[The following description only applies to Kawa in CVS or 1.8.1 or newer. If you're using an older Kawa, skip to the last sub-section.]
To allocate a Java array you can use the make
function.
For example, you allocate an array with room for 10 elements
each of each is a primitive int
you can do this:
(make <int[]> length: 10)
If you use the type “expression” <int[]>
as a function,
you can leave out the make
name, so this is equivalent:
(<int[]> length: 10)
You can specify the initial elements instead of the length:
(<object[]> 31 32 33 34)
This creates a 4-length array, initialized to the given values.
If you specify a length, you can also specify initial values for selected elements. If you specify an index, in the form of a literal integer-valued keyword, then following elements are placed starting at that position.
(<int[]> length: 100 10 12 80: 15 16 50: 13 14)
This creates an array with 100 elements. Most of them are initialized to the default value of zero, but elements with indexes 0, 1, 50, 51, 80, 81 are initialized to the values 10, 12, 13, 14, 15, 16, respectively.
You can access the elements of a Java array by treating it as a one-argument function, where the argument is the index:
(define primes (<integer[]> 2 3 5 7 11 13)) (primes 0) ==> 2 (primes 5) ==> 13
You can set an element by treating the array as a function
with a setter
:
(set! (primes 0) -2) (set! (primes 3) -7) primes ==> [-2 3 5 -7 11 13]
To get the number of elements of an array, you can treat
it as having a length
field:
primes:length ==> 6
Here is a longer example. This is the actual definition of the
standard gcd
function. Note the args
variable
receives all the arguments on the form of an Object
array.
(define (gcd #!rest (args :: <Object[]>)) :: <integer> (let ((n :: <int> args:length)) (if (zero? n) 0 (let ((result :: <integer> (args 0))) (do ((i :: <int> 1 (+ i 1))) ((>= i n) result) (set! result (<integer>:gcd result (<integer>: (args i)))))))))
The above example generates good code, thanks to judicious use of casts and type specifications. In general, if Kawa knows that a “function” is an array then it will generate efficient bytecode instructions for array operations.
[The following macros are deprecated in CVS, but you still have to use them in Kawa 1.8 or older.]
The following macros evaluate to procedures that can be used to manipulate primitive Java array objects. The compiler can inline each to a single bytecode instruction (not counting type conversion).
Syntax: primitive-array-new
element-type
Evaluates to a one-argument procedure. Applying the resulting procedure to an integer count allocates a new Java array of the specified length, and whose elements have type
element-type
.
Syntax: primitive-array-set
element-type
Evaluates to a three-argument procedure. The first argument of the resulting procedure must be an array whose elements have type
element-type
; the second argument is an index; and the third argument is a value (coercible toelement-type
) which replaces the value specified by the index in the given array.