[Tutor] extract a submatrix

[Eike Welk]

Hello Bala!

On Sunday July 11 2010 23:41:14 Bala subramanian wrote:
> I have a
> matrix of size 550,550. I want to extract only part of this matrix say
>  first 330 elements, i dnt need the last 220 elements in the matrix. is
>  there any function in numpy that can do this kind of extraction.

I demonstrate it with a integer matrix of dimension (5, 10):

In [3]: a = array(range(50)).reshape(5,10)

In [4]: a
Out[4]:
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],
       [30, 31, 32, 33, 34, 35, 36, 37, 38, 39],
       [40, 41, 42, 43, 44, 45, 46, 47, 48, 49]])

In [5]: a[:, 0:7]
Out[5]:
array([[ 0,  1,  2,  3,  4,  5,  6],
       [10, 11, 12, 13, 14, 15, 16],
       [20, 21, 22, 23, 24, 25, 26],
       [30, 31, 32, 33, 34, 35, 36],
       [40, 41, 42, 43, 44, 45, 46]])

In [6]: a[:, 7:]
Out[6]:
array([[ 7,  8,  9],
       [17, 18, 19],
       [27, 28, 29],
       [37, 38, 39],
       [47, 48, 49]])


The colons ":" denote slices. In a 2D array you can have slices in two 
dimensions. in the first dimension (downwards) I always select all elements. 
A good explanation of slices is here:
http://tiny.cc/ohl2g
http://stackoverflow.com/questions/509211/good-primer-for-python-slice-
notation

A nice list of Numpy's many functions and methods is here: (This is the Numpy 
page I use most often.)
http://tiny.cc/qzwoq
http://www.scipy.org/Numpy_Example_List_With_Doc#head-11717acafb821da646a8db6997e59b820ac8761a

The funny prompt is from IPython (ipython --pylab), a program that enhances 
Python's interactive mode, and keeps Matplotlib graphs alive.


Eike.