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

Answer 1

[Oleg Komarov]

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);

[Romain W]

thank you very much, it works just fine.

Answer 2

[Jan]

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

[Oleg Komarov]

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]

@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