Hello,
The cross() function was wished by http://bugzilla.scilab.org/9941.
It is being designed at https://codereview.scilab.org/#/c/11469/
through the Scilab Enhancement Proposal SEP#96
https://codereview.scilab.org/cat/11469%2C2%2CSEP/SEP_096_cross.odt
It's a good new! Despite this SEP has not been posted here, here are
some free
comments about its present implementation
Rationale
cross is an elementary function that exists in Matlab which is used to
return the vector cross product
of two column vectors of size 3-by-1. It may be useful in mathematics,
mechanics, etc...
Why refering to Matlab? If the simple fact that something exists in
Matlab should turn
it useful to implement in Scilab, this opens many ways to downgrade
Scilab. We should
publish a list, if needed. Matlab has its own drawbacks, due to its own
history. Is it really
useful to add them to Scilab, when Scilab has sometimes better solutions
and is fighting
its own pitfalls, bad designs, etc to reduce them?
If obviously features existing elsewhere can't be all bad, IMO to be
available elsewhere
is not an intrinsic argument.
cross() is actually expected in mechanics and many other fields of
physics: electromagnetism,
fluid dynamics, etc. and if it was'nt available elsewhere, it would be
useful anyway for these
reasons. So, IMO, no need to point other lab.
Syntax and semantics
Only one calling sequence is available:
C = cross(A,B)
A and B can be: real, complex, polynomial, sparse or boolean matrix of
size 3-by-1.
The extension to polynomials beyond decimals is excellent. AFAIK, it is
not available in
the other lab.
The processing of booleans should be more clearly indicated: * => AND ,
+ => OR
or %T => 1, %F => 0 (that is to say, as if bool2s() was applied before
processing. Since this
is not yet illustrated with an example, writting something about would
be useful, as well
as in the help page).
Rational fractions are not supported. It could be worthwhile to indicate
this limit.
Sparsity is an encoding type rather than a data type. It could be
indicated rather after data types.
The 3x1 size of required args is uselessly demanding and should be
extended (see below).
*Present implementation* (patch set 2)
function c=cross(a,b)
// This function returns the cross product of a and b
// (checking input arguments). Then: )
a=[a;a];
b=[b;b];
c=a(2:4).*b(3:5)-b(2:4).*a(3:5);
endfunction
It is tricky and efficient.
1) Unless willing to stick to the weak implementation of other labs,
Scilab's one can be
easily vectorized:
c = a(2:4,:).*b(3:5,:) - b(2:4,:).*a(3:5,:);
2) About accepted formats for input args, here is a suggestion:
- a and b must have the same format. One of their dimensions must
be 3.
If both dimensions have a size of 3, calculations are done
column-wise.
Else, calculations are done along the dimension of size 3.
- the result r has the same format as a and b (by the way, this is
the way Matlab
accepts a and b and returns r: if a and b are rows, r is a row).
3) (Option) If hypermatrices are provided, argument tests could be done
after
applying squeeze() to them. Hypermatices are not rare in physics.
This is even
rather the rule.
Hope to read comments from other potential users
Regards
Samuel
_______________________________________________
dev mailing list
[email protected]
http://lists.scilab.org/mailman/listinfo/dev
