Array: Difference between revisions
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This method is best for arrays that are not pre-defined. Placed before the 0th element of the array is its maximum size. This value's location relative to the 0th element is always the same, regardless of how long the array actually is. As such, the pointer to the 0th element can be offset by a fixed negative amount to get the size. Alternatively, rather than storing the size of the array, a pointer to the last element can also be stored in front of the array. This lets you use pointer arithmetic to calculate the array's size, by subtracting the pointer of the 0th element from the last.
===[[ATS]]===
The array types built into ATS are an extremely complicated topic, and I shall mention only three points of interest.
# ATS wants to prevent you from going outside the bounds of the array; and it wants to do so without runtime checks. There are numerous ways to get around that "desire", but the capability for strictness is there.
# ATS wants to distinguish between the initialized and uninitialized parts of an array. Again, you can get around this "desire", but the capability to be rigorous is there.
# ATS arrays are basically C arrays. An array is a contiguous block of memory at a particular machine address, with no other runtime structure.
An array is represented by a pointer to the beginning of the memory block, as in C; but, furthermore, there is a ''view''. The view exists only as a typechecking entity; there is no runtime code associated with it. Suppose you have a fully initialized array, called '''a''', of fourteen '''int''' starting at address '''p'''. Then the view for the array may be written <lang ATS>@[int][14] @ p</lang> or equivalently as <lang ATS>array_v (int, p, 14)</lang>
To access an element of the array, that view needs to be available to the typechecker. Let us suppose the view is in fact available, and that there are operator overrides to use the square brackets as (in this case one-dimensional) array indices. Then a program that says <lang ATS>let val x = a[3] in a[4] := x end</lang> should be compilable, but one that says <lang ATS>let val x = a[20] in a[30] := x end</lang> will trigger a type error, due to the out-of-bounds indices.
Above we assumed '''a''' was fully initialized. Working with an array that is not fully initialized can become quite complex, and I shall mention only that the view for '''a''' before it is initialized would be <lang ATS>@[int?][14]</lang> with a question mark. Fortunately there are routines in the prelude to fully initialize an array, for instance with a fill value such as zero or an empty string. There are also ways to fully deinitialize an array; to allocate an array in the heap, and to free one; to place an array in a stack frame; to make an array as a global variable.
The curious reader is encouraged to go to [http://www.ats-lang.org/ the ATS website] for more information.
===[[Fortran]]===
Arrays have been available from the start, for each of the allowed standard types of variables: integer, real, complex, logical, and character, and their different precisions. They can be indexed only with integer values, and definitely not with text strings as in say Snobol - though one could place a short text into a small CHARACTER variable that is equivalenced to an INTEGER variable and use that integer as in a "hash table" scheme, possibly to good effect. Indexing starts with one, and the size is fixed by the declaration at compile time, thus <lang Fortran> REAL A(66) !Declares an array, elements one to sixty-six.
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