Re: Surprise in array concatenation

["Jeffrey R. Carter" <spam@spam.com>]

Gene wrote:

> I completely agree with the marginal utility of other-than-1 least
> array indices.

Then you need a lot more experience. There is great utility to arrays with lower 
bounds other than 1, and to arrays with non-numeric bounds.

30 years ago, when I became a coder, I worked in dendroclimatology, attempting 
to determine the climate of the past from tree-ring information. We had code 
with big arrays of tree-ring data, all indexed from 1, of course, since this was 
in FORTRAN 66, and other arrays of climate data. The tree-ring data started with 
values from some year in the past, such as 1600, and the climate data in 1895 or 
so. The code was liberally sprinkled with calculations to convert between the 
logical indices (years) and the actual indices (1 .. # of years). Different 
calculations had to be used for the 2 data sets, I didn't capture the algorithms 
in functions because I didn't know any better, and sometimes I managed to use 
the wrong calculation. Ah, those were the days!

How much easier it would have been if I could simply have indexed these things 
from 1600 .. 1965 and 1895 .. 1965. (Having record types would have helped, too.)

This helps explain the popularity of C and its descendants: It gives a real 
feeling of accomplishment to do this kind of thing successfully, even though it 
contributes nothing to the solution of the real problem being addressed.

The problem in the example is that the code defines an array type with "Integer 
range <>", allowing any lower bound in Integer, and proceeds to assume that all 
values of the type have lower bound of 1. That's a simple implementation error. 
Had the implementor recognized that values of the type may have any lower bound 
in Integer, and used 'First rather than an incorrect magic number, the problem 
would not have appeared.

If there is a requirement that the values have a lower bound of 1, there's an 
Ada mechanism for that:

type A is array (Positive range <>) of Whatever;

type Lower_Bound_Always_1 (Length : Natural) is record
    Value : A (1 .. Length);
end record;

Even then, one should still use 'First.

-- 
Jeff Carter
"I fart in your general direction."
Monty Python & the Holy Grail
05