getting-started-with-arrays/script.rst
author Nishanth <nishanth@fossee.in>
Sun, 10 Oct 2010 15:49:44 +0530
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.. 4.1 LO: getting started with arrays (2) [anoop] 
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.. ------------------------------------------------
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.. * why arrays 
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..   + speed - simply say 
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..   + array level operations 
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.. * creating arrays 
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..   + direct data 
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..   + list conversion 
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..   + homogeneous 
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..   + builtins - identitiy, zeros, 
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.. * array operations 
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..   + =+ - * /= 
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===========================
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Getting started with Arrays
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===========================
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{{{ show the welcome slide }}}
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Welcome to the spoken tutorial on getting started with arrays.
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{{{ switch to next slide, outline slide }}}
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In this tutorial, we will learn about arrays, how to convert a list
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into an array and also why an array is preferred over lists. And array
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operations.
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{{{ switch to next slide on overview of array }}}
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Arrays are homogeneous data structures, unlike lists, arrays cannot
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have heterogeneous data elements, that is, it can have only one type
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of data type, either all integers, or strings, or float, and not a
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mix.
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Arrays are really fast in mathematical operations when compared to
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lists, it is at least 80 to 100 times faster than lists.
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{{{ switch to the next slide, creating arrays }}}
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Now let us see how to create arrays.
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I am assuming that you have your IPython interpreter running with the
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``-pylab`` option, so that you have the required modules loaded.
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To create an array we will use the function ``array()`` as,
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::
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    a1 = array([1,2,3,4])
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Notice that here we created a one dimensional array. Also notice the
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object we passed to create an array. Now let us see how to create a
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two dimensional array. Pause here and try to do it yourself before
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looking at the solution.
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This is how we create two dimensional arrays.
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    a2 = array([[1,2,3,4],[5,6,7,8]])
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Let us see an easy method of creating an array with elements 1 to 8.
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::
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    ar = arange(1,9)
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And it created a single dimensional array of elements from 1 to 8.
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::
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    print ar
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And how can we make it a two dimensional array of order 2 by 4. Pause
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here and try to do it yourself, try ``ar.tab`` and find a suitable
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method for that.
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We can use the function ``reshape()`` for that purpose and it can be
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done as,
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::
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    ar.reshape(2,4)
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    ar.reshape(4,2)
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    ar = ar.reshape(2,4)
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Now, let us see how to convert a list object to an array. As you have
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already seen, in both of the previous statements we have passed a
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list, so creating an array can be done so, first let us create a list
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``l1``
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::
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    l1 = [1,2,3,4]
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Now we can convert the list to an array as,
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    a3 = array(l1)
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{{{ switch to the next slide, problem statement of unsolved exercise 1 }}}
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Create a three dimensional array of the order (2,2,4).
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{{{ switch to the next slide, shape of an array }}}
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To find the shape of an array we can use the object ``.shape``, let us
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check the shape of the arrays we have created so far,
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::
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    a1.shape
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``a1.shape`` object is a tuple, and since a1 is a single dimensional
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array, it returned a tuple (4,).
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{{{ switch to the next slide, unsolved exercise 2 }}}
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Find out the shape of the other two arrays that we have created.
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{{{ Array can have only a single type of data }}}
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Now let us try to create a new array with a mix of elements and see
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what will happen,
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::
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    a4 = array([1,2,3,'a string'])
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Well, we expected an error as previously I said that an array can have
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only homogeneous elements, but it didn't give an error. Let us check
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the values in the new array created. In your IPython terminal type,
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::
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    a4
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Did you notice it, 
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{{{ highlight all the array elements one by one using mouse 
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movements }}}
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all the elements have been implicitly type casted as string, though
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our first three elements were integers.
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{{{ switch to the next slide, identity & zeros methods }}}
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An identity matrix is a square matrix in which all the diagonal
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elements are one and rest of the elements zero. We can create an
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identity matrix using the method ``identity()``.
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The function ``identity()`` takes an integer argument,
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    identity(3)
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As you can see the identity method returned a three by three square
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array with all the diagonal elements as one and the rest of the
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elements as zero.
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``zeros()`` function accepts a tuple, which is the order of the array
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we want to create, and it generates an array with all elements zero.
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{{{ switch to the next slide, problem statement of the solved exercise
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1 }}}
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Let us creates an array of the order four by five with all the
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elements zero. We can do it using the method zeros,
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::
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    zeros((4,5))
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Notice that we passed a tuple to the function zeros.
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{{{ switch to next slide, learning exercise }}}
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We learned two functions ``identity()`` and ``zeros()``, find out more
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about the functions ``zeros_like()``, ``ones()``, ``ones_like()``.
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{{{ switch to next slide, array operations }}}
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Try the following, first check the value of a1,
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::
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    a1
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``a1`` is a single dimensional array, and now try,
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::
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    a1 * 2
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It returned a new array with all the elements multiplied by 2.
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::
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    a1
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note that the value of a1 still remains the same.
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Similarly with addition,
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::
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    a1 + 2
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it returns a new array, with all the elements summed with two. But
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again notice that the value of a1 has not been changed.
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::
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    a1
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You may change the value of a1 by simply assigning the newly returned
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array as,
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::
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    a1 += 2
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Notice the change in elements of a,
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    a
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We can use all the mathematical operations with arrays, Now let us try
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this
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::
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   a1 = array([1,2,3,4])
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   a2 = array([1,2,3,4])
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   a1 + a2
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Returns an array with element by element addition,
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::
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    a1 * a2
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Returns an array with element by element multiplication, notice that
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it does not perform matrix multiplication.
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{{{ switch to next slide, recap slide }}}
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So this brings us to the end of this tutorial, in this tutorial we
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covered basics of arrays, how to create an array, converting a list to
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an array, basic array operations etc.
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{{{ switch to next slide, thank you }}}
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Thank you!
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..  Author: Anoop Jacob Thomas <anoop@fossee.in>
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    Reviewer 1:
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    Reviewer 2:
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    External reviewer: