pre-allocating structures

Question

Romain W 2012-3-1

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/30764-pre-allocating-structures

Hello everyone,

I'm trying to pre-allocate memory for a structure defined as follows:

% S   : Structure containing geometrical description of polygons.
%         S(i) contains all information relative to the i-th shape
%         S(i).P(j) gives access to the geometrical description of the j-th
%         element of the i-th shape. 
%               XData : S(i).P(j).x    : Vector
%               YData : S(i).P(j).y    : Vector
%               Hole  : S(i).P(j).hole : Binary value

I'd like to pre-allocate before the following loop:

for i=1:ne
   for j=1:2
       S(i).P(j).x = v_proj(f(i,j),1);
       S(i).P(j).y = v_proj(f(i,j),2);
       S(i).P(j).hole = 0;
   end
end

I'd like to have a number of shapes ne.

But I don't really know how to do that. Any help would be more than welcome.

Thank you very much,

% Romain

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Oleg Komarov 2012-3-1

2
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/30764-pre-allocating-structures#answer_39246

在 MATLAB Online 中打开

One way to preallocate:

P(1:2) = struct('x',zeros(10,1),'y',zeros(1,2),'hole',false);
ne = 10;
[S(1:ne).P] = deal(P);

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Romain W 2012-3-1

thank you very much, it works just fine.

请先登录，再进行评论。

Answer 2

Jan 2012-3-1

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/30764-pre-allocating-structures#answer_39237

在 MATLAB Online 中打开

You can create the last element at first to do an implicite pre-allocation:

for i=ne:-1:1
  for j=[2,1]
     S(i).P(j).x = v_proj(f(i,j),1);
     S(i).P(j).y = v_proj(f(i,j),2);
     S(i).P(j).hole = 0;
  end
end

2 个评论
显示无隐藏无

Oleg Komarov 2012-3-1

It doesn't work since structures use pointers:

S(20).P(20).x = 1:20;

S : 816.00 B

S(19).P(19).x = 1:20;

S : 1.37 KB

Jan 2012-12-13

编辑：Jan 2012-12-13

@Oleg: What does not work and why are the sizes of S interesting here? The code I've posted pre-allocates S in one step and the fields P for each S(:) in one step also. There is no way to pre-allocate all ne elements of P simulataneously, because they are distinct arrays. As far as I can see [S(1:ne).P] = deal(P) creates shared data copies for the contents of all S(:).P, such that writing values later on will consume additional time by creating deep data copies at first. Therefore your pre-allocation is faster only if no data are written and this is rarely needed in real world programs.

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

请先登录，再进行评论。