[GAP Forum] [Devel] Strange output

Sun Feb 10 11:25:14 GMT 2013

On 10 February 2013 14:25, Laurent Bartholdi
<laurent.bartholdi at gmail.com> wrote:
> The [a, b .. c] in GAP means { a, b, 2b-a, ..., a+i(b-a), ..., c } in
> mathematics.

I've been using GAP for 20 years, and I never saw or heard about this
one. Sorry.
IMHO it is confusing, counter-intuitive, goes against well-established practices
in other programming languages and CAS's, etc...

I can probably list a dozen programming languages / CAS's, where such
lists (or/and loops) are constructed by the triple of parameters
("beginning","end","step"), in this order.

May I humbly propose syntax [a .. c, b-a] instead?

Best,
Dmitrii

>
> GAP is not exactly a programming language; there are *objects* and their
> *representations*, and GAP is allowed to change a representation to another
> one in case it's provably more efficient (i.e. there are more or faster
> algorithms for it).
>
> Hope this helps,
> Laurent
>
>
> On Sun, Feb 10, 2013 at 5:58 AM, Dmitrii (Dima) Pasechnik <dima at ntu.edu.sg>
> wrote:
>>
>> On 10 February 2013 10:52, Gordon Royle <gordon.royle at uwa.edu.au> wrote:
>> > Well, thanks for the feedback, but I remain somewhat confused…
>>
>> I think this is definitely a bug, and must be fixed ASAP...
>>
>> Best,
>> Dima
>>
>> >
>> > Firstly,  why does GAP unilaterally choose to change the format .. .the
>> > program is simply a recursive program with a single Print statement, and yet
>> > the output changes format , and then changes back again, without me asking
>> > it to...
>> >
>> > [ 1, 5, 51 ]
>> > [ 1, 6 .. 11 ]
>> > [ 1, 6, 12 ]
>> >
>> >
>> >
>> >
>> > Secondly, why is
>> >
>> > 6 .. 11
>> >
>> > the "range representation" for
>> >
>> > 6, 11
>> >
>> > In most languages that support it, the ".." means "everything from the
>> > first to the last (or perhaps the first to the last-minus-one)"
>> >
>> >
>> > Actually, it means the same in GAP too:
>> >
>> > gap> for x in [6 .. 11] do
>> >> Print(x," ");
>> >> od;
>> > 6 7 8 9 10 11
>> >
>> > So, naively I would expect
>> >
>> > [1, 6 .. 11]
>> >
>> > to be a fancy way of saying
>> >
>> > [1, 6, 7, 8, 9, 10, 11]
>> >
>> >
>> > But it's not:
>> >
>> > gap> for x in [1, 6 .. 11] do
>> >> Print(x," ");
>> >> od;
>> > 1 6 11
>> >
>> >
>> > It seems dangerous to me to have the expression   a .. b mean something
>> > different to (a) other uses in the same language, (b) mathematical usage and
>> > (c ) other programming languages.
>> >
>> >
>> > Thirdly, why has this happened RIGHT NOW, when I've been using this
>> > program for years without ever seeing anything like this before…
>> >
>> >
>> > Thanks again
>> >
>> > Gordon
>> >
>> >
>> >
>> > Professor Gordon Royle
>> > School of Mathematics and Statistics
>> > University of Western Australia
>> > Gordon.Royle at uwa.edu.au<mailto:Gordon.Royle at uwa.edu.au>
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> >
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>
> --
> Prof. Dr. Laurent Bartholdi   \ laurent.bartholdi<at>gmail<dot>com
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