.. Objectives

.. ----------

.. Prerequisites

.. -------------

..  Author: Anoop Jacob Thomas <anoop@fossee.in>

    Internal Reviewer   : Puneeth 

    External Reviewer   :

    Checklist OK?       : <put date stamp here, if OK> [2010-10-05]

===========================

Getting started with Arrays

===========================

.. #[Puneeth: Prerequisites and Objectives are missing. Fill them in]

{{{ show the welcome slide }}}

Welcome to the spoken tutorial on getting started with arrays.

{{{ switch to next slide, outline slide }}}

In this tutorial, we will learn about arrays, how to convert a list into an

array and also why an array is preferred over lists. And array operations.

.. #[Puneeth: Fix the grammar above.]

{{{ switch to next slide on overview of array }}}

Arrays are homogeneous data structures, unlike lists, arrays cannot have

heterogeneous data elements, that is, it can have only one type of data

type, either all integers, or strings, or float, and not a mix.

.. #[Puneeth: Use multiple short sentences, rather than one long sentence

   I would've written something like this. 

   Unlike lists, arrays are homogeneous data structures. They can have only

   type of data, ....]

Arrays are really fast in mathematical operations when compared to lists,

it is at least 80 to 100 times faster than lists.

.. #[Puneeth: For what size of an array is that the comparison?

{{{ switch to the next slide, creating arrays }}}

Now let us see how to create arrays.

I am assuming that you have your IPython interpreter running with the

``-pylab`` option, so that you have the required modules loaded.

.. #[Puneeth: 'I am assuming' doesn't sound right. Ask them to open if it

.. is not open?]

To create an array we will use the function ``array()`` as,

::

    a1 = array([1,2,3,4])

Notice that here we created a one dimensional array. Also notice the object

we passed to create an array. Now let us see how to create a two

dimensional array. Pause here and try to do it yourself before looking at

the solution.

.. #[Puneeth: I don't think this question can be solved by an average

.. viewer. Questions during the tutorial, should generally be to re-iterate

.. concepts learnt? ]

.. #[Puneeth: Also, you didn't even point out that we are converting a

.. list, using the ``array`` function. Bring the later section about

.. converting a list, here. A separate section is not necessary, IMHO.]

This is how we create two dimensional arrays.

::

    a2 = array([[1,2,3,4],[5,6,7,8]])

.. #[Puneeth: Again, you could explain a bit about the fact that we are

.. converting a list of lists.]

Let us see an easy method of creating an array with elements 1 to 8.

::

    ar = arange(1,9)

.. #[Puneeth: say, creating the same array as before. for some time I got

.. confused .]

And it created a single dimensional array of elements from 1 to 8.

::

    print ar

.. #[Puneeth: be consistent with voice. say, we obtained... or something.]

And how can we make it a two dimensional array of order 2 by 4. Pause here

and try to do it yourself, try ``ar.tab`` and find a suitable method for

that.

{{{ switch to next slide, reshape() method }}}

We can use the function ``reshape()`` for that purpose and it can be done

as,

::

    ar.reshape(2,4)

    ar.reshape(4,2)

    ar = ar.reshape(2,4)

{{{ switch to next slide, creating array from list}}}

Now, let us see how to convert a list object to an array. As you have

already seen, in both of the previous statements we have passed a list, so

creating an array can be done so, first let us create a list ``l1``

::

    l1 = [1,2,3,4]

Now we can convert the list to an array as, 

::

    a3 = array(l1)

{{{ switch to the next slide, problem statement of unsolved exercise 1 }}}

Create a three dimensional array of the order (2,2,4).

.. #[Puneeth: s/order/shape or size ?]

{{{ switch to the next slide, shape of an array }}}

To find the shape of an array we can use the object ``.shape``, let us

check the shape of the arrays we have created so far,

.. #[Puneeth: s/object/method ?]

::

    a1.shape

``a1.shape`` object is a tuple, and since a1 is a single dimensional array,

it returned a tuple (4,).

.. #[Puneeth: first show a 2D array, so that it becomes easier to explain.

.. Also, the word ``tuple`` need not be mentioned. ]

{{{ switch to the next slide, unsolved exercise 2 }}}

Find out the shape of the other arrays that we have created.

.. #[Puneeth: solution missing.]

{{{ Array can have only a single type of data }}}

.. #[Puneeth: I guess, this whole section can be skipped. If you want to

.. keep this, just briefly mention that arrays are homogeneous in the

.. intro, don't explain it there.]

Now let us try to create a new array with a mix of elements and see what

will happen,

::

    a4 = array([1,2,3,'a string'])

Well, we expected an error as previously I said that an array can have only

homogeneous elements, but it didn't give an error. Let us check the values

in the new array created. In your IPython terminal type, 

::

    a4

Did you notice it,

{{{ switch to next slide, implicit type casting }}}

.. #[Puneeth: typecasting may be unnecessary. (Also too advanced?) an

.. average guy wouldn't use arrays with strings.]

.. #[Puneeth: You may want to mention that float is the default dtype.]

{{{ highlight all the array elements one by one using mouse movements }}}

all the elements have been implicitly type casted as string, though our

first three elements were integers.

.. #[Puneeth: when I type a4 it says some ``dtype`` etc. I don't understand

.. what it is, can you explain? ;)]

{{{ switch to the next slide, identity & zeros methods }}}

.. #[Puneeth: something needs to motivate this. why are we suddenly talking

.. of an identity matrix?]

An identity matrix is a square matrix in which all the diagonal elements

are one and rest of the elements zero. We can create an identity matrix

using the method ``identity()``.

The function ``identity()`` takes an integer argument,

::

    identity(3)

As you can see the identity method returned a three by three square array

with all the diagonal elements as one and the rest of the elements as zero.

.. #[Puneeth: You say array here, matrix there -- it's a bit messed up.

.. Clarify, explicitly.]

``zeros()`` function accepts a tuple, which is the order of the array we

want to create, and it generates an array with all elements zero.

{{{ switch to the next slide, problem statement of solved exercise 1 }}}

Let us creates an array of the order four by five with all the elements

zero. We can do it using the method zeros, ::

    zeros((4,5))

Notice that we passed a tuple to the function zeros.

{{{ switch to next slide, learning exercise }}}

We learned two functions ``identity()`` and ``zeros()``, find out more

about the functions ``zeros_like()``, ``ones()``, ``ones_like()``.

{{{ switch to next slide, array operations }}}

Try the following, first check the value of a1,

::

    a1

``a1`` is a single dimensional array, and now try,

::

    a1 * 2

It returned a new array with all the elements multiplied by 2.

::

    a1

note that the value of a1 still remains the same.

Similarly with addition,

::

    a1 + 2

it returns a new array, with all the elements summed with two. But

again notice that the value of a1 has not been changed.

::

    a1

You may change the value of a1 by simply assigning the newly returned

array as,

::

    a1 += 2

Notice the change in elements of a,

::

    a

We can use all the mathematical operations with arrays, Now let us try this

::

   a1 = array([1,2,3,4])

   a2 = array([1,2,3,4])

   a1 + a2

Returns an array with element by element addition,

::

    a1 * a2

Returns an array with element by element multiplication, notice that it

does not perform matrix multiplication.

{{{ switch to next slide, summary slide }}}

So this brings us to the end of this tutorial, in this tutorial we covered

basics of arrays, how to create an array, converting a list to an array,

basic array operations etc.

.. #[Puneeth: s/how to create an array/creating an array]

{{{ switch to next slide, thank you }}}

Thank you!

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