Help with Pre-allocating function values

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,

[Dyuman Joshi]

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]

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

Answer 1

[Stephen23]

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 
    ___    ____

    697    1209
    697    1336
    697    1477
    770    1209
    770    1336
    770    1477
    852    1209
    852    1336
    852    1477
    941    1209
    941    1336
    941    1477

[Mason]

Thank you