Can someone explain why applying map like this in Scala throws and IllegalArgumentException?

Question

Here's a copy of the interpreter commands:

scala>val myTable = Array(Array(1))
res30: Array[Array[Int]] = Array(Array(1))

scala> myTable.map(_.map(_.toString))
res31: Array[Array[java.lang.String]] = Array(Array(1))

scala> var result = 0
result: Int = 0

scala> myTable.head
res32: Array[Int] = Array(1)

//note how applying this works
scala> res32.map(elem => if(3> result) result = 3)
res34: Array[Unit] = Array(())

scala> result
res35: Int = 3

//this also works
scala> myTable.map(_.map(_.toString))
res31: Array[Array[java.lang.String]] = Array(Array(1))

//when you combine the double map application and the anonymous function from earlier:
scala> myTable.map(_.map(elem => if(3 > result) result = 3))
java.lang.IllegalArgumentException
    at java.lang.reflect.Array.newArray(Native Method)
    at java.lang.reflect.Array.newInstance(Array.java:52)
    at scala.reflect.ClassManifest$class.arrayClass(ClassManifest.scala:107)
    at scala.reflect.Manifest$$anon$9.arrayClass(Manifest.scala:152)
    at scala.reflect.Manifest$class.arrayManifest(Manifest.scala:46)
    at scala.reflect.Manifest$$anon$9.arrayManifest(Manifest.scala:152)
    at scala.reflect.Manifest$$anon$9.arrayManifest(Manifest.scala:152)
    at scala.reflect.ClassManifest$.arrayType(ClassManifest.scala:205)
    at .<init>(<console>:45)
    at .<clinit>(<console>)
at .<init>(<console>:11)
at .<clinit>(<console>)
at $export(<console>)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:592)
at scala.tools.nsc.interpreter.IMain$Request$$anonfun$10.apply(IMain.scala:828)
at scala.tools.nsc.interpreter.Line$$anonfun$1.apply$mcV$sp(Line.scala:43)
at scala.tools.nsc.io.package$$anon$2.run(package.scala:31)
at java.lang.Thread.run(Thread.java:662)

Is this expected behaviour?

score 5 · Answer 1 · answered Aug 08 '11 at 10:57

5

First things first, you should be using foreach, not map, but you already knew that :-)

You are trying to create an array of array of Unit, which gets translated into Void.TYPE in java. This results in the IllegalArgumentException.

answered Aug 08 '11 at 10:57

Matthew Farwell

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paradigmatic · Accepted Answer · 2011-08-08T11:05:23.360

Scala use java.lang.reflect.Array.newInstance to create arrays dynamically when map is called. ~~This method, does not accept a size of zero. since, your lambda is returning an empty array of type Array[Unit], which as a size of zero, you get the exception, when calling the outer map.~~

By the way, in this example, you really should use foreach since you are not trying to map the array to another array, but rather to use side-effects:

scala> myTable.foreach(_.foreach(elem => if(3 > result) result = 3))

scala> result
res21: Int = 3

Edit: I misunderstood the problem (See MatthieuF answer). The problem is not the size of the array but the type. It is possible to reproduce the problem with just:

scala> java.lang.reflect.Array.newInstance(classOf[Unit], 1)
java.lang.IllegalArgumentException
at java.lang.reflect.Array.newArray(Native Method)
at java.lang.reflect.Array.newInstance(Array.java:52)
at .<init>(<console>:8)
...

Yes it does (accept a size of zero). The method being used is: public static Object newInstance(Class> componentType, int length) throws NegativeArraySizeException. Not the other one — Matthew Farwell, Aug 08 '11 at 10:53
Try the following: java.lang.reflect.Array.newInstance(int.class, 0) — Matthew Farwell, Aug 08 '11 at 11:00

Can someone explain why applying map like this in Scala throws and IllegalArgumentException?

2 Answers2