Le 23/05/2019 à 09:34, Dang Ngoc Chan, Christophe a écrit :
Hello,
De : Carrico, Paul
Envoyé : jeudi 23 mai 2019 09:21
I share your suggestion in adding an example in the doc to prevent Scilab
users, but the question is "where" ?
I suggest the "Scilab Help >> Elementary Functions > extraction" page.
I am not sure that is the right place. In fact our discussion is about
the general concepts of *lvalue* and *rvalue* (in most languages,
compiled or interpreted, see e.g.
https://www.quora.com/What-is-lvalue-and-rvalue-in-C which starts with
general stuff valuable for Scilab also)
In this insertion expression:
x(i) = y
x(i) determines the identity of an object, and we need this because the
evaluation of the expression will modify it . In this case "x(i)" is a
*lvalue*. The "l" is to recall that such constructs are to the *left* of
the assignment operator
In this extraction expression:
y = x(i)
"x(i)" is a *rvalue*, i.e. a "result" which is _temporary_
materialized, and identity of x is lost (because we don't need it). Here
the "r" can be seen as "right" as well as "result" (I prefer "result"...)
Hence, in the expressions
x=rand(10,1);
[v,k]=find(x(3:10))
find() just sees an anonymous/temporary 8x1 vector. It is the user's
responsibility to do
k=k+2
to compute the actual index in original vector.
S.
Regards
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Christophe Dang Ngoc Chan
Mechanical calculation engineer
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Stéphane Mottelet
Ingénieur de recherche
EA 4297 Transformations Intégrées de la Matière Renouvelable
Département Génie des Procédés Industriels
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