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12.9) Array Indexing


Array indexing and slicing follows the same rules as for lists.

>>> a=np.array([3.3,2.4,5,-6.7,3])
>>> a[0]   # remember indexing starts at 0 in Python
3.3
>>> a[2]
5.0
>>> b=a[1:3]    # in slicing, the upper bound is excluded
>>> b
array([ 2.4,  5. ])

You can assign a new value to an element of an array, like you do with lists (i.e. arrays are mutable):

>>> a[3]=67.8
>>> a
array([  3. ,   2.4,   5. ,  67.8,   3.3])

How do you get the second from last element (-6.7) of a, where:

a=np.array([3.3,2.4,5,-6.7,3])

Which of the following answers is correct?

  1. a[2]
  2. a[-2]
  3. a[4]
  4. a[3]

Print out their values to see which ones are correct.