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--- other:python:jyp_steps [2016/01/29 15:41]
jypeter Added the numpy for matlab users references
+++ other:python:jyp_steps [2016/02/09 15:32]
jypeter Added the views side effect section
@@ Line 49: / Line 49: @@
 Where: [[http://docs.scipy.org/doc/|html and pdf documentation]]
-How to get started?
+==== Getting started ====
   - always remember that indices start at ''0'' and that the last element of an array is at index ''-1''!\\ First learn about //indexing// and //slicing// by manipulating strings, as shown in [[#part1|Part 1]] above (try '''This document by JY is awesome!'[::-1]'' and '''This document by JY is awesome!'[slice(None, None, -1)]'') 8-)
   - if you are a Matlab user (but the references are interesting for others as well), you can read the following:
@@ Line 59: / Line 60: @@
     - Numpy Reference Guide
     - Scipy Reference Guide
+==== Beware of the array view side effects ====
+<note warning>When you take a slice of an array, you get a **//View//** : an array that has a new shape but that still shares its data with the first array.
+That is not a problem when you only read the values, but **if you change the values of the //View//, you change the values of the first array** (and vice-versa)! If that is not what want, do not forget to **make a copy** of the data before working on it!
+//Views// are a good thing most of the time, so only make a copy of your data when needed, because otherwise copying a big array will just be a waste of CPU and computer memory. Anyway, it is always better to understand what you are doing... :-P
+Check the example below and the [[https://docs.scipy.org/doc/numpy-dev/user/quickstart.html#copies-and-views|copies and views]] part of the quickstart tutorial.
+<code python>
+>>> import numpy as np
+>>> a = np.arange(30).reshape((3,10))
+>>> a
+array([[ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9],
+       [10, 11, 12, 13, 14, 15, 16, 17, 18, 19],
+       [20, 21, 22, 23, 24, 25, 26, 27, 28, 29]])
+>>> b = a[1, :]
+>>> b
+array([10, 11, 12, 13, 14, 15, 16, 17, 18, 19])
+>>> b[3:7] = 0
+>>> b
+array([10, 11, 12,  0,  0,  0,  0, 17, 18, 19])
+>>> a
+array([[ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9],
+       [10, 11, 12,  0,  0,  0,  0, 17, 18, 19],
+       [20, 21, 22, 23, 24, 25, 26, 27, 28, 29]])
+>>> a[:, 2:4] = -1
+>>> a
+array([[ 0,  1, -1, -1,  4,  5,  6,  7,  8,  9],
+       [10, 11, -1, -1,  0,  0,  0, 17, 18, 19],
+       [20, 21, -1, -1, 24, 25, 26, 27, 28, 29]])
+>>> b
+array([10, 11, -1, -1,  0,  0,  0, 17, 18, 19])
+>>> c = a[1, :].copy()
+>>> c
+array([10, 11, -1, -1,  0,  0,  0, 17, 18, 19])
+>>> c[:] = 9
+>>> c
+array([9, 9, 9, 9, 9, 9, 9, 9, 9, 9])
+>>> b
+array([10, 11, -1, -1,  0,  0,  0, 17, 18, 19])
+>>> a
+array([[ 0,  1, -1, -1,  4,  5,  6,  7,  8,  9],
+       [10, 11, -1, -1,  0,  0,  0, 17, 18, 19],
+       [20, 21, -1, -1, 24, 25, 26, 27, 28, 29]])
+</code></note>
 ===== cdms2 and netCDF4 =====

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