how preallocate structure for better memory

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Salvatore Mazzarino 2012-9-22

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https://ww2.mathworks.cn/matlabcentral/answers/48807-how-preallocate-structure-for-better-memory

评论： Dyuman Joshi 2024-3-27

I had created a structure made so:

head.number = 3;
head.pck_rcv = [1 0 0];
heads(2).number = 5;
head(2).pck_rcv = [1 1 0];

and so on.

How can I preallocate a structure?

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Answer 1

Jan 2012-9-22

27
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https://ww2.mathworks.cn/matlabcentral/answers/48807-how-preallocate-structure-for-better-memory#answer_59659

编辑：Jan 2017-10-2

在 MATLAB Online 中打开

for k = n:-1:1    % Backwards!
  head(k).number = 3;
  head(k).pck_rcv = [1 0 0];
end

Now the final size of the struct array is created in the first iteration.

[EDITED] Alternatively:

head = struct('number', cell(1, 10), 'pck_rv', cell(1, 10));

Now head is a [1 x 10] struct array withe the fields 'number' and 'pck_rv'. Pre-allocating the contents of the fields is another job and you need a loop to do this. But now it can run in forward direction also.

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Jan 2012-12-13

编辑：Jan 2012-12-13

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@Jason: I've added a comment to Oleg's message now. There are two jobs to to when pre-allocating a struct array: 1. allocate the struct array itself, 2. allocate memory for all different fields. While point 1 can be solved in one step using either the struct('field', {data}) or the implicit pre-allocation by assigning the last element at first, the allocation of the contents of the fields cannot be done in one step. There is no way to pre-allocate n arrays simultaneously. Trying to do this results in "shared data copies", which means that all the fields share the same data value. At first this is fast and memory efficient. But when you change the values, an addition deep copy is performed for each element, which consumes more time than allocating the different arrays of the fields in a loop. For this reasons it is e.g. useless to "pre-allocate" the elements of a cell array, while pre-allocating the cell itself is strongly recommended:

n = 1e5;
tic; for k = 1:1000
  clear('c');
  for in = 1:n
    c{in} = 'qvw';
  end
end, toc
tic; for k = 1:1000
  clear('c');
  c = cell(1, n);  % important pre-allocation of cell
  c(:) = {'asd'};  % Useless pre-allocation of cell elements
  % Now all elements of C point to the same memory such that 
  % the characters 'asd' are found once only in the RAM.
  for in = 1:n
    c{in} = 'qvw';  % Now "unsharing" the string consumes time
  end
end, toc
tic; for k = 1:1000
  clear('c');
  c = cell(1, n);  % important pre-allocation of cell
  for in = 1:n
    c{in} = 'qvw';
  end
end, toc

This is very similar for structs, because the only difference to cells is the storage of the fieldnames in a CHAR array, while the organization of the elements is equal (therefore struct2cell and cell2struct are so fast).

Summary: Pre-allocate the struct in one step, pre-allocate the fields separately for each element of the struct in a loop, or assign them directly if possible.

Igor Gitelman 2022-5-20

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thanks! that

head = struct('number', cell(1, 10), 'pck_rv', cell(1, 10));

work fine!

Dyuman Joshi 2024-3-27

I am accepting Jan's answer as it provides a robust solution to the question posted.

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Answer 2

Azzi Abdelmalek 2012-9-22

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https://ww2.mathworks.cn/matlabcentral/answers/48807-how-preallocate-structure-for-better-memory#answer_59658

编辑：Azzi Abdelmalek 2012-9-22

在 MATLAB Online 中打开

heads=struct('numbers',zeros(10,1), 'pck_rcv',zeros(10,3))

%then

for k=1:n
heads.numbers(k)=2
heads.pck_rcv(k,:)=[1 2 3]
end

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Jan 2017-10-2

@Alexandra: I do not agree. Salvatore asked for a struct array: "head(2).numbers and so on". Azzi's suggestion creates a scalar struct only.

Alexandra Simpson 2017-10-2

True, I just tried it out and realised it wasn't what I wanted either! Thanks for the response.

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Answer 3

Walter Roberson 2012-12-13

2
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https://ww2.mathworks.cn/matlabcentral/answers/48807-how-preallocate-structure-for-better-memory#answer_68385

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head = struct('number', {3, 5}, 'pck_rcv', {[1 0 0], [1 0 1]})

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