# Help with Pre-allocating function values

2 views (last 30 days)
Mason on 12 Nov 2023
Moved: Dyuman Joshi on 12 Nov 2023
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 CommentsShow NoneHide None
Dyuman Joshi on 12 Nov 2023
Moved: Dyuman Joshi on 12 Nov 2023
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 on 12 Nov 2023
Moved: Dyuman Joshi on 12 Nov 2023
Stephen responded a little faster but, thank you for the additional information helps in the long run

Stephen23 on 12 Nov 2023
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 on 12 Nov 2023
Thank you

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