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c++arraysvectorgslcountif
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Reputation: 53

is there a count_if function for a gsl_vector? C/C++

I am using the gnu scientific library (GSL). Say I have a gsl_vector like this: 7 0 -6 5 8 0 10 -2

That's a vector containing positive numbers, negative numbers, and zeros as elements.

I want to count the number of non-zero elements or zero elements in this gsl_vector. I know there is a function called count_if for a C++ Vector. But I search through the gsl_vector.h and gsl_blas.h, there is no function equal to that. I can go though all the elements by assessing them though gsl_vector_get() and then ask the if question. 

int counter = 0;

for(int i = 0;i<length_of_the_gsl_vector;++i){ 
  if(fabs(gsl_vector_get(y,i))<0.5) ++counter;
}

return counter;

But I have been wondering for almost a day whether there is such a function already in GSL that is much more efficient.

Or maybe there is a count_if function for gsl_array ?

Upvotes: 4

Views: 574

Answers (2)

Romain Francois
Romain Francois

Reputation: 17642

You can get hold of the data pointer by using gsl_vector_ptr, then use std::count_if on pointers: 

struct Predicate{
    inline bool operator()(double x) const {
        return fabs(x) < 0.5 ;
    }
} ;

int res = std::count_if( 
    gsl_vector_ptr(y,0), gsl_vector_ptr(y,0) + size, 
    Predicate()
) ;

Upvotes: 2

CapelliC
CapelliC

Reputation: 60034

They state that GSL implements std::valarray model, I read such statement as authorizing me to use something like:

gsl_block vec;
/* initialize ... */
std::valarray<double> a(vec.data, vec.size);
/* use std:: galore */

Upvotes: 1

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