Subj : Re: any function to handle this kind of counting?
To   : comp.programming
From : Rob Thorpe
Date : Mon Aug 01 2005 10:49 am

Gerry Quinn wrote:
> In article <dcdi1t$g7m$1@justice.itsc.cuhk.edu.hk>, a@cuhk.edu.hk
> says...
> > Dear all,
> >
> > i have generated a 2000 X 1 vector, e.g.
>
> > and then i want to count, say, for "1", there are two A's, one E, ...
> >
> > it seems that a for loop can do the job but as we don't know how many
> > instances there are, i.e. don't know A, C, E and then what will occur in
> > this corresponding vector, is there any function which is devoted to handle
> > this kind of counting problem? thx!!
>
> Well, I guess there are libraries for linked lists, vectors etc.  There
> are also sorting functions that can often make such problems easier.
>
> The following is untested C++ to count them directly.
>
> I will treat both vectors as containing ints though I'll call them
> 'nums' and 'lets'.  It's trivial to convert the second to and from
> chars if required (e.g. just use val = letter - 'A'.)  Besides, they
> could be enums for all I know.  The algorithm should be modified if
> speed is at a premium and there are a large number of different letter
> values.
>
> 'targ' is the required number, 1 in the example.  One could modify it
> easily to return a complete solution for all numbers as a vector of
> vectors, using a similar methodology.
>
> If the data are in fact C-style arrays rather than vectors, it's just
> as easy to implement, but using an STL vector for the result makes the
> solution very easy in C++:
>
> #include <vector>
> using namespace std;
>
> class Datum
> {
> public:
>   int letter;
>   int count;
> };
>
> vector< Datum > result;
>
> ASSERT( nums.size() == lets.size() );
>
> for ( int iLet = 0; iLet < lets.size(); iLet++ )
> {
>   if ( nums[ iLet ] != targ )
>   {
>     continue;
>   }
>   int iRes = 0;
>   while ( iRes < result.size() )
>   {
>     if ( result[ iRes ].letter == lets[ iLet ] )
>     {
>       break;
>     }
>     iRes++;
>   }
>   if ( iRes == result.size() )
>   {
>     Datum datum;
>     datum.letter = lets[ iLet ];
>     datum.count = 0;
>     result.push_back( datum );
>   }
>   result[ iRes ].count++;
> }

The above is fine if you use a Microsoft C++ compiler.

In standard C++ there is no function or macro called "ASSERT", there is
a macro called "assert".

The two do opposite things:

In debug builds "ASSERT" will bomb if the condition in parens is true.
In debug builds "assert" will bomb if the condition in parens is not
true.
Neither are compiled in release builds.

I know this isn't a standard C++ only newsgroup, but I thought I would
mention the above since it's rather confusing.

.