How do I pre-allocate memory for a structure?

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WANYI ZHANG
WANYI ZHANG am 27 Mär. 2019
Bearbeitet: Steven Lord am 7 Jul. 2023
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 Kommentar
Stephen23
Stephen23 am 27 Mär. 2019
Bearbeitet: Stephen23 am 27 Mär. 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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Akzeptierte Antwort

KSSV
KSSV am 27 Mär. 2019
S = struct ;
for i = 1:10
S(i).Rand = rand ;
S(i).loop = i ;
end
  8 Kommentare
KSSV
KSSV am 27 Mär. 2019
dm=zeros(length(v_in),1);
phase_in = cell(length(v_in),1);
fluid_up = struct ;
for i=1:length(v_in)
phase_in{i} = refprop(v_in(i),T_in,r410a);
fluid_up(i).P1 = phase_in{i,1}.P;
fluid_up(i).rho = 1/phase_in{i,1}.vg;
dm(i) = orifice(fluid_up(i),P_d,orifice_params);
end
WANYI ZHANG
WANYI ZHANG am 27 Mär. 2019
Thanks a lot.
It is all worked.
I appreciate you.
Thanks again.

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Weitere Antworten (2)

Walter Roberson
Walter Roberson am 27 Mär. 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 Kommentare
Stephen23
Stephen23 am 27 Mär. 2019
Bearbeitet: Stephen23 am 27 Mär. 2019
Looping backwards is very easy:
for k = numel(v_in):-1:1 % backwards!!
S(k) = ... scalar structure
end
Just make sure that the output structure S does NOT exist in the workspace before the loop.
Walter Roberson
Walter Roberson am 27 Mär. 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 am 15 Jun. 2022
Bearbeitet: Steven Lord am 7 Jul. 2023
repmat offers another solution for preallocating structures:
[SL: Removed broken link to code generation documentation, added link to the main repmat function documentation page.]

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