pre-allocating structures

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Romain W
Romain W 2012-3-1
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

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采纳的回答

Oleg Komarov
Oleg Komarov 2012-3-1
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);

更多回答（1 个）

Jan
Jan 2012-3-1
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
Oleg Komarov
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
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

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