Entry

When using NumPy, how can I use an integer variable as an index for an array created using array()?

Nov 17th, 2007 09:34
Harry Parker, Ben, http://numpy.scipy.org/ http://numpy.scipy.org/numpydoc/numdoc.htm

A simple complete example:

In [1]: from numpy import array

In [2]: a = array([0,5,10,15,20])

In [3]: i = 2

In [4]: a[i]
Out[4]: 10


Following is a short excerpt from the "Array Basics" chapter of
"Numerical Python", found at http://numpy.scipy.org/numpydoc/numdoc.htm
, your best starting point for learning NumPy.  Just change every
reference of "numeric" in the sample code of this old document to
"numpy".  (See  http://numpy.scipy.org/  for the full story.)


Getting and Setting array values

Just like other Python sequences, array contents are manipulated with
the [] notation. For rank-1 arrays, there are no differences between
list and array notations:

>>> a = arrayrange(10)

>>> print a[0] # get first element

0

>>> print a[1:5] # get second through fifth element

[1 2 3 4]

>>> print a[-1] # get last element

9

>>> print a[:-1] # get all but last element

[0 1 2 3 4 5 6 7 8]

The first difference with lists comes with multidimensional indexing. If
an array is multidimensional (of rank > 1), then specifying a single
integer index will return an array of dimension one less than the
original array.

>>> a = arrayrange(9)

>>> a.shape = (3,3)

>>> print a

[[0 1 2]

[3 4 5]

[6 7 8]]

>>> print a[0] # get first row, not first element!

[0 1 2]

>>> print a[1] # get second row

[3 4 5]

To get to individual elements in a rank-2 array, one specifies both
indices separated by commas:

>>> print a[0,0] # get elt at first row, first column

0

>>> print a[0,1] # get elt at first row, second column

1

>>> print a[1,0] # get elt at second row, first column

3

>>> print a[2,-1] # get elt at third row, last column

8