Preallocate a variable of unknown size

Question

Akbar 2018-5-24

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/402356-preallocate-a-variable-of-unknown-size

评论： Akbar 2018-5-24

采纳的回答： Guillaume

nodes.mat

在 MATLAB Online 中打开

Please help to preallocate the variable "D". Its size is unknown.

clc;
D = {};
Di = [];
r = 1;
N = length(nodes.PrtCtyAll);
DstBlocks = cell(N,1);
for i = 1:N % Iterate through PrtCtyAll
for j = 1 : length(nodes.PrtCtyAll{i})
  DstBlocks{i}{j,1} = nodes.PrtCtyAll{i}(j).DstBlock;
end    
for k = 1 : length(DstBlocks{i})
  if (~isempty(DstBlocks{i}{k}) && length(DstBlocks{i}{k})==1)
      D{end+1,1} = DstBlocks{i}{k};
      r = r + 1;
  else
      for z = 1:length(DstBlocks{i}{k})
      D{end+1,1} = DstBlocks{i}{k}(z);
      end
  end
end
% Get Indices
for a = 1:length(D)
    Di(a) = find(abs(nodes.Handles-D{a}) < 0.1);
end
Si = find(abs(nodes.Handles-nodes.Handles(i)) < 0.1);
Di = [];
D = {};
end
clear i j k r z a N D Di Si

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Answer 1

Guillaume 2018-5-24

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/402356-preallocate-a-variable-of-unknown-size#answer_321781

在 MATLAB Online 中打开

That's some overly complicated code there. For example,

for j = 1 : length(nodes.PrtCtyAll{i})
  DstBlocks{i}{j,1} = nodes.PrtCtyAll{i}(j).DstBlock;
end

is simply

DstBlocks{i} = {nodes.PrtCtyAll{i}.DstBlock}.'; %no need for a loop

but even that is a waste of time. You're converting a structure array into a cell array for the sole purpose of iterating over its elements. Just iterate directly over the structure.

There is absolutely no point in making D a cell array since each cell is only going to contain a scalar. The whole calculation of D can be simplified into

D = [nodes.PrtCtyAll{i}.DstBlock].';

which does not require preallocation as the array is created and filled at the same time.

But even creating D is probably a waste of time. I don't really understand what you're trying to do with your a loop. What is sure is that at some point it's going to error since find will return more than one index which won't fit into Di(a). In any case, Di is never used and emptied shortly after.