On Tue, Feb 25, 2014 at 6:01 PM, Daniele Nicolodi <dani...@grinta.net>wrote:
> On 26/02/2014 00:04, JB wrote: > > At the risk of igniting a flame war...can someone please help me > understand > > the indexing behavior of NumPy? I will readily I admit I come from a > Matlab > > background, but I appreciate the power of Python and am trying to learn > more. > > > >>From a Matlab user's perspective, the behavior of indexing in NumPy seems > > very bizarre. For example, if I define an array: > > > > x = np.array([1,2,3,4,5,6,7,8,9,10]) > > > > If I want the first 5 elements, what do I do? Well, I say to myself, > Python > > is zero-based, whereas Matlab is one-based, so if I want the values 1 - > 5, > > then I want to index 0 - 4. So I type: x[0:4] > > The Python slicing syntax a:b defines the interval [a, b), while the > Matlab syntax defines the interval [a:b]. > > This post from Guido van Rossum (the creator of Python) explains the > choice of zero indexing and of this particular slice notation: > > https://plus.google.com/115212051037621986145/posts/YTUxbXYZyfi > > I actually find how Python works more straight forward: obtaining the > first n elements of array x is simply x[:n], and obtaining n elements > starting at index i is x[i:i+n]. > > To enlarge just a bit, as said, python indexing comes from C, Matlab indexing comes from Fortran/Matrix conventions. If you look at how Fortran compiles, it translates to zero based under the hood, starting with a pointer to memory one location before the actual array data, so C just got rid of that little wart. Chuck
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