The JScript Type System Part Eight: The Last Blog Entry About Arrays, I Promise



 

Recall
that I defined a type as consisting of two things: a set of
values, and a rule for associating values outside of that set with
values inside the set.  In JScript .NET,
assigning a value outside of a type to a variable annotated with that type restriction
does that coercion if possible

 

var s
: String = 123; // Converts 123 to a String

 

Similarly,
I already discussed what happens when you assign a JScript array to a hard-typed CLR
array variable

 

var sysarr
: int[] = [10, 20, 30]; // Create new int[3] and copy

 

and
what happens when you assign a one-dimensional CLR array to a JScript array variable:

 

var jsarr
: Array = sysarr; // Wrap sysarr

 

But
what happens when you assign a hard-typed
CLR array
to a variable annotated with a different CLR
array type?

 

var intarr
: int[] = [10, 20, 30];

var strarr
: String[] = intarr;

 

You
might think that this does the string coercion on every element, but in fact this
is simply not legal. Rather than creating a copy with every element coerced to the
proper type, the compiler simply gives up and says that these are not type compatible.
If you find yourself in this situation then you will simply have to write the code
to do the copy for you.  Something like
this would work:

 

function
copyarr(source : System.Array) : String[]

{

  var
dest : String[] = new String[source.Length];

  for(var
index : int in source)

    dest[index]
= source.GetValue(index);

  return
dest;

}

 

There
are a few notable things about this example. First, notice that this copies a rank-one
array of any element type to an array of strings. This is one of the times when it
comes in handy to have the
System.Array “any
hard-typed array” type!

 

Second,
notice that you can use the
for-in loop
with hard-typed CLR arrays. The for-in loop enumerates
all the indices of an array rather than the contents of the array.
Since CLR arrays
are always indexed by integers the index can be annotated as an int. The loop above
is effectively the same as

 

for (var
index : int = 0 ; index < source.Length ; ++index)

 

but
the
for-in syntax
is less verbose and possibly more clear.

 

Third,
you might recall that
GetValue (and SetValue)
take an array of indices because the array
might be multidimensional. But we’re not passing in an array here.  Fortunately,
you can also pass only the index if it is a single-dimensional array.

 

Generally
speaking, hard-typed array types are incompatible with each other. There
is an exception to this rule, which I’ll discuss later when I talk about what exactly
“subclassing” means in JScript .NET.


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