# Using a Variable between different functions

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Gözde Üstün on 21 Jun 2020
Commented: Gözde Üstün on 25 Jun 2020
Hello,
I have this two function. evAlice and evBob were defined in CHSH_ but I need to access themin basis function. But I could not know how can I access to evAlice and evBob in basis fucntion.
function [coin, I] = CHSH_(sigma_x,sigma_z)
ev1_of_alice = sigma_x;
ev2_of_alice = sigma_z;
ev1_of_bob = sigma_x+sigma_z;
ev2_of_bob = sigma_x-sigma_z;
evAlice=[sigma_x,sigma_z];
evBob=[sigma_x+sigma_z,sigma_x-sigma_z];
... %function is too long so I just put the relevant part
end
function [basis_mat] = basis(measurement_setting)
% Choose the correct basis depending on the
% measurement setting. If you change the observables,
% these are obtained by diagonalization
switch measurement_setting
case evAlice(0)
basis_mat = [1, 1; 1,-1]/sqrt(2);
case evAlice(1)
basis_mat = [1, 0; 0, 1];
case evBob(0)
basis_mat = [ 1+sqrt(2), 1-sqrt(2); 1, 1];
case evBob(1)
basis_mat = [-1-sqrt(2),-1+sqrt(2); 1, 1];
end
end

the cyclist on 21 Jun 2020
Edited: the cyclist on 21 Jun 2020
You can pass them out of the first function:
function [coin, I, evAlice, evBob] = CHSH_(sigma_x,sigma_z)
and into the second one by calling it like this
basis(measurement_setting, evAlice, evBob)
and defining it as
function [basis_mat] = basis(measurement_setting, evAlice, evBob)
Alternatively, you could define those variables as global in scope, in both functions, but that is generally a bad idea.

Show 1 older comment
the cyclist on 21 Jun 2020
It's really up to you, based on what you think makes the code simpler and easier to understand in the long run.
You might want to google "global variable bad idea matlab" and do a little reading, before you decide. :-)
Stephen Cobeldick on 22 Jun 2020
"Instad of that can I use global variable or is this better ?"
Global variables: slower, more buggy, harder to debug, obfuscate the code intent.
Input/output variables: faster, easier to debug, recommended by the MATLAB documentation as "best practice":
If you have lots of arguments then put them into a structure and pass that.
Gözde Üstün on 25 Jun 2020
Thank you very very much : -) @the cyclist and @Stephen Cobeldick

R2020a

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