Constants in MATLAB

Question

I've come into ownership of a bunch of MATLAB code and have noticed a bunch of "magic numbers" scattered about the code. Typically, I like to make those constants in languages like C, Ruby, PHP, etc. When Googling this problem, I found that the "official" way of having constants is to define functions that return the constant value. Seems kludgey, especially because MATLAB can be finicky when allowing more than one function per file.

Is this really the best option?

I'm tempted to use / make something like the C Preprocessor to do this for me. (I found that something called mpp was made by someone else in a similar predicament, but it looks abandoned. The code doesn't compile, and I'm not sure if it would meet my needs.)

Answer 1

Matlab has constants now. The newer (R2008a+) "classdef" style of Matlab OOP lets you define constant class properties. This is probably the best option if you don't require back-compatibility to old Matlabs. (Or, conversely, is a good reason to abandon back-compatibility.)

Define them in a class.

classdef MyConstants
    properties (Constant = true)
        SECONDS_PER_HOUR = 60*60;
        DISTANCE_TO_MOON_KM = 384403;
    end
end

Then reference them from any other code using dot-qualification.

>> disp(MyConstants.SECONDS_PER_HOUR)
        3600

See the Matlab documentation for "Object-Oriented Programming" under "User Guide" for all the details.

There are a couple minor gotchas. If code accidentally tries to write to a constant, instead of getting an error, it will create a local struct that masks the constants class.

>> MyConstants.SECONDS_PER_HOUR
ans =
        3600
>> MyConstants.SECONDS_PER_HOUR = 42
MyConstants = 
    SECONDS_PER_HOUR: 42
>> whos
  Name             Size            Bytes  Class     Attributes

  MyConstants      1x1               132  struct              
  ans              1x1                 8  double              

But the damage is local. And if you want to be thorough, you can protect against it by calling the MyConstants() constructor at the beginning of a function, which forces Matlab to parse it as a class name in that scope. (IMHO this is overkill, but it's there if you want it.)

function broken_constant_use
MyConstants(); % "import" to protect assignment
MyConstants.SECONDS_PER_HOUR = 42 % this bug is a syntax error now

The other gotcha is that classdef properties and methods, especially statics like this, are slow. On my machine, reading this constant is about 100x slower than calling a plain function (22 usec vs. 0.2 usec, see this question). If you're using a constant inside a loop, copy it to a local variable before entering the loop. If for some reason you must use direct access of constants, go with a plain function that returns the value.

For the sake of your sanity, stay away from the preprocessor stuff. Getting that to work inside the Matlab IDE and debugger (which are very useful) would require deep and terrible hacks.