thrust::set_difference_by_key

Defined in thrust/set_operations.h

template<typename DerivedPolicy, typename InputIterator1, typename InputIterator2, typename InputIterator3, typename InputIterator4, typename OutputIterator1, typename OutputIterator2, typename StrictWeakCompare>
thrust::pair<OutputIterator1, OutputIterator2> thrust::set_difference_by_key(const thrust::detail::execution_policy_base<DerivedPolicy> &exec, InputIterator1 keys_first1, InputIterator1 keys_last1, InputIterator2 keys_first2, InputIterator2 keys_last2, InputIterator3 values_first1, InputIterator4 values_first2, OutputIterator1 keys_result, OutputIterator2 values_result, StrictWeakCompare comp)

set_difference_by_key performs a key-value difference operation from set theory. set_difference_by_key constructs a sorted range that is the difference of the sorted ranges [keys_first1, keys_last1) and [keys_first2, keys_last2). Associated with each element from the input and output key ranges is a value element. The associated input value ranges need not be sorted.

In the simplest case, set_difference_by_key performs the “difference” operation from set theory: the keys output range contains a copy of every element that is contained in [keys_first1, keys_last1) and not contained in [keys_first2, keys_last2). The general case is more complicated, because the input ranges may contain duplicate elements. The generalization is that if [keys_first1, keys_last1) contains m elements that are equivalent to each other and if [keys_first2, keys_last2) contains n elements that are equivalent to them, the last max(m-n,0) elements from [keys_first1, keys_last1) range shall be copied to the output range.

Each time a key element is copied from [keys_first1, keys_last1) or [keys_first2, keys_last2) is copied to the keys output range, the corresponding value element is copied from the corresponding values input range (beginning at values_first1 or values_first2) to the values output range.

This version of set_difference_by_key compares key elements using a function object comp.

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

The following code snippet demonstrates how to use set_difference_by_key to compute the set difference of two sets of integers sorted in descending order with their values using the thrust::host execution policy for parallelization:

 #include <thrust/set_operations.h>
 #include <thrust/functional.h>
 #include <thrust/execution_policy.h>
 ...
 int A_keys[6] = {9, 6, 5, 4, 3, 1, 0};
 int A_vals[6] = {0, 0, 0, 0, 0, 0, 0};

 int B_keys[5] = {9, 7, 5, 3, 1};
 int B_vals[5] = {1, 1, 1, 1, 1};

 int keys_result[3];
 int vals_result[3];

 thrust::pair<int*,int*> end = thrust::set_difference_by_key(thrust::host, A_keys, A_keys + 6, B_keys, B_keys + 5,
A_vals, B_vals, keys_result, vals_result, thrust::greater<int>());
 // keys_result is now {0, 4, 6}
 // vals_result is now {0, 0, 0}

See also

set_union_by_key

See also

set_intersection_by_key

See also

set_symmetric_difference_by_key

See also

sort_by_key

See also

is_sorted

Parameters

  • exec – The execution policy to use for parallelization.

  • keys_first1 – The beginning of the first input range of keys.

  • keys_last1 – The end of the first input range of keys.

  • keys_first2 – The beginning of the second input range of keys.

  • keys_last2 – The end of the second input range of keys.

  • values_first1 – The beginning of the first input range of values.

  • values_first2 – The beginning of the first input range of values.

  • keys_result – The beginning of the output range of keys.

  • values_result – The beginning of the output range of values.

  • comp – Comparison operator.

Template Parameters

  • DerivedPolicy – The name of the derived execution policy.

  • InputIterator1 – is a model of Input Iterator, InputIterator1 and InputIterator2 have the same value_type, InputIterator1's value_type is a model of LessThan Comparable, the ordering on InputIterator1's value_type is a strict weak ordering, as defined in the LessThan Comparable requirements, and InputIterator1's value_type is convertible to a type in OutputIterator's set of value_types.

  • InputIterator2 – is a model of Input Iterator, InputIterator2 and InputIterator1 have the same value_type, InputIterator2's value_type is a model of LessThan Comparable, the ordering on InputIterator2's value_type is a strict weak ordering, as defined in the LessThan Comparable requirements, and InputIterator2's value_type is convertible to a type in OutputIterator's set of value_types.

  • InputIterator3 – is a model of Input Iterator, and InputIterator3's value_type is convertible to a type in OutputIterator2's set of value_types.

  • InputIterator4 – is a model of Input Iterator, and InputIterator4's value_type is convertible to a type in OutputIterator2's set of value_types.

  • OutputIterator1 – is a model of Output Iterator.

  • OutputIterator2 – is a model of Output Iterator.

  • StrictWeakCompare – is a model of Strict Weak Ordering.

Returns

A pair p such that p.first is the end of the output range of keys, and such that p.second is the end of the output range of values.

Pre

The ranges [keys_first1, keys_last1) and [keys_first2, keys_last2) shall be sorted with respect to comp.

Pre

The resulting ranges shall not overlap with any input range.