The JScript Type System, Part Five: More On Arrays In JScript .NET

"urn:schemas-microsoft-com:office:office" />As
I was saying the other day, CLR arrays and JScript arrays are totally different beasts.
It is hard to imagine two things being so different and yet both called the same thing.
Why did the CLR designers and the JScript designers start with the same desire --
create an array system -- and come up with completely different implementations?

the CLR implementers knew that dense, nonassociative hard-typed arrays are easy to
make fast and efficient. Furthermore,
such arrays encourage the programmer to keep
homogenous data in strictly bounded tables
. That makes large
programs that do lots of data manipulation easier to understand
. Thus, languages
such as C++, C# and Visual Basic have arrays like this, and thus they are the basic
built-in array type in the CLR.

associative, soft-typed arrays are not particularly fast but they are far
more dynamic and flexible
than Visual Basic-style arrays. They make it easy to
store heterogeneous data in any table without
worrying about picky details
like exactly how big that table is. In other words,
they are scripty. Languages such as JScript
and Perl have arrays like this.

.NET has both very dynamic, scripty arrays
and more strict CLR arrays, making it suitable
for both rapid development of scripts and programming in the large
. But like I
said, making these two very different kinds of arrays work well together is not trivial.

.NET supports the creation of multidimensional hard-typed arrays. As with single-dimensional
arrays, the array size is not part of the
. To annotate a variable as containing a hard-typed multidimensional array
the syntax is to follow the type with brackets containing commas. For example, to
annotate a variable as containing a two dimensional array of Strings you would say:

var multiarr
: String[,];

number of commas between the brackets plus one is equal to the rank of the array.
(By this definition if there are no commas between the brackets then it is a rank-one
array, as we have already seen.)

multidimensional array is allocated with the new keyword
as you might expect:

= new String[4,5];

= "hello";

that hard-typed array elements are always
accessed with a comma-separated list of integer indices
. There must always be exactly
one index for each dimension in the array
. You can't use the ragged array syntax [0][0].

are certain situations in which you know that a variable or function argument will
refer to a hard-typed CLR array but you do not actually know the element type or the
rank, just that it is an array. Should you find yourself in one of these (rather rare)
situations there is a special annotation for a CLR array of unknown type and rank:

var sysarr
: System.Array;

= new String[4,5];

= new double[10];

you can see, a variable of type System.Array may
hold any CLR array of any type and rank. However, there is a drawback. Variables of
type System.Array may
not be indexed directly because the rank is not known
. This is illegal:

var sysarr
: System.Array;

= new String[4,5];

= "hello"; // ILLEGAL, System.Arrays
are not indexable

to index a System.Array you
must call the GetValue and SetValue methods
an array of indices:

var sysarr
: System.Array;

= new String[4,5];


rank and size of a System.Array can
be determined with the Rank, GetLowerBound and GetUpperBound members.

about this a bit now, I suppose that we could have
detected at compile time that a System.Array was
being indexed, and constructed the call to the getter/setter appropriately for you,
behind the scenes. But apparently we
didn't. Oh well.

time: mixing and matching JScript and CLR arrays.