tuto.map_reduce.rst

Tutorial: Higher-order functions (Map, Reduce, Fold)

Arraymancer supports efficient higher-order functions on the whole tensor or on an axis.

map, apply, map2, apply2

a.map(x => x+1)

or

proc plusone[T](x: T): T =
  x + 1
a.map(plusone) # Map the function plusone

Note: for basic operation, you can use implicit broadcasting instead a +. 1

apply is the same as map but in-place.

map2 and apply2 takes 2 input tensors and respectively, return a new one or modify the first in-place.

proc `**`[T](x, y: T): T = # We create a new power `**` function that works on 2 scalars
  pow(x, y)
a.map2(`**`, b)
# Or
map2(a, `**`, b)

reduce on the whole Tensor or along an axis

reduce apply a function like + or max on the whole Tensor[T] returning a single value T.

For example: - Reducing with + returns the sum of all elements of the Tensor. - Reducing with max returns the biggest element of the Tensor

reduce can be applied along an axis, for example the sum along the rows of a Tensor.

fold on the whole Tensor or along an axis

fold is a generalization of reduce. Its starting value is not the first element of the Tensor.

It can do anything that reduce can, but also has other tricks because it is not constrained by the Tensor type or starting value.

For example: - Reducing with was_a_odd_and_what_about_b and a starting value of true returns true if all elements are odd or false otherwise

Just in case

proc was_a_odd_and_what_about_b[T: SomeInteger](a: bool, b: T): bool =
  return a and (b mod 2 == 1) # a is the result of previous computations, b is the new integer to check.