Issues using ListBuffer in Scala

Question

package com.listbuffer.ex

import scala.collection.mutable.ListBuffer

object IUEReclass{
   def main(args: Array[String]) {

     val codes = "XY|AB"
     val codeList = codes.split("|")
     var lb = new ListBuffer[String]()

     codeList.foreach(lb += "XYZ")

       val list = lb.toList

   }

I'm getting following exception.

[ERROR] C:\ram\scala_projects\Fidapp\src\main\scala\com\listbuffer\ex\ListBufferEx.
scala:38: error: type mismatch;
[INFO]  found   : scala.collection.mutable.ListBuffer[String]
[INFO]  required: String => ?
[INFO]
                lb += "XYZ"
[INFO]`enter code here`
                          ^
[ERROR] one error found

Nader Ghanbari · Accepted Answer · 2015-04-14 07:58:49Z

4

The type of codeList is Array[String], which is because split method on Strings will return an Array[String].

Now you have a Array[String] on which you are calling the foreach method, so what you should pass to this function is a function from a String to Unit. What you are giving it instead is a ListBuffer[String], because += method on a ListBuffer will return a ListBuffer. This type inconsistency will cause the compile error.

Details on `foreach` method

From the Scala docs of foreach method:

Applies a function f to all elements of this array.

In this case elements of this array are of type String so the provided function to foreach should accept inputs of type String.

Alternatives

If adding all elements of codeList to the ListBuffer is your intention, as mentioned by Paul in comments, you can do it with

codeList.foreach(code => lb += code)

or

codeList.foreach(lb += _)

Alternatively you can use the appendAll method from ListBuffer:

lb.appendAll(codeList)

which

Appends the elements contained in a traversable object to this buffer.

according to the Scala Docs.

edited Apr 14, 2015 at 7:58

answered Apr 13, 2015 at 6:44

Nader Ghanbari

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2 Comments

The Archetypal Paul Over a year ago

This is mostly right. But "What you are giving it instead is a function from ListBuffer[String]" is wrong - no function is being passed, but an expression. If the OP really wants to add XYZ for each element of codelList,. then codeList.foreach { e => lb += "XYZ" } will work (i..e a function of the correct type that ignores its argument). Probably the OP wants { e => lb += e} though (or appendAll, which does the same)

Nader Ghanbari Over a year ago

Thanks for pointing this out, will edit the answer accordingly.

卢声远 Shengyuan Lu · Accepted Answer · 2015-04-13 07:52:16Z

0

Use .insertAll():

lb.insertAll(0, codeList)
val list = lb.toList

answered Apr 13, 2015 at 7:52

卢声远 Shengyuan Lu

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1 Comment

The Archetypal Paul Over a year ago

That doesn't do what the OP's code does, though (which adds "XYZ" codeList.length times). It's probably what the OP actually wants, though :) And appendAll is probably better.

Ramakishore Yelamanchilli · Accepted Answer · 2015-04-14 08:10:36Z

0

Thanks to Paul, I'm able to fix it.

I just changed the code to have

codeList.foreach { e => lb += "XYZ" }

Thanks much everyone who took time to look at the issue!!

Regards

Ram

answered Apr 14, 2015 at 8:10

Ramakishore Yelamanchilli

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