[Numpy-discussion] Help Understanding Indexing Behavior

[Chris Barker]

On Wed, Feb 26, 2014 at 5:32 AM, Slavin, Jonathan
<jslavin at cfa.harvard.edu>wrote:

> This behavior is a property of python slicing.  It takes some getting used
> to, but has its advantages.
>

quite a few, actually!

They key with slicing is to think of the index as pointing to the space
between the elements:


0  1  2  3  4  5
   |   |   |   |   |   |

but the reason (and beauty) of this is that it results in a number of nifty
properties:


len( seq [i:j] ) == j-i

seq[i:j] + seq[j:k] == seq[i:k]

len( seq[:i] ) == i

len( seq[-i:] ) == i


and if you have an array representing a axis, for instance, and want to
know the value of a given index:

x = x_0 + i*dx

or the index of a given value:

i = (x - x_0) / dx

Notice that I don't have a single +1 or -1 in all of that -- this make sit
easier to undersand and a lot less error prone.

-Chris

-- 

Christopher Barker, Ph.D.
Oceanographer

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