how to accumulate 2D array elements in Scala? - scala

Question

I have 2D array as:
Array(Array(1,1,0), Array(1,0,1))
and I would like to accumulate values over column so my final output look like
Array(Array(1,1,0), Array(2,1,1))
If this is 1D array, I can simply use 'scan' but I'm having trouble with using scan in 2D array.
can anyone help on this issue?

Answer 1

Here's one way to do it:
val t = Array(Array(1,1,0), Array(1,0,1))
val result = t.scanLeft(Array.fill(t(0).length)(0)) ((x,y) =>
x.zip(y).map(e => e._1 + e._2)).drop(1)
//to see the results
result.foreach(e => println(e.toList))
gives:
List(1, 1, 0)
List(2, 1, 1)
The idea is to create an array filled with zeros (using Array.fill) and then scan the 2D array using that as an accumulator. In the end, drop(1) gets rid of the zero-filled array.
EDIT:
In response to the comment, this solution works for a matrix of any size. The zip function takes care of element-wise addition.
EDIT 2 (Step by step explanation):
You already know about scan or a one-dimensional array. The idea is essentially the same.
We initialize the accumulator with zero. In this case, zero means an array of zeros. Array.fill is used to create an array filled with zeros.
Instead of a single addition, we need to add arrays element-wise. This is what the combination of zip and map do. There are a lot of examples available on the Internet about how these methods work.
Finally, we drop the zero element using Scala's drop(1). The result is an array of arrays containing accumulated values.

Answer 2

I would solve it as for given row r, sum all previous rows.
val accumatedMatrix =
for(row <- array.indices)
yield array.take(row + 1).foldLeft(Array(0, 0, 0)) {
case (a, b) => Array(a(0) + b(0), a(1) + b(1), a(2) + b(2))
}
input:
val array = Array(
Array(1,1,0),
Array(1,0,1)
)
output:
1,1,0
2,1,1
Instead of summing all the previous rows repeatedly you can improve it to memoize as well.

Answer 3

Pretty much the same approach as a 1D array:
a.tail.scan(a.head)((acc, value) =>
Array(acc(0) + value(0), acc(1) + value(1), acc(2) + value(2))
)
For arbitrary number (as long as they all have the same number):
a.tail.scan(a.head)((acc, value) =>
acc zip value map {case (a,b) => a+b}
)