comp.lang.c++ Google Group

How to get an insertion hint for an unordered associated container?

2009-11-21T19:34:15Z

I am trying to figure out if I can use tr1/C++0x std::unordered_map to
speed-up an algorithm I did before using an ordered map.
The problem is quite common:
BEGIN
Given a key 'k', if no equivalent of 'k' is on the container, create
value and insert it; otherwise, update part of the value that is already

Re: Article on possible improvements to C++

2009-11-21T19:02:03Z

It's of no use if the client code wants to know about the failure in few
higher-level callers. Of course you can create a class for every
overloaded + operator that can overflow, create a method "onOverflow()"
in it etc. but what level of cooperation would it require from the
client code to add and remove interested parties?

Re: Math/CompSci Interview Question - Thoughts?

2009-11-21T18:46:21Z

In article <6a8340b8-19b0-4fda-96d5-e744a ead1...@m26g2000yqb.googlegrou p
s.com>, Andrew Tomazos <and...@tomazos.com> writes
This sort of problem is covered by articles on Data Stream Processing in
CACM Oct 2009. (CACM is a lot more interesting these days than it was
some years ago). There are some very neat ideas in there, of which the

Re: Math/CompSci Interview Question - Thoughts?

2009-11-21T18:33:38Z

On Sat, 21 Nov 2009 08:12:53 -0800 (PST), Andrew Tomazos
[snip code]
I hope "masters-equivilant" isn't on your resume.

Probably not, but that is not necessarily a good thing.
That's an interesting question with an interesting
presupposition. The first thing to understand is that this is a
puzzle rather than a programming problem. The presupposition is

Re: Why do some code bases don't use exceptions?

2009-11-21T17:46:55Z

I was referring to an "unprecedented amount". I mean if we disregard
the known trolls. I do not regard the OP as a troll, so it made me very
sad to see so many bad assumptions.

Re: Defining hash-keys?

2009-11-21T17:34:39Z

* James Kanze:
It depends. Like how exactly the hash table handles collisions (search for next
available, overflow list, what?), and whether values tend to be bundled in small
intervals. I think doing the randomization thing is safest, but considering how
it might lead to excessive paging I'm not even sure about that -- depends also

Re: Math/CompSci Interview Question - Thoughts?

2009-11-21T17:29:14Z

There is an old analysis of this problem by Fischer and Salzberg.
M.J. Fisher and S.L. Salzberg (1982)
Finding a majority among n votes.
Journal of Algorithms 3, pp 143-152.
If 2k elements contain a majority element (= an element that occurs at
least k+1 times), then you can find it with 3k-2 element comparisons

Re: Math/CompSci Interview Question - Thoughts?

2009-11-21T17:24:10Z

If what you're asking is whether anybody having a master in CS and
maths would have been able to come up with this solution in the
interview time, I guess we can answer that definitely no, otherwise
there would be no point in trying to select candidates with this test.
Obviously, it's because some people (with or without a master diploma,

Re: Why do some code bases don't use exceptions?

2009-11-21T16:50:02Z

Just a linguistic nit, but formally, "unprecedented" means that
there is no precedent---no one has done it before. Given this
definition, I wouldn't call posting a web page on a subject you
know nothing about exactly "unprecedented":-).
Not that that that changes anything with regards to your
argument.

Re: Why do some code bases don't use exceptions?

2009-11-21T16:45:48Z

What is the perceived problem? And why is aborting (as a
default behavior) a bad solution? Violating an exception
specification is a contract violation, so aborting *is* about
the only appropriate default behavior. The question is: what
problem is solved by supporting contracts in which a function

Re: Defining hash-keys?

2009-11-21T16:34:43Z

std::map will take an std::vector<int> as a key. No need to
convert to string. (The access will still be O(lg n), rather
than O(1), but that's often enough.)

Re: Defining hash-keys?

2009-11-21T16:29:31Z

That's a lot more work than necessary. The better string hash
functions actually treat the string as a sequence of unsigned
char---small unsigned integers. Depending on the algorithm and
your data types (and values), either just converting the
elements to unsigned (if this results in no loss of information

Re: Defining hash-keys?

2009-11-21T16:25:44Z

Don't forget the most important characteristic of the hash
function: if a == b, then hash(a) == hash(b). This could easily
be a problem with floats (where +/- O might hash differently, if
the hash function was done naïvely) or unordered collections
(where different ordering of the same values might hash

Math/CompSci Interview Question - Thoughts?

2009-11-21T16:12:53Z

I was posed the following question in a technical interview for a
Software Engineering position by a major multinational NASDAQ company:
[Paraphrasing] "You are given an array of 1,000,000 32-bit integers.
One int value x occurs 500,001 times or more in the array. Specify an
algorithm to determine x."

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