# Save values from a parfor-loop

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dominik ballreich 2012 年 1 月 13 日
Hello,
I've got a question regarding my little parfor-example.
Y=rand(100,3)
matlabpool open
parfor p=1:2
for q=1:2
for e=1:3
Z=sum(Y(:,e)+p*q)
end
end
end
matlabpool close
For every combination of p and q there are 3 different values of Z which I would like to save within a matrix. For example in this manner:
1 1 1 1 1 1
1 1 1 2 2 2 . . .
Z(1) Z(2) Z(3) Z(4) Z(5) Z(6)
If it was not a parfor-loop but a simple for-loop, I would try something ike this:
Y=rand(100,3)
z=1
for p=1:2
for q=1:2
for e=1:3
Z=sum(Y(:,e)+p*q)
end
ergebnis(1,z)=p
ergebnis(2,z)=q
ergebnis(3,z)=Z
z=z+1
end
end
But of course that's not possible when working with parfor-loops.
It would be great, if someone helped me out.
Thank's a lot
Domninik
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Alex 2012 年 1 月 13 日
As a recommendation, if you know the size of a matrix beforehand, always initialize it before the loop. Like in your example, ergebnis = zeros(3,12), should be placed before your second loop example. This prevents memory loops that increase in size.

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### 回答 (2 件)

Alex 2012 年 1 月 13 日
It looks like parfor sends one variable to each parallel operation, meaning that the variable references must be clear to begin with.
I.e. Z(i) = #, Z(i,j) = #, and Z(i,j,k) = # are valid but Z(i + j) is not valid.
So, that leaves you with two choices, that I know of.
1. Calculate Z as a 2x2x3 matrix
Y=rand(100,3)
Z = zeros([2,2,3]);
matlabpool open
parfor p=1:2
for q=1:2
for e=1:3
Z=sum(Y(:,e)+p*q)
end
end
end
matlabpool close
Now, you have a 2x2x3 matrix that you would need to deference into a vector using tools such as repmat or loops.
2. calculate z as a vector
The other option is to have a single Parallel loop and d3ereference the index's that you want within the loop. An example follows.
Y =rand(10,3);
Z = zeros(1,12);
parfor i = 1:12
l = mod(i,3);
if(l == 0);
l = 3;
end
j = floor( ( i -l) / 6);
k = floor( ( i - l - j) / 3);
Z( i) = sum(Y(:,l)+j*k)
end
matlabpool close
This leaves you with your desired Z vector. However, the inner loop calculations are messier.
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dominik ballreich 2012 年 1 月 13 日
Hallo Alex,
thank you very much for your help. I tried to implement the idea of your first example into my problem. intervall=zeros([length(kaufsignal),length(verkaufsignal),length(massmooth),anzahl_bootstraps]) <-would be Z = zeros([2,2,3]);
intervall=zeros([length(kaufsignal),length(verkaufsignal),length(massmooth),anzahl_bootstraps]); matlabpool open
parfor p=1:length(kaufsignal)
for q=1:length(verkaufsignal)
for u=1:length(massmooth)
%Bootstrap-Sektion
for e=1:anzahl_bootstraps
x=verkaufsignal(1,p);y=verkaufsignal(1,q);z=massmooth(1,u);
R=Rsi(Y(:,e), z);
R=R';
vermoegen=zeros(long,1);
drin=3;
vermoegen(1)=kapital;
for i=2:long
if R(i)>x && R(i-1)<=x &&drin~=1;
drin=1;
vermoegen(i)=vermoegen(i-1)*t_k;
elseif R(i)<y && R(i-1)>=y &&drin~=0;
drin=0;
vermoegen(i)=vermoegen(i-1)*t_k;
elseif drin==1
vermoegen(i)=vermoegen(i-1)*(Y(i,e)/Y(i-1,e));
elseif drin==0
vermoegen(i)=vermoegen(i-1)*(1+(1-(Y(i,e)/Y(i-1,e))));
else vermoegen(i)=vermoegen(i-1);
end
end
Renditen_Vermoegen=[0;log(vermoegen(2+einpendeln:end)./vermoegen(1+einpendeln:end-1))];
%Performancemessung
mittelwert=mean(Renditen_Vermoegen.*weight);
intervall(p,q,u,e)=mittelwert;
end
end
end
end
matlabpool close
But Matlab tells me "The variable intervall in a parfor cannot be classified." (intervall)
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Alex 2012 年 1 月 20 日
I don't know what the issue is. The following works for me.
Y=rand(100,3)
Z = zeros([2,2,3]);
matlabpool open
parfor p=1:2
for q=1:2
for e=1:3
Z(p,q,e)=sum(Y(:,e)+p*q)
end
end
end
matlabpool close

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