Map pattern

The map pattern applies an operation to every element in one or multiple data sets, generating a new data set as an output.

The interface to the map pattern is provided by function grppi::map(). As all functions in GrPPI, this function takes as its first argument an execution policy.

grppi::map(exec, other_arguments...);

Map variants

There are several variants:

• Unary map: A map taking a single input sequence.
• N-ary map: A map taking multiple input sequences.

Key elements in a map

The key element in a map pattern is the Transformer operation.

The transformer may be a UnaryTransformer or a MultiTransformer.

A UnaryTransformer is any C++ callable entity that takes a data item and transforms it. The input type and the output type may differ. Thus, a unary transformer op is any operation that, given an input x of type T and output type U, makes valid the following:

U res = op(x);

A MultiTransformer is any C++ callable entity that takes data items, one of each input sequence, and transforms them into an output value. The input types and the output type may differ. Thus, a multi-transformer op is any operation that, given inputs x1, x2, ... , xN of types T1, T2, ... , TN and an output type U, makes valid the following:

U res = op(x1,x2,...,xN)

Details on map variants

Unary map

An unary map takes a data set and transforms each element in the data set by applying an unary function and generating a new data set.

The only interface currently offered for this pattern is based in iterators (following the C++ standard library conventions):

• The input data set is specified by two iterators.
• The output data set is specified by an iterator to the start of the output sequence.

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Example: Doubling values in a vector.

vector<double> v = get_the_vector();
vector<double> w(v.size());
map(exec, begin(v), end(v), begin(w),
    [](double x) { return 2 *x; }
);

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N-ary map

A n-ary map takes multiple data sets and transforms a tuple of elements from those data sets by applying a n-ary function and generating a new data set.

The only interface currently offered for this pattern is based in iterators:

• The first data set is specified by two iterators.
• The output data set is specified by an iterator to the start of the output sequence.
• All the other input data sets are specified by iterators to the start of the input data sequences.

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Example: Computing the addition of three vectors.

vector<double> v1 = get_first_vector();
vector<double> v2 = get_second_vector();
vector<double> v3 = get_third_vector();
vector<double> w(v1.size());
map(exec, begin(v1), end(v1), begin(w),
  [](double x, double y, double z) { return x+y+z; },
  begin(v2), begin(v3)
);

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