Assigning each value as next item in Array in Scala

Question

I'm new to Scala and am having trouble iterating through an array to assign values to a Matrix. My programming background is mostly Python, so I'm having difficulty seeing the functional way to do this task.

I've got an Array, val averages = Array(1,2,3,4,5,6,7,8,9), and I've got an empty 3x3 matrix, M, made with Scala library breeze. I need to take the values from the Array and put them into the matrix.

Normally, I would try something like:

 var index = 0
 for(i <- 0 until M.cols)
        for(j <- 0 until M.rows)
            M(i,j) = averages(index)
            index += 1

This doesn't work, and results in a matrix simply filled with "1"s. I assume this is because Scala is evaluating M(i,j) = averages(index) before it gets to index += 1. I've experimented with different types of for loops for averages, but can't quite get the result I want:

1 2 3
4 5 6
7 8 9

I know that in Scala with the Random collection you can do this, and fill the matrix with random variables:

 val r = new scala.util.Random(0)
 for(i <- 0 until M.rows)
    for(j <- 0 until M.cols)
        M(i, j) = r.nextDouble()

But looking at the Scala documentation, Array doesn't have the .next method.

At any rate, even using .next is very iterative, and not a functional solution. Can anyone help me 1) Figure out the proper way to loop through averages and/or 2) Figure out a functional way to do this? Thanks!

val averages = Array(1,2,3,4,5,6,7,8,9).toIterator Now it has a next. — jwvh
– jwvh, Commented Jan 10, 2017 at 20:47
You just need braces (to make a block) around both statements you want executed in your first for-loop. Scala uses {}, not indentation like Python, for scoping — The Archetypal Paul
– The Archetypal Paul, Commented Jan 10, 2017 at 20:54
You should check library's Scaladoc also, probably there is already a method that you need, like Matrix.create(3, 3, array)... — insan-e
– insan-e, Commented Jan 10, 2017 at 21:05

John · Accepted Answer · 2017-01-10 20:57:34Z

2

Are you sure you're not just having a brackets issue? The code you provided above looks like it ought to be:

 var index = 0
 for(i <- 0 until M.cols)
    for(j <- 0 until M.rows) {
        M(i,j) = averages(index)
        index += 1
    }

If you don't include those braces, only the first line of the second for loop will run. Java/Scala rely on these braces to group things. You can't rely on indenting like Python. If you don't want to use the index parameter, some math works too:

for(i <- 0 until M.cols
    for(j <- 0 until M.rows) {
        M(i, j) = 1 + i + j * M.cols

answered Jan 10, 2017 at 20:57

John

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jwvh · Accepted Answer · 2017-01-10 22:16:25Z

2

Copying data from one existing collection to another is, by definition, relying on side effects to accomplish your goals. A more FP approach is to build a new one directly from the old.

I don't have Breeze installed but, according to the scaladocs, something like this should be possible.

Matrix((0 until colLen).map(x => averages.slice(x*rowLen, (x+1)*rowLen)))

edited Jan 10, 2017 at 22:16

answered Jan 10, 2017 at 21:22

jwvh

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