Re: A Better Choice?

Victor Bazarov <v.bazarov@comcast.invalid>
Wed, 02 Oct 2013 19:20:13 -0400
On 10/2/2013 5:38 PM, Jorgen Grahn wrote:

On Wed, 2013-10-02, Victor Bazarov wrote:

On 10/2/2013 2:48 PM, Gert-Jan de Vos wrote:

On Sunday, 29 September 2013 00:15:35 UTC+2, Mike Copeland wrote:

I have the following data declaration that is causing compiler

     char DBI[60001] = {'\0'} ;

     The purpose of this data is to store a character value ('a'..'I') for
each corresponding record that has been read from a database. The
records are identified by a value that ranges from 1-60000. I can't use
a bitset here, as I need to store the character value associated with
each record. I don't want to be limited by the current supported range

For mapping a number to a character with any number of entries, this would
be my first choice:

std::map<int, char> dbi;

I wonder what the difference with std::vector<char> would be if *all*
sixty thousand and one numbers in the range need a character. I mean,
the total size of a vector is 60001+sizeof(std::vector<char>) plus the
overhead of allocating the block, which is ~16 bytes. For a map it
would be 60000 * (sizeof(__node_type) + overhead), yes? Whatever
__node_type is, that is. And given that it usually needs to keep at
least three pointers, the value, and [as in VC++] a couple of chars,
that's like 27 bytes... So, is that, like, 40 times the memory? I
don't think it's worth considering unless the array is *really* sparse.

Yes, using a std::map would be a waste of memory. On the other hand:
(a) it would not be noticeable on a typical system!

I am guessing it depends very much on the definition of "noticeable" and
on the definition of "typical".

(b) using std::map (or unordered_map) ought to make for more readable
     code, if this is indeed a value->value mapping problem

Really? More readable, how? Instead of writing

     mytable[recordnum] = thischar;

you will write

     mytable[recordnum] = thischar;


So I agree with G-JdV about first choice.


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