Swapping array values with for and yield scala

Question

I am trying to swap every pair of values in my array using for and yield and so far I am very unsuccessful. What I have tried is as follows:

val a = Array(1,2,3,4,5) //What I want is Array(2,1,4,3,5)

for(i<-0 until (a.length-1,2),r<- Array(i+1,i)) yield r

The above given snippet returns the vector 2,1,4,3(and the 5 is omitted)

Can somebody point out what I am doing wrong here and how to get the correct reversal using for and yields?

Thanks

This is from "Scala for the impatient". Exercise 3.2

zbstof
– zbstof

2013-04-05 10:12:15 +00:00
Commented Apr 5, 2013 at 10:12 — zbstof
– zbstof, Commented Apr 5, 2013 at 10:12
Next time, please mention that it's a homework exercise.

Abhijit Sarkar
– Abhijit Sarkar

2018-07-20 09:05:21 +00:00
Commented Jul 20, 2018 at 9:05 — Abhijit Sarkar
– Abhijit Sarkar, Commented Jul 20, 2018 at 9:05

Rogach · Accepted Answer · 2012-04-15 06:39:40Z

41

a.grouped(2).flatMap(_.reverse).toArray

or if you need for/yield (much less concise in this case, and in fact expands to the same code):

(for {b <- a.grouped(2); c <- b.reverse} yield c).toArray

answered Apr 15, 2012 at 6:39

Rogach

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2 Comments

dhg Over a year ago

I like the use of reverse. Nice.

Rob Over a year ago

The exercise says do a loop, then do a for/yield, not a functional solution. Nice though...

dhg · Accepted Answer · 2012-04-15 15:22:07Z

17

It would be easier if you didin't use for/yield:

a.grouped(2)
  .flatMap{ 
    case Array(x,y) => Array(y,x)
    case Array(x) => Array(x)
  }.toArray // Array(2, 1, 4, 3, 5)

edited Apr 15, 2012 at 15:22

answered Apr 15, 2012 at 0:19

dhg

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4 Comments

sc_ray Over a year ago

This is probably a mature way of doing things. I am learning scala and was wondering if the same could be accomplished using for/yield.

dhg Over a year ago

@sc_ray, The for/yield construction makes things nicer a lot of the time, but this isn't a great match for it.

Timur Milovanov Over a year ago

If original task comes from Horstman's book, it ask us to swap elements in array, NOT CREATE NEW ONE. But theese solutions are more elegant than yield(ing)

Timur Milovanov Over a year ago

Sory, there are both task: with swap elements with existing array and while creating new one (with map-function)

Joseph Lust · Accepted Answer · 2014-01-28 21:58:01Z

9

I don't know if the OP is reading Scala for the Impatient, but this was exercise 3.3 .

I like the map solution, but we're not on that chapter yet, so this is my ugly implementation using the required for/yield. You can probably move some yield logic into a guard/definition.

for( i <- 0 until(a.length,2); j <- (i+1).to(i,-1) if(j<a.length) ) yield a(j)

I'm a Java guy, so I've no confirmation of this assertion, but I'm curious what the overhead of the maps/grouping and iterators are. I suspect it all compiles down to the same Java byte code.

answered Jan 28, 2014 at 21:58

Joseph Lust

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2 Comments

Sarah Phillips Over a year ago

Nice. This seems to be more in the spirit of the exercise.

Kuldeep Singh Over a year ago

Good answer for the question asked.

Haris Osmanagić · Accepted Answer · 2014-03-17 09:17:46Z

1

Another simple, for-yield solution:

def swapAdjacent(array: ArrayBuffer[Int]) = {
    for (i <- 0 until array.length) yield (
        if (i % 2 == 0)
            if (i == array.length - 1) array(i) else array(i + 1)
        else array(i - 1)
    )
}

answered Mar 17, 2014 at 9:17

Haris Osmanagić

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Comments

Basile du Plessis · Accepted Answer · 2014-05-20 16:06:34Z

0

Here is my solution

  def swapAdjacent(a: Array[Int]):Array[Int] =
    (for(i <- 0 until a.length) yield
      if (i%2==0 && (i+1)==a.length) a(i) //last element for odd length
      else if (i%2==0) a(i+1)
      else a(i-1)
    ).toArray

https://github.com/BasileDuPlessis/scala-for-the-impatient/blob/master/src/main/scala/com/basile/scala/ch03/Ex03.scala

answered May 20, 2014 at 16:06

Basile du Plessis

1

Comments

ChinnoDog · Accepted Answer · 2015-10-07 17:19:33Z

0

If you are doing exercises 3.2 and 3.3 in Scala for the Impatient here are both my answers. They are the same with the logic moved around.

/** Excercise 3.2 */
for (i <- 0 until a.length if i % 2 == 1) {val t = a(i); a(i) = a(i-1); a(i-1) = t }
/** Excercise 3.3 */
for (i <- 0 until a.length) yield { if (i % 2 == 1) a(i-1) else if (i+1 <= a.length-1) a(i+1) else a(i) }

edited Oct 7, 2015 at 17:19

answered Oct 7, 2015 at 17:06

ChinnoDog

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Comments

Carlos Caldas · Accepted Answer · 2016-09-26 19:48:08Z

0

for (i <- 0 until arr.length-1 by 2) { val tmp = arr(i); arr(i) = arr(i+1); arr(i+1) = tmp }

I have started to learn Scala recently and all solutions from the book Scala for the Impatient (1st edition) are available at my github:

Chapter 2 https://gist.github.com/carloscaldas/51c01ccad9d86da8d96f1f40f7fecba7

Chapter 3 https://gist.github.com/carloscaldas/3361321306faf82e76c967559b5cea33

answered Sep 26, 2016 at 19:48

Carlos Caldas

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Comments

Artem Vlasenko · Accepted Answer · 2017-01-11 20:32:46Z

0

I have my solution, but without yield. Maybe someone will found it usefull.

def swap(x: Array[Int]): Array[Int] = {
    for (i <- 0 until x.length-1 by 2){
      var left = x(i)
      x(i) = x(i+1)
      x(i+1) = left
    }
    x
  }

answered Jan 11, 2017 at 20:32

Artem Vlasenko

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Comments

Seth B · Accepted Answer · 2021-02-14 01:51:30Z

0

Assuming array is not empty, here you go:

val swapResult = for (ind <- arr1.indices) yield {
  if (ind % 2 != 0) arr1(ind - 1)
  else if (arr1(ind) == arr1.last) arr1(ind)
  else if (ind % 2 == 0) arr1(ind + 1)
}

edited Feb 14, 2021 at 1:51

Seth B

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answered Feb 14, 2021 at 0:46

Samip

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3 Comments

Samip Over a year ago

@jwvh I found few of the solution were throwing exceptions in case of odd numbers of variables hence the modification was done. I can see fi there is an empty array defined than the method .indices might will produce compile time error.

jwvh Over a year ago

.indices on an empty Array does not produce a compile time error. It doesn't produce a run time error. It works correctly without error.

Samip Over a year ago

@jwvh Thank you just figured that in case if we don't define the type of array during declaration i.e. val arr1: Array[] = Array() then .indices works but when we depend on type inference i.e. val arr1 = Array() then the error exists

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