Matrix increasing more than it should during iteration

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Mahmood Haddara
Mahmood Haddara 2022-11-15
评论： Mahmood Haddara 2022-11-20
Ran in:
I have an array (which I have called mu) and I am performing some operations on it to create a new array mu_new. I am then repeatedly iterating. I would like to make sure that the sum of mu and mu_new remains the same throughout my operations, so I have created a running sum called der to keep track of the changes between the two arrays. My problem is, after just 2 iterations the value I have for der is not the same as the value I get when I subtract the sums of the two arrays. Can anyone see why this would be the case?
Note: I removed some operations in the code to simplify it for debugging, so it is not a problem that der doesn't equal 0 here. I just want to see why der is not equal to the difference in sums of the two arrays. There are obviously some prespecified objects such as M, zeta, gx, etc.
M = 25;
mu=zeros(2*M+1,2*M+1,2*M+1);
mu(2*M+1,2*M+1,2*M+1)=1;
mu_new=mu;
zeta = rand(size(mu));
gx = rand(size(mu));
%%% iterate code below. On the second iteration der-sum(mu_new-mu,'all') is
%%% significantly differnet from 0
for i=1:2
der=0;
for x=(M+1):(2*M+1)
for y=(M+1):(2*M+1)
for z=(M+1):(2*M+1)
x1=M+1-(x-M-1);
y1=M+1+(y-M-1)-(x-M-1);
z1=M+1+(z-M-1)-(x-M-1);
x2=M+1+(x-M-1)-(y-M-1);
y2=M+1-(y-M-1);
z2=M+1+(z-M-1)-(y-M-1);
x3=M+1+(x-M-1)-(z-M-1);
y3=M+1+(y-M-1)-(z-M-1);
z3=M+1-(z-M-1);
leave=gx(x,y,z)+zeta(x,y,z)+gx(x1,y1,z1)+zeta(x1,y1,z1)...
+gx(x2,y2,z2)+zeta(x2,y2,z2)+gx(x3,y3,z3)+zeta(x3,y3,z3);
mu_new(x,y,z)=(1-leave)*mu(x,y,z);
der=der-leave*mu(x,y,z);
if x<2*M+1 && y<2*M+1 && z<2*M+1
mu_new(x+1,y+1,z+1)=mu(x+1,y+1,z+1)...
+(gx(x,y,z)+zeta(x,y,z))*mu(x,y,z);
der=der+(gx(x,y,z)+zeta(x,y,z))*mu(x,y,z);
end
if x==(M+1) && y<2*M+1 && z<2*M+1
mu_new(x1+1,y1+1,z1+1)=mu(x1+1,y1+1,z1+1)...
+(gx(x1,y1,z1)+zeta(x1,y1,z1))*mu(x,y,z);
der=der+(gx(x1,y1,z1)+zeta(x1,y1,z1))*mu(x,y,z);
else
mu_new(x-1,y,z)=mu(x-1,y,z)...
+(gx(x1,y1,z1)+zeta(x1,y1,z1))*mu(x,y,z);
der=der+(gx(x1,y1,z1)+zeta(x1,y1,z1))*mu(x,y,z);
end
end
end
end
der-sum(mu_new-mu,'all')
mu_new=mu;
end
ans = 0
ans = 0
%der-sum(mu_new-mu,'all')
  6 个评论
显示 4更早的评论
Torsten
Torsten 2022-11-18
In your original code you set
mu = mu_new
instead of
mu_new = mu
at the end of your code.
In this case,
der-sum(mu_new-mu,'all')
was not 0 in the second run.
I must admit that I cannot understand why.

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采纳的回答

Yogesh
Yogesh 2022-11-18
Hi @Mahmood Haddara
With the information you have provided above, I have verified your code for M in range [1, 100] and its working as expected. If there is a problem in successive runs make sure you are resetting all variables to required values as they will acquire some value in workspace.
Hope that helps!
  2 个评论
Mahmood Haddara
Mahmood Haddara 2022-11-20
Hi Yogesh,
As Torsten mentioned above, if we set
mu = mu_new
At the end instead of
mu_new = mu
We get der not equal to 0 on the second run. Do you have any idea why this is the case?

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