Re: Determine index from array reference?

From:
Patricia Shanahan <pats@acm.org>
Newsgroups:
comp.lang.java.programmer
Date:
Thu, 25 Jan 2007 23:03:37 GMT
Message-ID:
<dHauh.15186$w91.9151@newsread1.news.pas.earthlink.net>
Knute Johnson wrote:

Mark Rafn wrote:

Lew wrote:

If you plan to use the result of your "indexOf()" to locate another
object, rather than just println() it, you might consider using a
Map <JTextField, OtherType>. That would have the benefit of constant
time lookup (if you use HashMap) instead of O(n). You also avoid
bugs caused by "parallel" Lists going non-Euclidean.


Winner! Keeping parallel lists or arrays, and searching one to find
the index
in another is error-prone, hard-to-understand, and makes you look like a
non-OO neanderthal programmer. The first two are actually good
reasons to
consider alternatives, and the third is a gratuitous epithet which may
provide
a bit of motivation to change, or to explain things well enough to
know why
it doesn't apply in this case.

Knute Johnson <nospam@rabbitbrush.frazmtn.com> wrote:

I don't think that saves me anything over the original scheme and
just makes it more complicated.


how is
  OtherType thing = fieldMap.get(thisField)
more complicated than
  OtherType thing =
secondArray[Arrays.asList(firstArray).indexOf(thisField)]

It's just more sane to use actual data structures that do what you
need than
to sling around pairs of arrays that your code happens to know are in
sync.

It's also faster, if these lists get to be of any significant size.
ArrayList.indexOf() is a linear search of the list. HashMap.get() is
constant-time.
--
Mark Rafn dagon@dagon.net <http://www.dagon.net/>


You guys are obviously not listening to this neanderthal programmer. I
only have one list. I can't use a Map because you can't get to the key
with the value. I could use a Map to a Map but that would have the
dreaded two lists.


I'm still very unclear about what the actual accesses are. Why do you
want the index? Presumably, there is some bigger operation involved that
uses the index.

Generally, if you find yourself thinking in terms of "you can't get to
the key with the value" it means you have the map the wrong way round.
However, without knowing what the index is for, it is impossible to say.

Patricia

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