user180940 user180940 - 1 year ago 74
Scala Question

How can I functionally iterate over a collection combining elements?

I have a sequence of values of type A that I want to transform to a sequence of type B.

Some of the elements with type A can be converted to a B, however some other elements need to be combined with the immediately previous element to produce a B.

I see it as a small state machine with two states, the first one handling the transformation from A to B when just the current A is needed, or saving A if the next row is needed and going to the second state; the second state combining the saved A with the new A to produce a B and then go back to state 1.

I'm trying to use scalaz's

s but I fear I'm overcomplicating it, and I'm forced to return a dummy B when the input has reached EOF.

What's the most elegant solution to do it?

Answer Source

What about invoking the sliding() method on your sequence?

You might have to put a dummy element at the head of the sequence so that the first element (the real head) is evaluated/converted correctly.

If you map() over the result from sliding(2) then map will "see" every element with its predecessor.

val input:  Seq[A] = ??? // real data here (no dummy values)
val output: Seq[B] = (dummy +: input).sliding(2).flatMap(a2b).toSeq
def a2b( arg: Seq[A] ): Seq[B] = {
    // arg holds 2 elements
    // return a Seq() of zero or more elements
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