# How to fast process a calculation within a matrix without using a loop?

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Leon 2019 年 3 月 29 日

I have a huge matrix A, and what i need to do is to go through each of the matrix element and do this below simple calculation:
[m n] = size(A);
for i=1:m
for j=1:n
B(i,j) = 10^-A(i,j);
end
end
The thing is that it is slow. Is there a way I can avoid the loop and make it faster?
Thank you!
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Rik 2019 年 3 月 29 日

Whenever the size of an array increases, Matlab has to create a copy in memory. This takes a lot of time if you repeat that very often. Your original code would cause the output to be extended m+n-1 times.
Pre-allocation would solve this issue by creating an array in advance of the correct size. If array processing is not possible, it is generally worth it to pre-allocate the output array.
Leon 2019 年 3 月 29 日
Got you!
Many thanks!

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### 採用された回答

Rik 2019 年 3 月 29 日
Yes, use the power of array processing in Matlab:
B=10.^-A;
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Leon 2019 年 3 月 29 日
Many thanks!
Will that be a problem, if my data also contains NaNs?
Rik 2019 年 3 月 29 日
It will do the same as your loop for NaN values, so the B matrix will contain NaN everywhere where A has a NaN.

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