Pre-allocating speed for an array that change size on every loop iteration

I have got a big matrix (1000x1000 for instance) and I want to get all the elements of the even columns. So I create that loop, but because of the lack of speed, it never gets finished.

Matrix = randi([0, 1], [1000,1000]);
[row,column]=size(Matrix);
VectorValue=[];
 
 for i=1:1:row
    for j=1:2:column   
        ValueEven=Matrix(i,j); %Get the value of actual even column
        VectorValue=[VectorValue, ValueEven];  %Put that value in an array of all the values of the even columns     
    end
 end

Answer 1

[Walter Roberson]

Matrix(:,2:2:end)

would give you a 1000 x 500 matrix extracted from the even columns. You could reshape() that if there was reason to do so.

[Alex]

Thank you, it works. But I have many other functions with that same issue and I would like to know how to solve it by Pre-allocating.

[Walter Roberson]

cols_out = floor(size(Matrix,2)/2);
output = zeros(size(Matrix,1), cols_out, 'like', Matrix);

for idx = 1:cols_out
    output(:,idx) = Matrix(:,idx*2);
end