Re: std::set, index of element
"Michael DOUBEZ" <michael.doubez@free.fr> wrote in message
news:49705232$0$10251$426a74cc@news.free.fr...
Hicham Mouline wrote:
Hello
we have a perf critical function that calculates for 1 thing, many
operations, and returns for each 1 result and 1 code.
codes and results are outputs, operationSet is the input.
[...]
I wish to index the operations set in operationSet, so that I can extract
a size_t index to use for codes and results.
The calling contexts have a static number of operations you pass in the
set, but there are many different calling contexts.
1 context would look like:
CalcOperationSet operationSet; operationSet.set( OP1 );
operationSet.set( OPthree ); operationSet.set( OPtwo );
double results[3]; CalcCode codes[3];
// somehow associate OP1 with 0, OPthree with 1 and OPtwo with 2
// of course, OP1 with 0 depends on the context , here we have a set with
just 3 elements
// can we determine the order of insertion
for (size_t s=0; s<1000; ++s)
Calculate(results, codes, operationSet, things[s] );
How would we know?
That depends on the type of CalcOperationSet.
If you have an iterator in CalcOperationSet you can easily associate
operation to result and for the other way around, from an index, you can
use std::advance to advance your iterator but performances depends on the
type of iterator (which must be at least a FowardIterator).
--
Michael
CalcOperationSet is a std::set, and so finding the index of the operation
inside the set,
using std::distance is not ideal.
I just switched to std::map< CalcOperation, size_t >, wrapper by
CalcOperations
and I actually store the index of each operation.
It is twice as fast than std::set in this usage case.
So
CalcOperations operations;
operations.set( OP1 ); operations.set( OPthree ); operations.set(
OPtwo );
double results[3]; CalcCode codes[3];
for (size_t s=0; s<1000; ++s) {
Calculate( codes, results, operations, things[s] ) ;
// use results and codes
}
Calculate( Code codes[], double results[], const CalcOperations& operations
, int thing )
{
if ( operations.IsSet( OP1 )
{
results [ operations.IndexOf(OP1 ) ] = ....
}
if ( operations.IsSet( OPthree )
{
}
// I have now factorized redundance runtime between OPxxx and OPyyy
// I can in 1 call to Calcuate get both results rather than call the
single version of Calculate
// a first time, then a second time and so on, which needlessly runs some
costly code more than once.
}
Your comments are appreciated,
regards,