MATLAB Insanity

There are complete blogs devoted to the quirks of MATLAB. Anyways, here is another post on the topic and how some minor quirks produce a hard to find bug. Look at the following code:

rand(rngStreams{obj.rngId}, 2, 1)

It is supposed to produce two random numbers generated from a random number generation stream which has been initialized with a specific seed (to always get the same sequence). However, when debugging this code I realized that the numbers were generated from the default stream and not rngStreams{rngId}. Thus, effectively ignoring the seed used for initialization. Can you spot what may cause the problem?

Here it comes: Someone forgot to initialize obj.rngId. As it is declared as a property on an object there is no warning and the property gets initialized with a default value. So far I consider this an acceptable behavior. However, the default value is an empty matrix [] and this leads to some funny behavior together with the cell array rngStreams.

If you use an empty array to index a cell array, MATLAB will return … nothing?! Not null, none, nil or an empty cell array, but just nothing. And with nothing the code snippet above becomes:

rand(2, 1)

And that's the latest point in time where an error should be produced. Obviously, the writer of the code intended to call the function with three arguments. But whatever, if one of the arguments goes just missing what harm does it do? We just leave the program producing the wrong random numbers and the user thinking everything's fine!

The sad thing is that this kind of behavior with MATLAB is not a one time experience for me. I stumble again and again over similar behavior where some code seems to be working but is not. Moreover, all of these problems could be prevented without loss if MATLAB was stricter on the number of arguments passed to a function and types (i.e. returning an empty cell array instead of nothing).

Unfortunately it is not. So, use Python or R or whatever. But no MATLAB!