Difference between revisions of "cpp/algorithm/exclusive scan"

Revision as of 12:11, 22 March 2016

Computes an exclusive prefix sum operation using binary_op (or std::plus<>() for overloads (1-2)) for the range [first, last), using init as the initial value, and writes the results to the range beginning at d_first. "exclusive" means that the i-th input element is not included in the i-th sum.

Formally, assigns through each iterator i in [d_first, d_first + (last - first)) the value of the generalized noncommutative sum of init, *j... for every j in [first, first + (i - d_first)) over binary_op,

where generalized noncommutative sum GNSUM(op, a1, ..., aN) is defined as follows:

• if N=1, a1
• if N > 1, op(GNSUM(op, a1, ..., aK), GNSUM(op, aM, ..., aN)) for any K where 1 < K+1 = M ≤ N

In other words, the summation operations may be performed in arbitrary order.

The behavior is nondeterministic if binary_op is not associative.

Overloads (2,4) are executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

binary_op shall not invalidate iterators or subranges, nor modify elements in the ranges [first, last) or [d_first, d_first + (last - first)). Otherwise, the behavior is undefined.

Parameters

Return value

Iterator to the element past the last element written.

Complexity

O(last - first) applications of the binary operation

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 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.

Example

See also