Hi Michael,
Am 16.10.2014 um 22:13 schrieb Michael Dietrich:
> Hello,
>
> I'm getting started with X10 and am a bit stuck with the handling of
> Rails (I think I might get the same problem with Arrays and so on).
>
> Let's say I create the following:
>
> val bla = new Rail[Long](10);
> bla(0) = 2343;
> bla(1) = 4534;
> bla(2) = 3424;
> for(i in 0..9)
> bla(i)=42;
> Console.OUT.print("Content: ");
> Console.OUT.println(bla);
>
> I get 42 ten times as an output.
> My first question: At least the first three values of my Rail are set.
> As far as I understood "val" makes them become immutable. So why can I
> turn them all into 42?
The semantics are not like in C(++) (where the Array AND its entry are
immutable), but only the reference to the array is immutable, like in
Java. Meaning: you can modify single entries, but you cannot write
bla = new Rail[Long] (...);
after the first initialization.
>
> My second question: If I change the for-loop into
>
> for(b in bla)
> b=42;
>
> I get the expected error message by the compiler though it's actually
> no formal change. Or is there any?
>
> bye
> Michael
You should wirte the loop as
for (b:Long in bla) { b = 42;}
which results in this compiler-error:
Final variable might already have been initialized.
Final variable: b
Writing
for (var b:Long in bla) { b = 42; }
Leads to:
Enhanced for loop may not have var loop index. var b: Long{amb}
Final variable might already have been initialized.
Final variable: b
In other words: if you use primitive data types (int, long, double,
...), you will not be able to change them with a foreach-loop. Even if
you were able to change the value of b, you would change ONLY the value
of b, since primitives are copied and not referenced.
If, on the other hand, your Rail holds objects, the loop-variable (b)
will hold references. While b is still a val, you would be able to
change attributes of b (just like you were able to change entries in the
Rail bla despite it being a val) and since b is only a reference to the
object in the Rail, you would get the desired result.
I hope i was able to help. Feel free to ask further questions.
Cheers,
Marco
>
>
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