How can I fix "Attempted to access Y(0); index must be a positive integer or logical." MATLAB for loop error?

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shackelferd
shackelferd 2015-9-7
回答： Walter Roberson 2015-9-7
I'm trying to use a for loop in MATLAB and I keep getting an error message. The instructions I'm following are to create a 10000X1 vector using a random number generator and label it "e", and then a second 10000X1 vector that only contains zeros. Then using a loop I have to simulate the process Y(t)=(1-a)*Y(t-1)*e(t) where a=0.1. The code I have been attempting to run is:
Y=zeros(10000,1);
e=randn(10000,1);
for t=1,2,...,10000
a=0.1;
Y(t)=(1-a)*Y(t-1)*e(t);
end
I keep getting an error message that says: "Attempted to access Y(0); index must be a positive integer or logical." Any help is appreciated

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回答（1 个）

Walter Roberson
Walter Roberson 2015-9-7
for t=1,2,...,10000
is not valid syntax. Please post your actual code.
Your formula has
Y(t)=(1-a)*Y(t-1)*e(t);
when t = 1, the Y(t-1) part is going to be trying to access Y(0). In MATLAB you cannot index an array at location 0.
I suspect the formula you need is
Y(t+1) = (1-a)*Y(t)*e(t);
with the "for" running from 1 to 9999. However, it is possible that instead you want the formula that you have now but with t starting from 2 in the loop.
The difference between the two formulas has to do with whether e(1) should ever be used and if so whether it should be used with Y(1) or with Y(2)

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