Hi Frederico,
Rouding stuff is not a simple issue, but it's clear that your are not
doing the computations in the same order, which could explain the
difference. Here is a simpler example showing that the result depends on
the order in which computations are done:
--> x=rand(10,1,"normal");
--> sum([1;x])-sum([x;1])
ans =
-2.220D-16
Floating point addition is NOT associative. See e.g.
https://en.wikipedia.org/wiki/Round-off_error
S.
Le 25/04/2022 à 10:00, Federico Miyara a écrit :
Dear All,
Here I have the following script, where I compute the same thing using
two iteration methods:
alfa = rand(1,10);
Rpr = rand();
R = rand();
Z = rand(10,200);
G = zeros(1,200);
for i=1:200
A = 0;
B = 0;
for k = 1:10
A = A + Rpr*alfa(k) / (Z(k,i) + alfa(k)*(1 - alfa(k))*R);
B = B + Rpr*(1 - alfa(k)) / (Z(k,i) + alfa(k)*(1 - alfa(k))*R);
end
G(i) = (1 + A) / (1 + B);
end
A = 1;
B = 1;
for k = 1:10
A = A + Rpr*alfa(k) ./ (Z(k,:) + alfa(k)*(1 - alfa(k))*R);
B = B + Rpr*(1 - alfa(k)) ./ (Z(k,:) + alfa(k)*(1 - alfa(k))*R);
end
G1 = A ./ B;
[M, ind] = max(abs(G- G1))
However, the last line shows that there are differences between both
methods. They are tiny (near %eps), but I can't find nor imagine which
is the reason.
NOTE: I kept the original formulas as in the script where the problem
arised but changed the data. When I tried to use a simpler formula
there was no difference.
Any idea?
Regards,
Federico Miyara
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Stéphane Mottelet
Ingénieur de recherche
EA 4297 Transformations Intégrées de la Matière Renouvelable
Département Génie des Procédés Industriels
Sorbonne Universités - Université de Technologie de Compiègne
CS 60319, 60203 Compiègne cedex
Tel : +33(0)344234688
http://www.utc.fr/~mottelet
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