CODEWITHSUNDEEP
scalafold
ntviet18
ntviet18

Reputation: 792

Fold with previous element

Given val as: Seq[Int] = ...

A lot of times I need to apply an operation to two consecutive elements, e.g.

By the way I don't like

for (i <- 1 until as.size) {
  // do something with as(i) and as(i - 1)
}

Or by another

as.tail.foldLeft((0, as.head)) { (acc, e) =>
  // do something with acc._2 and e 
  // and try to not forget returning (_, e) 
}

How do I writer better code for this scenario?

Upvotes: 3

Views: 1208

Answers (3)

Jordan Cutler
Jordan Cutler

Reputation: 582

Can make your own fold which supports previous element. Safe with 1 or zero element collections.

  def foldLeftWithPrevious[A, B](as: Seq[A], accumulator: B)(f: (B, A, A) => B): B = {
    @scala.annotation.tailrec
    def foldLeftInner(list2: Seq[A], previous: A, accumulator: B, f: (B, A, A) => B): B = {
      if (list2.isEmpty) accumulator
      else foldLeftInner(list2.tail, list2.head, f(accumulator, previous, list2.head), f)
    }

    if (as.length <= 1) accumulator
    else foldLeftInner(as.tail, as.head, accumulator, f)
  }

Feel free to test it with this snippet.

val foldLeftTest = Seq(1)
  foldLeftWithPrevious(foldLeftTest, 0)((accum, previous, current) => {
    println("accum = " + accum)
    println("previous = " + previous)
    println("current = " + current)
    println("accum will be... " + accum + " + " + previous + " + " + current)
    println("which is... " + (accum + previous + current))
    accum + previous + current
  })

Upvotes: 1

jwvh
jwvh

Reputation: 51271

Here's one way to supply the head of your sequence to every subsequent element.

val sq:Seq[Int] = Seq(. . .)

sq.headOption.fold(sq){hd =>
  sq.tail.map(/*map() or fold() with the hd value*/)
}

Note that this is safe for collections of 1 or zero elements.

Upvotes: 1

Andrey Tyukin
Andrey Tyukin

Reputation: 44908

You could zip the sequence as with its own tail:

for ((prev, curr) <- as zip as.tail) {
  // do something with `prev` and `curr`
}

Or you could use sliding:

for (window <- as.sliding(2)) {
  val prev = window(0)
  val curr = window(1)
  // do something with `prev` and `curr`
}

Upvotes: 5

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