Array-like object indexing

Array-like object indexing

[Ryan]

I've recently been playing with making all the standard data structures
in Ada to prepare myself for writing my first major project.  Currently 
I'm writing a resizable-array package.  I am trying to make it as 
similar to built-in arrays as possible, with functions for First, Length 
and other useful operations and attributes.  It is a generic package 
instantiated with a type for the elements to be stored and a type for 
indexing.  I've run into a small problem with respect to the indexing type.

I'm not sure how to represent the boundaries of an empty array without 
raising a constraint error in some cases.  If an array is being indexed 
by Natural, for example, and the first index should be 0 (as makes sense 
in many situations), then what should the First and Last functions 
(intended to work like their analogous attributes) return?  I cannot 
return 0 and -1 because -1 would raise a constraint error.

Should the responsibility for this rest on the package or the user? 
Would it be a better to return 1 and 0, or to force a user of the 
package to instantiate it with an indexing type that starts at -1 
instead even though -1 should never be used to actually look into the 
array?  What are your opinions on this?

Thank you,
Ryan Tarpine

Re: Array-like object indexing

[R. Tim Coslet]

Have First and Last return Index_Type'Base. At least for numeric types this
will eliminate most cases of the Constraint_Error that you don't want.
(Natural'Base does include -1 in its 'Range even though Natural doesn't)

Unfortunately you can't declare your own functions to return "universal"
types, like the attributes return.

-- 
        R. Tim Coslet
        r_tim_coslet@pacbell.net

Technology, n. Domesticated natural phenomena.


> From: Ryan <rtarpine@hotmail.com>
> Newsgroups: comp.lang.ada
> Date: Sat, 17 Aug 2002 17:22:33 -0700
> Subject: Array-like object indexing
> 
> I've recently been playing with making all the standard data structures
> in Ada to prepare myself for writing my first major project.  Currently
> I'm writing a resizable-array package.  I am trying to make it as
> similar to built-in arrays as possible, with functions for First, Length
> and other useful operations and attributes.  It is a generic package
> instantiated with a type for the elements to be stored and a type for
> indexing.  I've run into a small problem with respect to the indexing type.
> 
> I'm not sure how to represent the boundaries of an empty array without
> raising a constraint error in some cases.  If an array is being indexed
> by Natural, for example, and the first index should be 0 (as makes sense
> in many situations), then what should the First and Last functions
> (intended to work like their analogous attributes) return?  I cannot
> return 0 and -1 because -1 would raise a constraint error.
> 
> Should the responsibility for this rest on the package or the user?
> Would it be a better to return 1 and 0, or to force a user of the
> package to instantiate it with an indexing type that starts at -1
> instead even though -1 should never be used to actually look into the
> array?  What are your opinions on this?
> 
> Thank you,
> Ryan Tarpine
> 

Re: Array-like object indexing

[Simon Wright]

Ryan <rtarpine@hotmail.com> writes:

> I've recently been playing with making all the standard data
> structures in Ada to prepare myself for writing my first major
> project.  Currently I'm writing a resizable-array package.  I am
> trying to make it as similar to built-in arrays as possible, with
> functions for First, Length and other useful operations and
> attributes.  It is a generic package instantiated with a type for
> the elements to be stored and a type for indexing.  I've run into a
> small problem with respect to the indexing type.
> 
> I'm not sure how to represent the boundaries of an empty array
> without raising a constraint error in some cases.  If an array is
> being indexed by Natural, for example, and the first index should be
> 0 (as makes sense in many situations), then what should the First
> and Last functions (intended to work like their analogous
> attributes) return?  I cannot return 0 and -1 because -1 would raise
> a constraint error.

The non-Ada C++ STL approach is to denote the end of the range by a
value notionally one past the last valid element. It's not obvious
that Last is the right name for an operation returning such a value,
cf My_Array'Last.

Re: Array-like object indexing

[Ryan]

Simon Wright wrote:
> Ryan <rtarpine@hotmail.com> writes:
> 
> The non-Ada C++ STL approach is to denote the end of the range by a
> value notionally one past the last valid element. It's not obvious
> that Last is the right name for an operation returning such a value,
> cf My_Array'Last.

Of course, that's just for iterators (which I am wrestling with 
separately :).  The STL name did always confuse me (Why is 
My_String.end() actually *past* the last character of My_String?).  I 
like First and Last being the exact indicies because it lets me ask "if 
Index not in First(My_Array)..Last(My_Array)".  I've (since originally 
asking) decided to put the responsibility on the user, but now I'm going 
to try the Index_Type'Base suggestion I received.

Thanks,
Ryan

Re: Array-like object indexing

[Darren New]

Ryan wrote:
> separately :).  The STL name did always confuse me (Why is
> My_String.end() actually *past* the last character of My_String?).

Because C starts counting at zero, and once you make the convention that
ranges are half-open, including the first index and excluding the last, then
a bunch of stuff works a lot better.

-- 
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   ** http://images.fbrtech.com/dnew/ **

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