MATLAB: Making a new variable

variables

newVar=[sum(first(:,1),sum(second(:,1),sum(third(:,1)];

numel()

What do you need it for?

There's no such thing as a dynamic array but you can grow an array with concatenation which is usually not recommended (or was not recommended).

If you have:

A = rand(10,1);

You can grow it as:

A = [A; rand(5,1)];

Such practice very often can be avoided.

Read the first 4 chapters of the getting started guide to see more examples of basi array operations.