Re: I cannot use the hash_map::operator[] to read the value in the hash map?

 James Kanze <>
Thu, 02 Aug 2007 11:36:00 -0000
On Aug 2, 7:25 am, wrote:

On Aug 2, 1:47 pm, xz <> wrote:

like km1.put(2, 40); in Java
like int a = (int) hm1.get(2); in Java

I think hash map is so basic data structure, why is it such a headache
to use this famous HASH MAP?
In java, life is simple......

This is simple? This is hideous compared to the natural operator[]

Still, the STL version is worse:

    std::map< X, int >::const_iterator it = hm1.find( key ) ;
    int a = it == hm1.end() ? 42 : *hm1 ;

The real problem in reading a map (or a hashmap) is how to
handle missing values. Sometimes, an exception is appropriate,
but it's not a good general solution. So you need something out
of band.

And should I include <hash_map> or <hash_map.h> ?

I thought it should be

but turns out:

error: hash_map: No such file or directory

Whatever the documentation for your system tells you. The
latest draft says <unordered_set>, but this is fairly recent,
and it's likely that your implementation still uses something
pre-standard. Which it *should* document.

If you want to be a programmer, you should at least know how to do a
recursive directory search for filenames including *hash*. Really,
kids these days.... ;-P

Given that the correct name is unordered_set, searching for a
file whose name includes *hash* isn't going to help. More
generally, of course, recursive directory search is almost
certainly the wrong way to go about it here.

FWIW, many people just use std::map, which has a similar interface,
the benefit of sorting the elements, is standardised and portable
(just #include <map>), and is unlikely to have a noticeable
performance difference unless you're inserting tens of thousands or
more elements for frequent lookup or the elements are difficult to
compare. Basically, map does a binary lookup which typically requires
approximately log2(N) element comparisons, or 10 for a thousand
elements, 20 for a million....

The advantage of std::map is that it has been in the standard
from the very beginning, and so is standard. The various
pre-standard implementations of hash_map varied somewhat, which
created no end of portability problems. In the near future, of
course, there's std::unordered_map. But I don't think you can
count on it yet.

Note too that std::map guarantees O(n ln n). unordered_set will
be *typically* O(n), but only if you have a good hashing
function on the key. And good hashing functions are not
necessarily trivial to come by.

James Kanze (GABI Software)
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