How do I pre-allocate memory for a structure?

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Can anyone help me to fix pre-allocation memory for a structure output?
I have a structure function which have 18 elements, it has two imputs and give results 16 outputs and two inputs.
If I want to pre-allocate the output memory how can I do it.
Here is the code:
phase_in = ?????; % what should be in question marks ( I did zeros(length(v_in),refprop))
for i=1:length(v_in)
phase_in(i) = refprop(v_in(i),T_in,r410a);
end
r410 is my structure function, and v_in and T_in are inputs, I will calculate 100 times to calculate phase_in.
If I skip pre-allocation, the output will give me 1*1000 structure. In this case, I tried to pre-allocate phase_in=zeros(100,18),
but it gave me error'' Conversion to double from struct is not possible.''
Thanks a lot in advance.
  1 Comment
Stephen23
Stephen23 on 27 Mar 2019
Edited: Stephen23 on 27 Mar 2019
By far the fastest and simplest an solution is to allocate the last element first, for example by simply looping over the indices backwards (as Walter Roberson explained in their answer). This topic has been discussed on the MATLAB blogs:
and also in many other Answers threads (with examples) e.g.:
Do NOT use a cell array of structures, this just makes your data storage more complex and less efficient, and means that you cannot use a simple comma-separated list for accessing the structure data:

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Accepted Answer

KSSV
KSSV on 27 Mar 2019
S = struct ;
for i = 1:10
S(i).Rand = rand ;
S(i).loop = i ;
end
  8 Comments
WANYI ZHANG
WANYI ZHANG on 27 Mar 2019
Thanks a lot.
It is all worked.
I appreciate you.
Thanks again.

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More Answers (2)

Walter Roberson
Walter Roberson on 27 Mar 2019
Loop backwards from length down to 1 so that the highest offset is assigned to first so it will not need to grow the array afterwards.
  2 Comments
Walter Roberson
Walter Roberson on 27 Mar 2019
(Or if it does exist, that it is a compatible structure and no larger than what will be needed.)

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Michael
Michael on 15 Jun 2022

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