Using C++, and hopefully the standard library, I want to sort a sequence of samples in ascending order, but I also want to remember the original indexes of the new samples.<
I wrote generic version of index sort.
template
void argsort(RAIter iterBegin, RAIter iterEnd, Compare comp,
std::vector& indexes) {
std::vector< std::pair > pv ;
pv.reserve(iterEnd - iterBegin) ;
RAIter iter ;
size_t k ;
for (iter = iterBegin, k = 0 ; iter != iterEnd ; iter++, k++) {
pv.push_back( std::pair(k,iter) ) ;
}
std::sort(pv.begin(), pv.end(),
[&comp](const std::pair& a, const std::pair& b) -> bool
{ return comp(*a.second, *b.second) ; }) ;
indexes.resize(pv.size()) ;
std::transform(pv.begin(), pv.end(), indexes.begin(),
[](const std::pair& a) -> size_t { return a.first ; }) ;
}
Usage is the same as that of std::sort except for an index container to receive sorted indexes. testing:
int a[] = { 3, 1, 0, 4 } ;
std::vector indexes ;
argsort(a, a + sizeof(a) / sizeof(a[0]), std::less(), indexes) ;
for (size_t i : indexes) printf("%d\n", int(i)) ;
you should get 2 1 0 3. for the compilers without c++0x support, replace the lamba expression as a class template:
template
class PairComp {
public:
Compare comp ;
PairComp(Compare comp_) : comp(comp_) {}
bool operator() (const std::pair& a,
const std::pair& b) const { return comp(*a.second, *b.second) ; }
} ;
and rewrite std::sort as
std::sort(pv.begin(), pv.end(), PairComp(comp)()) ;