◆ count_if() [1/2]

template<typename DerivedPolicy , typename InputIterator , typename Predicate >
__host__ __device__ thrust::iterator_traits<InputIterator>::difference_type thrust::count_if ( const thrust::detail::execution_policy_base< DerivedPolicy > &  exec,
InputIterator  first,
InputIterator  last,
Predicate  pred 

count_if finds the number of elements in [first,last) for which a predicate is true. More precisely, count_if returns the number of iterators i in [first, last) such that pred(*i) == true.

The algorithm's execution is parallelized as determined by exec.

execThe execution policy to use for parallelization.
firstThe beginning of the sequence.
lastThe end of the sequence.
predThe predicate.
The number of elements where pred is true.
Template Parameters
DerivedPolicyThe name of the derived execution policy.
InputIteratormust be a model of Input Iterator and InputIterator's value_type must be convertible to Predicate's argument_type.
Predicatemust be a model of Predicate.

The following code snippet demonstrates how to use count to count the number of odd numbers in a range using the thrust::device execution policy:

#include <thrust/count.h>
struct is_odd
__host__ __device__
bool operator()(int &x)
return x & 1;
// fill a device_vector with even & odd numbers
thrust::device_vector<int> vec(5);
vec[0] = 0;
vec[1] = 1;
vec[2] = 2;
vec[3] = 3;
vec[4] = 4;
// count the odd elements in vec
int result = thrust::count_if(thrust::device, vec.begin(), vec.end(), is_odd());
// result == 2
See also