MATLAB: How to avoid getting fooled by ‘implicit expansion’

implicit expansion

I am trying my very best not to be a grumpy old man here, but I just wasted the better part of an hour because of the addition of 'implicit expansion', and I need to hear from the proponents of this feature how to live with it. The offending bit of code was:
smoothed=smooth(EEG.data(iC,:),EEG.srate*60,'moving');
deviation=EEG.data(iC,:)-smoothed;
this bit of code kept giving me a 'memory error', which was odd, since 'deviation' should be a lot smaller than 'EEG'. However, EEG is very large, and I don't have room for two of them in my memory, so I figured there might be some intermediate step in calculation that was tripping me up, or perhaps windows was hogging the resources, or something else out of my control. It took two restarts of matlab, and one reboot of the pc, and finally a complicated rewrite of the script (which still didn't fix it) to finally realize that 'smooth' (which is a matlab built-in) was changing the dimensions, such that I was subtracting a column vector from a row vector.
What is the good coding practice for this not to occur? I would have thought that having a language that complained when you performed an ill-defined operation was the good solution to this problem, but I can see from my google search that apparently a lot of people think that 'implicit expansion' is a great good. How do you avoid pitfalls such as this? (please note that if I hadn't been running out of memory, I might have made it a good deal further down the script before noticing that something was off).
Should I just never trust any command to preserve the dimensions of my arrays, even if it's an inbuilt one?

Best Answer

 Should I just never trust any command to preserve the dimensions of my arrays,
 even if it's an inbuilt one?
The shapes of the output of built-in function have been subject to changes in the past and I expect this to happen in the future also. Therefore I catch my assumptions about shapes in a "unit-test" like function. So when I write
smoothed = smooth(EEG.data(iC,:), EEG.srate*60, 'moving');
I add this to the test function:
y = smooth(rand(1, 100), 5, 'moving');
if ~isrow
  error('SMOOTH does not reply a row vector for a row vector input.');
end
Nevertheless, it is a lot of work to do this for all assumptions. In many cases I'm not even aware of what I assume, e.g. for strncmp('hello', '', 2), which has changed its behavior in the past also.
In your case it would have been smart and efficient, if
deviation = EEG.data(iC,:) - smoothed
causes an error. Unfortunately the implicit expansion tries to handle this smartly, but it is smarter than the programmer in many cases. When it is intended, the implicit expansion is nice and handy, but it is an invitation for bugs also. All we can do is to live with it, because it is rather unlikely that TMW removes this feature. But I cannot be bad to write this as an enhancement request to them.
To answer the actual question:
 How do I avoid getting fooled by 'implicit expansion'?
Use Matlab < R2016b, at least for testing your code.
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