how to use an array element as a for loop index variable in scala

Question

I am learning scala, and I am trying to do the equivalent of this C++ code in scala, but I get a compile error. Here's the c++ code:

for(reels[0] = 0; reels[0] < 10; reels[0]++)
  doStuff();

I have tried to implement this loop in scala like:

var reels: Array[Int] = new Array[Int](5)

for(reels(0) <- 0 until 10)
  doStuff

This results in a compile error on the for loop:

variable reels is not a case class, nor does it have an unapply/unapplySeq member

If I replace reels(0) with a regular Int variable, it works fine. Why is that, and more importantly, how would I iterate the array element. More specifically, what I want to do is nest 5 loops, and have each loop iterate exactly one of the reels one at a time. I do not want to define 5 separate variables to get around the problem.

Thanks in advance.

This SO post has nice explanation of <- scroll down past accepted answer to longer version with more explanation stackoverflow.com/questions/3754089/scala-for-comprehension — Barry
– Barry, Commented Dec 23, 2014 at 21:25
While James DW already answered your question, I would strongly advise you to start trying to think in a functional way. I am saying so, as you firstly tried using Array[T] and store it in a var, which are both mutable structures. Learning Scala by applying Object-oriented or Imperative paradigms you are used from C++ might feel comfortable. Scalas true power, especially if it comes to lists like in your example, lies in collection combinators or (tail)recursive calls though. — Alexander Weber
– Alexander Weber, Commented Dec 23, 2014 at 23:26
Actually, I don't believe my question has been answered. What I have is 5 reels, but for simplicity, let's say 2 reels, each with 10 stops. I want to say: reels(0) goes from 0 to 9, and then reels(1) goes from 0 to 9. I don't want a variable to go from 0 go 1 (ie go to each reel). Thanks. — Victor
– Victor, Commented Dec 24, 2014 at 7:42

James DW · Accepted Answer · 2015-02-17 22:54:22Z

1

This might be what you want:

// create your list of numbers
val reels = 0 to 9
// do something with each one.
reels.foreach {
  doStuff
}

EDIT: Based on your comment above you may want to try a for comp.

 def doStuff(v: Int) = println(v)
 val reel = Seq(0 to 9, 0 to 9)
 for {
   outer <- reel
   inner <- outer
 }
  yield doStuff(inner)

edited Feb 17, 2015 at 22:54

answered Dec 23, 2014 at 23:19

James DW

1,81316 silver badges21 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Maxim · Accepted Answer · 2014-12-23 21:47:20Z

0

reels.foreach { item =>
  //do something i.e. - println(item)
}

edited Dec 23, 2014 at 21:47

answered Dec 23, 2014 at 21:27

Maxim

7,3681 gold badge34 silver badges45 bronze badges

Comments

Barry · Accepted Answer · 2014-12-24 13:55:51Z

0

Based on your comment above is this what you are looking for? No explicit variable going from reels1 at index 0 to reel2 at index 1?

val reels1: Array[Int] = new Array[Int](10)
val reels2: Array[Int] = new Array[Int](10)

val reels = Seq(reels1,reels2)
 reels.foreach{ reel =>
  for(item <- reel){
   doStuff
   }
  }

answered Dec 24, 2014 at 13:55

Barry

1,8202 gold badges26 silver badges46 bronze badges

4 Comments

Victor Over a year ago

No, not quite. I don't want reels1 to be an array. I want reels to be an array. I want reels(1) to iterate through the values 1 to 10, and reels(2) to iterate through the values 1 to 10. I don't need to save all the values 1 to 10 at the same time on reels. I want the array, "reels", to hold two numbers at a time, and I want to set those two numbers by two nested for loops.

Victor Over a year ago

After typing your code into the scala interpreter, this is close to what I want, but not quite there. But thank you. It is helpful.

Barry Over a year ago

OK great - I can modify further but not sure I am clear exactly what you want. Do you want Array of Arrays? So reel is Array that contains two items where each item itself is an array? Then you want to iterate through every item in the top level array and all of its sub-arrays?

Victor Over a year ago

Okay, I think this will illustrate what I want to do better. Imagine I have a 2 dimensional matrix, and I want to traverse every element on the matrix. Say the matrix is a 10x10 to keep consistent with the example. I want to define an array (A point) that that traverses every element of the matrix. So, at any one time, the point has 2 coordinates. A row, and a column. I want two for loops to iterate the coordinates on the point so that every element in the matrix is visited. Thank you for your help.

Victor · Accepted Answer · 2014-12-27 08:40:34Z

0

I found an answer, but it seems somewhat cumbersome to me. Perhaps because I am not used to doing things this way. What I wanted was something like this:

{Array.tabulate(10, 10...){(reel1, reel2, ... reel5) => /*This is what I wanted to avoid doing. define a variable for each dimension I will be using*/ do_stuff(reel1, reel2,... reel5)}}

answered Dec 27, 2014 at 8:40

Victor

455 bronze badges

Comments

Community · Accepted Answer · 2017-05-23 12:05:42Z

0

May be this approach will help you:

var reels: Array[Int] = new Array[Int](5)

def doStuff(): Unit = {
  print(reels(0) + " ")
}
(0 until 10).foreach(i => { reels(0) = i; doStuff })

// produces
// 0 1 2 3 4 5 6 7 8 9

Also pay attention to @Barry's comment link. Especially to @missingfaktor's answer.

edited May 23, 2017 at 12:05

CommunityBot

11 silver badge

answered Dec 23, 2014 at 21:18

user5102379

1,5129 silver badges9 bronze badges

Collectives™ on Stack Overflow

how to use an array element as a for loop index variable in scala

5 Answers 5

Comments

Comments

4 Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

Comments

Comments

4 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related