Subj : Re: any function to handle this kind of counting?
To   : comp.programming
From : Arthur J. O'Dwyer
Date : Fri Jul 29 2005 05:21 pm


On Fri, 29 Jul 2005, Ross wrote:
>
> i have generated a 2000 X 1 vector [of integers]
> and this vector indeed has a corresponding vector [of letters]
> 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!!

The simple answer [UNTESTED CODE], in C:

     int how_many(const int *idxs, const char *chs, size_t n,
                  int idx, char ch)
     {
         int i;
         int count = 0;
         for (i=0; i < n; ++i)
           if (idxs[i] == idx && chs[i] == ch)
             ++count;
         return count;
     }

If you're looking for a more efficient answer (e.g., make a table of all 
(idxs * chs) possible queries and index into that), you're going to have
to be more specific about what your input is. What are the ranges of your 
lists of numbers and letters? 1 to 10? A to Z? Arbitrary strings?

   Once you've nailed that down, it should be easy for you to come up with 
the algorithm for making the table on your own. (Which is a roundabout way 
of saying I think it's a homework problem.)

HTH,
-Arthur

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