Preallocating and filling long array with shorter arrays

3 views (last 30 days)
Nik Rocky on 20 Mar 2023
Commented: Nik Rocky on 21 Mar 2023
Hello dear community,
I have a question:
If I want to fll an array with single values using preallocation, I do:
input = zeros(1,1000000);
for k = 2:1000000
input(k) = input(k-1) + 5; % 5 is a newdata value in this example
end
so, in input will be stored values 0, 5, 10, 15, 20....
*********************************************************************
now, I have a data input from arrays (ex.) 5 values long, and I want to concatenate them to one big input array after every loop:
newdata:
4 5 7 8 9, 5 8 7 9 5, 1 7 8 9 6, 1 2 3 6 5
to one array input in few steps:
4 5 7 8 9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
............
4 5 7 8 9 5 8 7 9 5 1 7 8 9 6 1 2 3 6 5
If I create a big zeroarray, and then create an input variable in a loop, I just create every time a new variable in a loop. How can I use the method from single variable by arrays to use preallocation?
input = [zeros(20,[])]; %it has no sence
for i = 1 : total_frames
input = [input;newdata];
end
Thank you!
P.S. Actually, I'm working with big arrays (48000 * 100000000), the short example is just for better understanding. I want to make my algorithm time efficient.

Stephen23 on 20 Mar 2023
Edited: Stephen23 on 20 Mar 2023
The simple apparoach is to use a matrix or array, for example:
N = 10;
M = nan(5,N);
for k = 1:N
M(:,k) = rand(5,1);
end
display(M)
M = 5×10
0.5932 0.1375 0.9838 0.9635 0.7082 0.7918 0.3489 0.1323 0.9478 0.5962 0.6731 0.8178 0.3265 0.6184 0.2314 0.0253 0.0699 0.5711 0.7934 0.5818 0.8518 0.1121 0.2469 0.3151 0.5395 0.7872 0.9489 0.0019 0.1219 0.3131 0.3972 0.2027 0.3039 0.0395 0.6487 0.6918 0.8420 0.1117 0.2279 0.0301 0.5613 0.1273 0.4150 0.2947 0.1517 0.5669 0.3289 0.1059 0.8498 0.1112
"Actually, I'm working with big arrays (48000 * 100000000)"
Nik Rocky on 21 Mar 2023
Great answer, thank you very much!
Here is comparsion old to new:
spf = 48000;
frames = 200;
data=[nan(spf*frames,[])];
tic
for k = 1 : frames
input = rand(48000,1);
data = [data;input];
end
toc
Elapsed time is 3.206472 seconds.
spf = 48000;
frames = 200;
data = nan(spf,frames);
tic
for k = 1:frames
data(:,k) = rand(spf,1);
end
data = reshape(data.', [],1);
toc
Elapsed time is 0.093135 seconds.
In the end I have a big (spf*frames) x 1 array, that I need
Feel the difference =)

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