3d matrix initialization and find the specific values after the for loops

Hello my Vf is 42444 *1*1000 matrix. I used this code for finding values for the location of 312 and incrementing of 324 from the 42444 values. But why the final matrix A is 131 * 3*1000. I am expecting 131*1*1000.

Answer 1

[Voss]

You give A three columns when you do this:

A=[m,n,p]; % size 1x3

Perhaps you meant:

A=zeros([m,n,p]);

But that would give A too many rows.

Anyway, the whole thing can be done in one line:

A = Vf(312:324:end,:,:);

[Saki]

I got the things what i want.

No need to assign a initial empty A 3d matix.

[Voss]

It's a good idea to clear (if in a script) or preallocate A before your loops.

Or avoid all of that and do:

A = Vf(312:324:end,:,:);

Demonstration:

Loop method

% Vf= is a 3d matix of 42444 * 1 * 1000;   
Vf = rand(42444,1,1000);
tic
m = size(Vf, 1);
n = size(Vf, 2);
p = size(Vf, 3);
for j = 1:p
    index = 1;
    for i = 312:324:m
        % Assign the value from Vf to the corresponding location in A
        A(index, :, j) = Vf(i, :, j);
        index = index + 1;
    end
end
toc

Elapsed time is 0.176245 seconds.

Direct method

tic
A_test = Vf(312:324:end,:,:);
toc

Elapsed time is 0.002738 seconds.

The result is the same

isequal(A,A_test)

ans = logical
   1

and the "Direct" method is much simpler code and about 65 times faster.