Undefined variable error while the variable was indeed defined right before the use

I am sorry to ask this again, but the previous answer was not even helpful.

My question is basically the same as describe here (see link) , but the answer did not explain things well and did not provide realization. I did try to add function handler, not working at all.

Attached code and error below:

%%Calculations
%To and Tw are previsouly imported tables
To.v = To.L / To.T; % unit = m/s
Tw.v = Tw.L / To.T; % unit = m/s
eta_oil = 2.675E-10* exp(5975.4/(16+273))
eta_water = 0.0011076 %unit = Pa*s
rho_oil = 0.878E3 % kg/m^3
rho_water = 1E3 %kg/m^3
rho_n = 1.13E3
rho_b = 8.45E3

To.Re = calcRe(To,0)

function Re = calcRe(T,type)
% Re = Dp*Vp*rho_f/eta_f
if type==0 % oil
    Re = T.D .* T.v * rho_oil/eta_oil
else
    Re = T.D .* T.v * rho_water/eta_water
end
end

The error is Undefined function or variable 'rho_oil'.

Workspace has eta_oil shown. I have no idea why the "snapshot" does not take eta_oil. Oh by the way, I also tried to declare the variable as global. Cannot resolve with effort. Please help ;-;

[madhan ravi]

 _I have no idea why the "snapshot" does not take eta_oil._

What does that mean?

[Kunhuan Liu]

Oh the term snapshot was mentioned in the answer for the previous question (see link).

Answer 1

[Stephen23]

@Kunhuan Liu: the reason is very simple: every function has its own workspace with its own variables:

https://www.mathworks.com/help/matlab/matlab_prog/base-and-function-workspaces.html

Variables do NOT magically jump from one workspace to another workspace: to get variables into the function workspace, you will need to do this explicitly, e.g. by defining them as input/output arguments. There are some exceptions to this, e.g. anonymous functions and nested functions. The answer you linked to refers to an anonymous function, which is unrelated to the code that you show in your question. But you could certainly use an anonymous function in your code:

To.L = 1;
To.T = 1;
To.D = 1;
Tw.L = 1;
To.v = To.L / To.T; % unit = m/s
Tw.v = Tw.L / To.T; % unit = m/s
eta.oil   = 2.675E-10*exp(5975.4/(16+273));
eta.water = 0.0011076; %unit = Pa*s
rho.oil   = 0.878E3; % kg/m^3
rho.water = 1E3; %kg/m^3
rho_n = 1.13E3;
rho_b = 8.45E3;

Define and call and anonymous function:

>> fun = @(T,type)T.D .* T.v * rho.(type)/eta.(type);
>> fun(To,'oil')
ans =  3440.7

Or you could define a normal function and explicitly pass the required variables:

function Re = calcRe(T,type,rho,eta)
% Re = Dp*Vp*rho_f/eta_f
Re = T.D .* T.v * rho.(type)/eta.(type);
end

and then simply call it with those inputs:

>> calcRe(To,'oil',rho,eta)
ans =  3440.7

Do NOT use global variables. The best practice is to use input/output arguments or nested functions (as my answer shows):

https://www.mathworks.com/help/matlab/matlab_prog/share-data-between-workspaces.html

[Kunhuan Liu]

I see. So the scope of a function in Matlab is such that it does not access any variables outside of its own area. That will explain everything. Thanks!