I've been learning Python lately, and although I'm loath to say so, it's a pretty sweet language. I do miss my semicolons, but I'll learn to do without; it'll never replace Perl for text munging, but then what will?
That said, I ran into some semi-unexpected behaviour today, and haven't found much by way of documentation. Python has an excellent construct for building strings, arrays, what have you by way of multiplication. For example:
>>> 'a'*3 'aaa' >>> *3 [1,1,1]
Sweet! I know it's old hat, and a crapton of other languages do it, but this is the first time I've gotten to use it in practice. It's epic for things like:
>>> x =  * 20
Creating an all-0 array is a snap!
Of course, if everything was peachy-keen, there wouldn't be much point in this. As a math guy, 0'd-out arrays are fine, but 0'd-out multi-dimensional arrays are even better. Let's say I want a matrix of 0's:
>>> x = [  * 3 ] * 3 >>> x [[0,0,0],[0,0,0],[0,0,0]]
Win! Alright, now I want to set the bottom-right corner to 1. Just because.
>>> x = 1 >>> x [[0,0,1],[0,0,1],[0,0,1]]
What? Fail. I'll readily admit to not being a member of the Python dev project, so this is pure speculation, but here's the guess: the
* operator works on values for fundamental types, including strings. For all other types, it operates on references. Thus constructing a 2d array as above, the first “row” is the same as the second is the same as the third, and updates barf the entire structure.
Until I find something better, it's time to go with the naive solution:
>>> x =  >>> for i in range(0,3): ... x.append(*3) ... >>> x = 1 >>> x [[0,0,0],[0,0,0],[0,0,1]]
Lesson learned. Drop suggestions if you have them!
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