there are no keys in a set, but I think it's natural
      to see them as values, no? Isn't a set a collection of values?
      And, in general, if the collection has some structure, it may also
      allow you to access these values by supplying a key/index...

      Of course, mathematically, both lists and dictionaries are
      mappings, i.e. sets of pairs... I would say that the "values" in
      the case of a mapping is the range, while the keys are the
      domain. To generalise to non-mappings, i.e. plain sets, I guess
      one has to choose whether it is more natural to view the
      elements as keys or values. Since it seems plain weird to have
      keys without values, but not to have values without keys,
      I would think the latter is preferrable.

      So, to return to my description in the previous mail; if you iterate
      over the keys of x, then x[key] should give you the value. If this
      is not possible, you are iterating over the values.

      So, "for element in set" would clearly be iterating over the
      values, since "set[element]" wouldn't work. Or if it did, then
      you were in fact iterating over the keys, but I'm not sure what
      set[element] would be...

      So, to conclude, since we in fact from a statement

      for x in collection:
      dosomething(x)

      can not conclude whether x is a key of a value, and this is
      clearly interesting to know, I don't think it's meaningless
      to request that there be some way of finding out.

      Of course one might test collection[x] in s try-block, but that
      seems a bit awkward.

      --

      Magnus Lie Hetland http://www.hetland.org

      "Reality is what refuses to disappear when you stop
      believing in it" -- Philip K. Dick
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