* Arrays

*** Outline

***** Introduction

******* Arsenal Required

********* working knowledge of lists

***** Initializing matrices

******* Entering element-wise

******* using special functions

***** Accessing and Changing elements

***** {{Simple operations??}}

*** Script

    Welcome to the Tutorial on arrays. 

    As mentioned in [the previous tutorial] arrays are much faster and

    more efficient. In this tutorial we shall look at creating arrays,

    accessing elements and changing them. 

    ---

    Let's start with creating simple arrays. We've already seen how to

    We enter a multi-dimensional array this way -

    In []: c = array([[11,12,13],

                      [31,32,33]])

    To see what c is, we just type c in the prompt. 

    In []: c

    To see the dimensions of the array c, we use c.shape

    In []: c.shape 

    Now let us look at some special methods of creating an

    array. There are various functions that allow us to create special

    arrays. 

    the range command, except that it returns an array and accepts

    In []: a = arange(10)

    In []: a

    This is the array we just created. 

    In []: a.shape

    We could also use a.shape to change the shape of the array a. 

    In []: a.shape = 2,5

    In []: a

    In []: b = ones((3, 4))

    In []: b 

    In []: b = ones_like(a)

    zeros and zeros_like are similar commands that can give you arrays

    with all zeros. empty and empty_like give you empty arrays (arrays

    with no initialization done.)

    The identity command can be used to obtain a square array with

    ones on the main diagonal. 

    In []: identity(3)

    To obtain a 2-D array, that is not necessarily square, eye command

    can be used. Look at the documentation of eye (using eye?) for

    more info. 

    ---

    accessing elements and changing them. 

    Let's work with the c, array which we had already created. 

    In []: c 

    In []: c[1][2]

    Note that this is similar to accessing an element inside a list of

    lists. Also, note that counting again starts from 0. 

    In []: c[1, 2]

    Now, we can also change the element using a simple assignment. 

    In []: c[1, 2] = -23

    Let's look at accessing more than one elements at a time. We begin

    with accessing rows. 

    In []: c[1] gives us the second row. (counting starts from 0)

    To get a column, we use a syntax that is similar to the one used

    to access a single element. 

    In []: c[:,1], gives us the first column. 

    In []: c[1,:]

    --- 

    You may have observed the similarity of the semi-colon notation to

    To get every alternate row of c, starting from the first one, we say

    In []: c[::2,:]

    To get every alternate row of c, starting from the second one, we

    In []: c[1::2,:]

    In []: c[:,::2]

    In []: c[::2,::2]

    ---

    We come to the end of this tutorial on arrays. In this tutorial,

    you've learnt how to create arrays and access, change elements. 

    Thank you. 

*** Notes