MATLAB Answers

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

Asked by Jim Riggs
on 17 Sep 2019
Latest activity Edited by Jim Riggs
on 29 Sep 2019 at 0:08
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.
Re = LibraryConstant('Earth_Radius');
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?

  1 Comment

Two very good suggestions.
Thanks to @James Tursa and @Steven Lord

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2 Answers

Answer by Steven Lord
on 17 Sep 2019
 Accepted Answer

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

  6 Comments

Ah, I suspect you're using a release that predates the ability to define string arrays using double-quotes.
Ah yes. I have run into that issue before here in the forum.
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.

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Answer by James Tursa
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
e.we_units = 'radians/sec';
e.flatinv = 298.257223563;
:
end
I put the units as strings so that simply displaying the structure will show me what the units are.

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