mapreduce(f, op, itr)
.. mapreduce(f, op, itr)
Like ``mapreduce(f, op, v0, itr)``. In general, this cannot be used
with empty collections (see ``reduce(op, itr)``).Examples
The mapreduce(f, op, v0, itr) function is used to apply a function f to each element in an iterable itr, and then reduce the result using the binary function op. The initial value v0 is a neutral element for op and will be returned for empty collections.
julia> mapreduce(x -> x^2, +, 1:3)
14In this example, the function f squares each element in the range 1:3, and then the + operator is used to sum up the squared values, resulting in 14.
The mapreduce function is functionally equivalent to calling reduce(op, v0, map(f, itr)), but it generally executes faster since no intermediate collection needs to be created.
Note that the associativity of the reduction is implementation-dependent, and some implementations may reuse the return value of f for elements that appear multiple times in itr. If you need guaranteed left or right associativity and invocation of f for every value, consider using mapfoldl or mapfoldr instead.
julia> mapfoldl(x -> x^2, +, 0, 1:3)
14In this example, mapfoldl is used to ensure left associativity and invocation of f for every value in the range 1:3. The result is the same as with mapreduce.
It's important to note that the behavior of v0 for non-empty collections is unspecified. Therefore, it's recommended to use v0 only for empty collections and not rely on its usage for non-empty collections.
See Also
foldl, foldr, mapfoldl, mapfoldr, mapreduce, mapreducedim,User Contributed Notes
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