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Asked: 2026-05-11T14:28:46+00:00

I have a class containing a number of double values. This is stored in

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I have a class containing a number of double values. This is stored in a vector where the indices for the classes are important (they are referenced from elsewhere). The class looks something like this:

Vector of classes

class A {   double count;   double val;   double sumA;   double sumB;    vector<double> sumVectorC;   vector<double> sumVectorD; }  vector<A> classes(10000);

The code that needs to run as fast as possible is something like this:

vector<double> result(classes.size()); for(int i = 0; i < classes.size(); i++) {   result[i] += classes[i].sumA;   vector<double>::iterator it = find(classes[i].sumVectorC.begin(), classes[i].sumVectorC.end(), testval);   if(it != classes[i].sumVectorC.end())     result[i] += *it; }

The alternative is instead of one giant loop, split the computation into two separate loops such as:

for(int i = 0; i < classes.size(); i++) {   result[i] += classes[i].sumA; } for(int i = 0; i < classes.size(); i++) {  vector<double>::iterator it = find(classes[i].sumVectorC.begin(), classes[i].sumVectorC.end(), testval);   if(it != classes[i].sumVectorC.end())     result[i] += *it; }

or to store each member of the class in a vector like so:

Class of vectors

vector<double> classCounts; vector<double> classVal; ... vector<vector<double> > classSumVectorC; ...

and then operate as:

for(int i = 0; i < classes.size(); i++) {   result[i] += classCounts[i];   ... }

Which way would usually be faster (across x86/x64 platforms and compilers)? Are look-ahead and cache lines are the most important things to think about here?

Update

The reason I’m doing a linear search (i.e. find) here and not a hash map or binary search is because the sumVectors are very short, around 4 or 5 elements. Profiling showed a hash map was slower and a binary search was slightly slower.

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1 Answer

  1. As lothar says, you really should test it out. But to answer your last question, yes, cache misses will be a major concern here.

    Also, it seems that your first implementation would run into load-hit-store stalls as coded, but I’m not sure how much of a problem that is on x86 (it’s a big problem on XBox 360 and PS3).

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