Using type to define a multidimensional array in scala

Question

I'm trying to define a type for a matrix (two dimensional array). I have this:

scala> type DMatrix[T] = Array[Array[T]]
defined type alias DMatrix

and then I define de DMatrix:

scala> def DMatrix = Array.ofDim[Double](2,2)
DMatrix: Array[Array[Double]]

So far so good. The problem now is how to work with th DMatrix. I've tried some examples but nothing happens:

scala> DMatrix(0)(0) = 1.0

scala> DMatrix
res40: Array[Array[Double]] = Array(Array(0.0, 0.0), Array(0.0, 0.0))

scala> DMatrix(0)
res41: Array[Double] = Array(0.0, 0.0)

scala> DMatrix(0) = Array(1.0,2.1)

scala> DMatrix(0)
res43: Array[Double] = Array(0.0, 0.0)

so, the question is how to use this DMatrix type?

thanks in advance

I want to fill the DMatrix with values, e.g. in position (0)(0) = 2.3, in position (1)(0) = 3.6 and after that I want to use those values invoking the DMatrix — MLeiria
– MLeiria, Commented Dec 29, 2016 at 13:09

Tzach Zohar · Accepted Answer · 2016-12-29 13:26:17Z

There's just a tiny but crucial mistake here - in:

scala> def DMatrix = Array.ofDim[Double](2,2)

You've used def instead of val to declare DMatrix: that means that the expression is evaluated anew everytime you access it, so when you modify the values in the arrays, the result is "thrown away" in favor of a new DMatrix instance.

Changing it to val would fix the issue and you'll see all changes:

scala> val DMatrix = Array.ofDim[Double](2,2)
DMatrix: Array[Array[Double]] = Array(Array(0.0, 0.0), Array(0.0, 0.0))

scala> DMatrix(0)(0) = 1.0

scala> DMatrix
res1: Array[Array[Double]] = Array(Array(1.0, 0.0), Array(0.0, 0.0))

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