Idiomatic way to build up an array/list based on prior index in Scala

Question

I am trying to build up a time series using an autoregressive formula in Scala. The equation looks to the prior index of the target array and the current index of two other arrays. Example dummy equation: z_i = z[i-1] - x[i] + y[i]

It is straightforward in an imperative style (see C++ example below) but I am trying to figure out the idiomatic way in Scala. I have created a solution with a while loop but it feels clunky and un-Scalaesque.

#include <vector>
#include <iostream>

int main() {
  std::vector<int> x {1, 2, 3, 4};
  std::vector<int> y {10, 9, 8, 7};
  std::vector<int> z {0, 0, 0, 0};

  for(int i = 1; i < x.size(); i++) {
      z[i] = z[i-1] - x[i] + y[i];
  }

  for(auto i: z) {
      std::cout << i << std::endl;
  }
}

// Desired result
[0, 7, 12, 15]

jwvh · Accepted Answer · 2019-03-26 01:15:28Z

2

Instead of modifying an existing z, you'll want to build it from x and y.

val x = Vector(1, 2, 3, 4)
val y = Vector(10, 9, 8, 7)
val z = (x.tail zip y.tail).scanLeft(0){case (nz,(nx,ny)) => nz - nx + ny}
//z: Vector[Int] = Vector(0, 7, 12, 15)

answered Mar 26, 2019 at 1:15

jwvh

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