A GNU Octave Feature

At work they recently moved me to a new project. It is a Matlab-based data analysis thing. I haven't really touched Matlab in over a year (the last time I used Matlab at work), and, instead, use GNU Octave at home when the language is appropriate. I got so used to Octave that I found a pretty critical feature missing from Matlab's implementation: treat an expression as if it were of the type of its output.

Let's say we want to index into the result of a function. Take, for example, the magic square function, magic(). This spits out a magic square of the given size. In Octave we can generate a 4x4 magic square and chop out the middle 2x2 portion in one line.

octave> magic(4)(2:3,2:3)
ans =

   11   10
    7    6

Or more possibly clearly,

octave> [magic(4)](2:3,2:3)
ans =

   11   10
    7    6

Try this in Matlab and you will get a big, fat error. You have to assign the magic square to a temporary variable to do the same thing. I kept trying to do this sort of thing in Matlab and was thinking to myself, "I know I can do this somehow!". Nope, I was just used to having Octave.

Where this really shows is when you want to reshape a matrix into a nice, simple vector. If you have a matrix M and want to count the number of NaN's it has, you can't just apply the sum() function over isnan() because it only does sums of columns. You can get around this with a special index, (:).

So, to sum all elements in M directly,

octave> sum(M(:))

In Octave, to count NaN's with isnan(),

octave> sum(isnan(M)(:))

Again, Matlab won't let you index the result of isnan() directly. Stupid. I guess the Matlab way to do this is to apply sum() twice.

Every language I can think of handles this properly. C, C++, Perl, Ruby, etc. It is strange that Matlab itself doesn't have it. Score one more for Octave.

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Chris Wellons

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