Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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Function scan

boost::mpi::scan — Compute a prefix reduction of values from all processes in the communicator.

Synopsis

template<typename T, typename Op> 
  void scan(const communicator & comm, const T & in_value, T & out_value, 
            Op op);
template<typename T, typename Op> 
  T scan(const communicator & comm, const T & in_value, Op op);
template<typename T, typename Op> 
  void scan(const communicator & comm, const T * in_values, int n, 
            T * out_values, Op op);

Description

scan is a collective algorithm that combines the values stored by each process with the values of all processes with a smaller rank. The values can be arbitrarily combined, specified via a function object op. The type T of the values may be any type that is serializable or has an associated MPI data type. One can think of this operation as a gather to some process, followed by an std::prefix_sum() over the gathered values using the operation op. The ith process returns the ith value emitted by std::prefix_sum().

When the type T has an associated MPI data type, this routine invokes MPI_Scan to perform the reduction. If possible, built-in MPI operations will be used; otherwise, scan() will create a custom MPI_Op for the call to MPI_Scan.

Parameters:

comm

The communicator over which the prefix reduction will occur.

in_value

The local value to be combined with the local values of other processes. For the array variant, the in_values parameter points to the n local values that will be combined.

op

The binary operation that combines two values of type T into a third value of type T. For types T that has ssociated MPI data types, op will either be translated into an MPI_Op (via MPI_Op_create) or, if possible, mapped directly to a built-in MPI operation. See is_mpi_op in the operations.hpp header for more details on this mapping. For any non-built-in operation, commutativity will be determined by the is_commmutative trait (also in operations.hpp).

out_value

If provided, the ith process will receive the value op(in_value[0], op(in_value[1], op(..., in_value[i]) ... )). For the array variant, out_values contains a pointer to storage for the n output values. The prefix reduction occurs independently for each of the n values referenced by in_values, e.g., calling scan on an array of n values is like calling scan n separate times, one for each location in in_values and out_values.

Returns:

If no out_value parameter is provided, returns the result of prefix reduction.


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