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std::transform_reduce

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
(C++17)
transform_reduce
(C++17)
C library
 
Defined in header <numeric>
template<class InputIt, class UnaryOp, class T, class BinaryOp>

T transform_reduce(InputIt first, InputIt last,

                   UnaryOp unary_op, T init, BinaryOp binary_op);
(1) (since C++17)
template<class ExecutionPolicy,

         class InputIt, class UnaryOp, class T, class BinaryOp>
T transform_reduce(ExecutionPolicy&& policy,
                   InputIt first, InputIt last,

                   UnaryOp unary_op, T init, BinaryOp binary_op);
(2) (since C++17)
1) Applies unary_op to each element in the range [first; last) and reduces the results (possibly permuted and aggregated in unspecified manner) along with the initial value init over binary_op.
2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

The behavior is non-deterministic if binary_op is not associative or not commutative.

The behavior is undefined if unary_op or binary_op modifies any element or invalidates any iterator in [first; last).

Contents

[edit] Parameters

first, last - the range of elements to apply the algorithm to
init - the initial value of the generalized sum
policy - the execution policy to use. See execution policy for details.
unary_op - unary FunctionObject that will be applied to each element of the input range. The return type must be acceptable as input to binary_op
binary_op - binary FunctionObject that will be applied in unspecified order to the results of unary_op, the results of other binary_op and init.
Type requirements
-
InputIt must meet the requirements of InputIterator.

[edit] Return value

Generalized sum of init and unary_op(*first), unary_op(*(first+1)), ... unary_op(*(last-1)) over binary_op,

where generalized sum GSUM(op, a
1
, ..., a
N
)
is defined as follows:

  • if N=1, a
    1
  • if N > 1, op(GSUM(op, b
    1
    , ..., b
    K
    ), GSUM(op, b
    M
    , ..., b
    N
    ))
    where
  • b
    1
    , ..., b
    N
    may be any permutation of a1, ..., aN and
  • 1 < K+1 = M ≤ N

in other words, the results of unary_op may be grouped and arranged in arbitrary order.

[edit] Complexity

O(last - first) applications each of unary_op and binary_op.

[edit] Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

  • If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the three standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Notes

unary_op is not applied to init

If the range is empty, init is returned, unmodified

[edit] Example

transform_reduce can be used to parallelize std::inner_product:

#include <vector>
#include <iterator>
#include <functional>
#include <iostream>
#include <numeric>
#include <execution_policy>
#include <boost/iterator/zip_iterator.hpp>
#include <boost/tuple.hpp>
 
int main()
{
    std::vector<double> xvalues(10007, 1.0), yvalues(10007, 1.0);
 
    double result = std::transform_reduce(
        std::par,
        boost::iterators::make_zip_iterator(
            boost::make_tuple(std::begin(xvalues), std::begin(yvalues))),
        boost::iterators::make_zip_iterator(
            boost::make_tuple(std::end(xvalues), std::end(yvalues))),
        [](auto r) { return boost::get<0>(r) * boost::get<1>(r); }
        0.0,
        std::plus<>()
    );
    std::cout << result << '\n';
}

Output:

10007

[edit] See also

sums up a range of elements
(function template) [edit]
applies a function to a range of elements
(function template) [edit]
(C++17)
similar to std::accumulate, except out of order
(function template) [edit]