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Karim Belabas on Mon, 08 May 2023 17:24:53 +0200
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Re: boolean values for [] and List()
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- To: Max Alekseyev <maxale@gmail.com>
- Subject: Re: boolean values for [] and List()
- From: Karim Belabas <Karim.Belabas@math.u-bordeaux.fr>
- Date: Mon, 8 May 2023 17:23:25 +0200
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* Max Alekseyev [2023-05-08 16:55]:
> Why is there a discrepancy between empty vector and empty list?
>
> ? if([], print("yes"), print("no"))
> no
> ? if(List(), print("yes"), print("no"))
> yes
Side effect of other definitions and design choices: PARI's '==' (here == 0)
operator is dodgy, use '===' if you want more consistent semantics.
The definition for a == b when it doesn't obviously makes sense is to
check whether a - b evaluates to 0, where an evaluation error is
interpreted as 'NO'. A t_LIST is not an arithmetic type and doesn't
support subtraction, so List() - 0 is an error (whereas [] - 0 is [],
which evaluates to 0 by another convention).
In short, a t_LIST is always 'non-0'.
Cheers,
K.B.
--
Pr. Karim Belabas, U. Bordeaux, Vice-président en charge du Numérique
Institut de Mathématiques de Bordeaux UMR 5251 - (+33) 05 40 00 29 77
http://www.math.u-bordeaux.fr/~kbelabas/