Re: need help learning Ada for a modula-2 programmer

[Mike H <postmaster@ada-augusta.demon.co.uk>]

In message <878utybjxc.fsf@adaheads.sparre-andersen.dk>, Jacob Sparre 
Andersen <jacob@jacob-sparre.dk> writes
>Generalising the array index beyond type Integer takes up ten lines in
>my first take on the task.  (Recursion _might_ make it shorter.)
>
-- ===================================================
-- Below is the 'body' of a generic function whose formal specification 
is ...
--
-- generic
--    type Item is private;
--    type Index is (<>);
--    type Array_type is array (index range <>)of Item;
-- function Shift_right_circular
--    (A     : Array_type;
--    Places : Natural) return Array_type;
--
-- ===================================================
--
function Shift_right_circular
   (A      : Array_type;
    Places : Natural) return Array_type is

    -- Split A into two slices A(I1 .. I2) & A(I3 .. I4). The position
    -- of the split is between I2 and I3. I3 is offset from I1 by 
distance P.
    -- I2 is the predecessor of I3.
    P      : Natural := Places rem A'LENGTH;
    I2, I3 : Index; -- Note: type of Index will be known from the 
instantiation

begin

    if P > 0 then
       -- The two slices of A are rejoined as (I3 .. I4) & (I1 .. I2).
       P  := Index'POS(A'FIRST) + P;
       I3 := Index'VAL(P);
       I2 := Index'PRED(I3);
       return A(I3 .. A'LAST) & A(A'FIRST .. I2);
    else
         -- P = 0 is an anomalous case (I3 has no predecessor so the line
         -- "I2 := Index'PRED(I3)" would raise a constraint error), so 
....
       return A;
    end if;

end Shift_right_circular;

--
In PLAN (ICL 1900 assembler) the mnemonic was SRC
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Rightpondian Mike