Help with Pre-allocating function values

Question

Mason 2023-11-12

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2046060-help-with-pre-allocating-function-values

移动：Dyuman Joshi 2023-11-12

Does anyone know how I might be able to go about preallocating f = []; I keep getting hit with the same "arrays dont match" error, I also noticed that if f isnt cleared it tends to add an extra column to the array. Ive noticed this particular portion of my function is extremely slow and it would help alot with the dial im making;

lf = [697 770 852 941]; % Low frequency group
hf = [1209 1336 1477];  % High frequency group
f = [];
for c = 1:4
  for r = 1:3
      f = [f [lf(c); hf(r)]];
  end
end

Thanks,

2 个评论
显示无隐藏无

Dyuman Joshi 2023-11-12

移动：Dyuman Joshi 2023-11-12

在 MATLAB Online 中打开

Dynamically growing arrays is detrimental to the performance of the code. You should Preallocate arrays according to the final output size.

However, you can vectorize this operation -

lf = [697 770 852 941]; % Low frequency group
hf = [1209 1336 1477];  % High frequency group
%% Original method
f = [];
for c = 1:4
  for r = 1:3
      f = [f [lf(c); hf(r)]];
  end
end
f
f = 2×12
         697         697         697         770         770         770         852         852         852         941         941         941
        1209        1336        1477        1209        1336        1477        1209        1336        1477        1209        1336        1477

%% Vectorization method
[x,y] = meshgrid(lf, hf);
F = [x(:) y(:)].'
F = 2×12
         697         697         697         770         770         770         852         852         852         941         941         941
        1209        1336        1477        1209        1336        1477        1209        1336        1477        1209        1336        1477
%Comparison
isequal(f, F)
ans = logical
   1

Mason 2023-11-12

移动：Dyuman Joshi 2023-11-12

Stephen responded a little faster but, thank you for the additional information helps in the long run

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Stephen23 2023-11-12

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2046060-help-with-pre-allocating-function-values#answer_1351185

在 MATLAB Online 中打开

The MATLAB approach:

lf = [697,770,852,941];
hf = [1209,1336,1477];
[X,Y] = meshgrid(lf,hf);
f = [X(:),Y(:)]
f = 12×2
         697        1209
         697        1336
         697        1477
         770        1209
         770        1336
         770        1477
         852        1209
         852        1336
         852        1477
         941        1209

Or

T = combinations(lf,hf) % note output is a table!
T = 12×2 table
    lf      hf 
    ___    ____

  1209
  1336
  1477
  1209
  1336
  1477
  1209
  1336
  1477
  1209
  1336
  1477