Python deleted my vector values15 Feb 2010
Sometimes scripting languages can be a real annoyance. Why? Because when you get as much help as you do with for instance Python, you also lose a lot of control.
Being used to scripting languages like PHP, I made the funny mistake today of initializing a set of arrays like:
a = v = r = zeros((n,2),float)
This seemed like a really good idea, saving me from typing two extra lines(!). As the sucker I am for short code I was happy with my newfound shortcut. What I didn’t realize is that Python, in comparison to PHP, treats assignments like these as pointers instead of variables.
I believed this would create three arrays with a lot of zeros in two dimensions as I would expect from PHP, but the result was that I instead created one array with loads of zeros in two dimensions, with three pointers a, v and r all pointing to the same array.
When I then started setting the values for each of these arrays using Euler’s method, the result was that I got a lot of nonsense in what I thought was three separate arrays.
As a reminder to myself and everyone else out there; Python is not PHP. If you want to initialize three arrays like this in Python, you’ll have to stick to the long version:
a = zeros((n,2),float) r = zeros((n,2),float) v = zeros((n,2),float)
Or, you could at least save yourself from having to edit each assignment if you ever need to change the code by writing:
a = zeros((n,2),float) r = a.copy() v = a.copy()