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Pre-allocate memory gives zeros all the time, how to avoid?

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want2know
want2know 2013-11-7
关闭： MATLAB Answer Bot 2021-8-20
In the beginning of a function, we have to pre-allocate memory by doing something like this: integ_signal=zeros(5,4);
Using the below code as an example, if I comment out this line: integ_signal=zeros(5,4);, I will get the desired answer which is 10.1 instead of 0.1. But, in Matlab Function block, I have to pre-allocate memory, how could I pre-allocate memory at the same time still won't assign zero to the integ_signal all the time?
integ_signal(1,1)=10;
integ_signal=zeros(5,4);
a=zeros(10);
arr=zeros(5,4);
count=2;
a=1;
integ_signal(count,1)= integ_signal(count-1,1);
arr(count,1) = integ_signal(count,1);
arr(count,1) = arr(count,1) + abs(I(1,a)-I(1,a+1));
(Desired answer, without pre-allocating memory) >> arr arr =
0 0 0 0
10.1000 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
(Wrong answer, with pre-allocating memory) >> arr
arr =
0 0 0 0
0.1000 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0

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回答（2 个）

Azzi Abdelmalek
Azzi Abdelmalek 2013-11-7
In your Matlab function, declare a variable arr as persistent
persistent arr
if isempty(arr)
arr=zeros(5,4)
end
You write
a=zeros(10),
a=1
The result is a=1 , then the line a=zeros(10) is unnecessary
  1 个评论
want2know
want2know 2013-11-7
ok, i tried this in Matlab workspace, but I got this Error using persistent A PERSISTENT declaration is only allowed in a M-file function. So, I was thinking to try this in my m-file, but due to the error I posted earlier (<http://www.mathworks.co.uk/matlabcentral/answers/105217-subscripted-assignment-dimension-mismatch-size-1-x-1-size-1-x)>, I could not check the outcome of using persistent yet.
Simon
Simon 2013-11-7
Hi!
Maybe your missing 10 because the first two rows are in the wrong order?

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