Re: Variables....

[Tasos Papastylianou]

Γειά σου Τάσο!

The short answer is, yes. Any variable needs to be declared before you can use it.

To answer your question on why the matrix is not a 3 x 3 matrix in your scenario:
When you do global a, you force a variable a to appear on your base workspace, but it is given the value [] (i.e. an empty set). Note that this is not the same as zero!

Therefore, adding 2 to the empty set (i.e. [] + 2) still returns an empty set (since you're adding '2' to all elements of the empty set, which hasn't got any). This is different to saying 0 + 2 (i.e. [0] + 2) because in this cause you're adding '2' to all elements of the set; this time the set contains one element, which happens to be zero, so your result will not be an empty set now.

In your specific example:

octave-cli:1> a = []
a = [](0x0)
octave-cli:2> X = [(a+2) 3 4; 5 (a-4) 8; 3 2 (a-1)]
X =

   3   4
   5   8
   3   2

octave-cli:3> a = 0;
octave-cli:4> X = [(a+2) 3 4; 5 (a-4) 8; 3 2 (a-1)]
X =

   2   3   4
   5  -4   8
   3   2  -1

Hope this helps :)
Tasos.

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