How do I pre-allocate memory for a structure?

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.

[Stephen23]

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:

https://blogs.mathworks.com/loren/2008/02/01/structure-initialization/

and also in many other Answers threads (with examples) e.g.:

https://www.mathworks.com/matlabcentral/answers/12912-how-to-create-an-empty-array-of-structs

https://www.mathworks.com/matlabcentral/answers/48807-how-preallocate-structure-for-better-memory

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:

https://www.mathworks.com/help/matlab/matlab_prog/access-multiple-elements-of-a-nonscalar-struct-array.html

https://www.mathworks.com/help/matlab/matlab_prog/comma-separated-lists.html

Answer 1

[KSSV]

S = struct ;
for i = 1:10
    S(i).Rand = rand ;
    S(i).loop = i  ;
end

[WANYI ZHANG]

I did not really understand on your answers.

Can you tell me what do I have to put in ''struct'',

is it just struct? or just my struct function name?

Thanks

[WANYI ZHANG]

phase_in = struct;
for i=1:length(v_in)
    phase_in(i) = refprop(v_in(i),T_in,r410a);
end

it gave me this error:

''Subscripted assignment between dissimilar structures.

Error in lab4_task3 (line 34)

phase_in(i) = refprop(v_in(i),T_in,r410a);''

[KSSV]

What is the ouput of this?

refprop(v_in(i),T_in,r410a)

[WANYI ZHANG]

That is 1*100struct

[KSSV]

YOu better store them in a cell:

phase_in = cell(length(v_in),1);
for i=1:length(v_in)
    phase_in{i} = refprop(v_in(i),T_in,r410a);
end

[WANYI ZHANG]

That is very helpfull, it gave me as structure output.

Thank you so much.

Can I ask you one more quetion continue that.

I think it is better to put the code I write rather than expalining.

dm=zeros(length(v_in),1);
phase_in = cell(length(v_in),1);
fluid_up.P1= zeros(length(v_in),1);
fluid_up.P1= zeros(length(v_in),1);

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;          %underlined
    fluid_up(i).rho = 1/phase_in{i,1}.vg;     %underlined
    
    dm(i) = orifice(fluid_up(i),P_d,orifice_params);   
end

In matlab script these two bold variables still have underlying wave line,

How can I fix it.

Thanks a lot!

[KSSV]

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]

Thanks a lot.

It is all worked.

I appreciate you.

Thanks again.

Answer 2

[Walter Roberson]

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.

[Stephen23]

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]

(Or if it does exist, that it is a compatible structure and no larger than what will be needed.)

Answer 3

[Michael]

repmat offers another solution for preallocating structures:

[SL: Removed broken link to code generation documentation, added link to the main repmat function documentation page.]

[Daniel]

Broken link