What is the best way to insure that all of my functions are using the same constant values?

Jim Riggs (view profile)

on 17 Sep 2019
Latest activity Edited by Jim Riggs

Jim Riggs (view profile)

on 29 Sep 2019 at 0:08

Steven Lord (view profile)

I want to make sure that my functions are all using the same values for physical constants, like earth radius, elipsoidal flattening, etc. and avoid hard-coding a bunch of constant values in each function.
Some people advocate using a function that returns constant values, e.g.
f = LibraryConstant('Ellipsoid_flattening');
ev = LibraryConstant('electron_volt');
...
The function "LibraryConstant" is a big case-select structure that returns the requested value.
Or, you might define a bunch of global constants, but this seems like an undesirable approach.
What about creating a structure or table that contains all of the constants - this would have to be passed as an additinal argumernt to each function.
What would you recommend as an efficient method?

Jim Riggs

Jim Riggs (view profile)

on 18 Sep 2019
Two very good suggestions.
Thanks to @James Tursa and @Steven Lord

Steven Lord (view profile)

on 17 Sep 2019

classdef myconstants
properties(Constant)
g = 9.8;
g_units = 'm/s/s';
c = 299792458;
c_units = 'm/s';
end
end
This is similar to James Tursa's struct based approach, but with one major difference: you don't need to call a function to create the struct in your workspace. You just reference the property with the name.
>> x = myconstants.c
x =
299792458
You can even give your properties help text. [This is documented in the documentation for the help function.]
classdef myconstants
properties(Constant)
% g is the gravity of Earth
% (https://en.wikipedia.org/wiki/Gravity_of_Earth)
g = 9.8;
g_units = 'm/s/s';
% c is the speed of light in a vacuum
c = 299792458;
c_units = 'm/s';
end
end
>> help myconstants.g

Steven Lord

Steven Lord (view profile)

on 19 Sep 2019
Ah, I suspect you're using a release that predates the ability to define string arrays using double-quotes.
Jim Riggs

Jim Riggs (view profile)

on 19 Sep 2019
Ah yes. I have run into that issue before here in the forum.
Jim Riggs

Jim Riggs (view profile)

on 29 Sep 2019 at 0:00
In addition to the "doc" feature that I commented on above, I have just discovered that the auto complete function works seamlessly with the class. So, using my example of the UnitConversion class, if I type
UnitConversion.s <tab>
I get a drop-down menu of all class properties that begin with "s": {slug2lbm | slug2kg | slug2g | slug2grn } This also is a great feature of using classes.
Also, typing
UnitConversion. <tab>
will bring up a list of every property of the class. These features will be a big help in finding the value that I want.

James Tursa (view profile)

on 17 Sep 2019

I use a function that returns a structure, containing the values and the unit descriptions. Your code can either pass this structure around, or call the function. E.g.,
function e = earth
e.re = 6378.137; % equatorial radius
e.re_units = 'km';
e.we = 7.2921150e-5; % rotation rate