Cannot pre-allocate an empty matrix?

Hello, I'd like to create a matrix that increases with every loop by concatenation the matrix to itself, starting from an empty matrix as shown below

Q1s    = [];
alphas = [];

for y = ymin:0.5:ymax 

    x = (xmin:2.5e-13:xmax)';

    Q1s    = vertcat(Q1s,x);
    alphas = vertcat(alphas,ones(size(x))*y);
end

Now I'm getting the famous "consider pre-allocating for speed" message. But I need Q1 and alpha to be empty matrices to begin with because I'm using vertcat. Is there a way to go about this?

Answer 1

[Stephen23]

There are many ways:

1. tell the editor to ignore the warning: right click, "suppress ... on this line"
2. use a preallocated cell array, concatenate after the loop.
3. figure out the final size (the total number of elements you require is just numel(x)*numel(y)).
4. use simpler, more efficient code:

[Q1s,alphas] = ndgrid(xmin:2.5e-13:xmax,ymin:0.5:ymax);
Q1s = Q1s(:);
alphas = alphas(:);

Those three lines give the exactly same output as your code, yet avoids all the bother of loops and expanding arrays and all that jazz.