st-scripts: comparison getting_started_with

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-:88a01948450d
+:d33698326409
+.. Objectives
+.. ----------
+.. At the end of this tutorial, you will be able to
+.. 1. Create arrays using data
+.. #. Create arrays from lists
+.. #. Basic array operations
+.. #. Creating identity matrix using ``identity()`` function.
+.. #. Learn about ``zeros()``, ``zeros_like()``, ``ones()``,
+``ones_like()`` functions.
+.. Prerequisites
+.. -------------
+..   1. should have ``ipython`` and ``pylab`` installed.
+..   #. getting started with ``ipython``.
+..   #. getting started with lists.
+..  Author: Anoop Jacob Thomas <anoop@fossee.in>
+Internal Reviewer   : Puneeth
+External Reviewer   :
+Language Reviewer   : Bhanukiran
+Checklist OK?       : <11-11-2010,Anand, 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 the data structure called an array, how to convert
+a list into an array, operations on arrays and also why an array is preferred
+to lists.
+.. #[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, they can have only one type of data
+as their entries, be them all integers, strings, or maybe floats, but 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 of a given length are comparatively much faster in mathematical
+operations than lists of the same length, because of the fact that they are
+homogeneous data structures.
+.. #[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.
+Run your IPython interpreter with ``-pylab`` option, to load the required
+modules to work with arrays.
+{{{ take terminal and run the following command }}}
+::
+ipython -pylab
+.. #[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 we created a one dimensional array here. Also notice the object
+we passed to create an array. We passed a list 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.
+{{{ switch to next slide, creating two dimensional arrays }}}
+.. #[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.]
+We create two dimensional array by converting a list of lists to an array
+as,
+::
+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.]
+Now let us use ``arange()`` function to create the same array as before.
+::
+ar = arange(1,9)
+.. #[Puneeth: say, creating the same array as before. for some time I got
+.. confused .]
+And we obtained a one dimensional array with 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 shape (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 method ``.shape``, let us
+check the shape of the arrays we have created so far,
+.. #[Puneeth: s/object/method ?]
+::
+a2.shape
+``a2.shape`` object is a tuple, and it returned a tuple (2, 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.]
+It can be done as,
+::
+a1.shape
+a3.shape
+ar.shape
+{{{ 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 would expect an error as it has been previously mentioned that arrays handle
+elements with the same datatype, but it didn't raise 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 strings, though our
+first three elements were meant to be 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?]
+Now let us see how to create an identity matrix of a given size, that is a
+two-dimensional array  in  which all the diagonal elements are ones and rest of the
+elements are zeros. We can create an identity matrix using the function
+``identity()``.
+The function ``identity()`` takes an integer argument which specifies the
+size of the desired matrix,
+::
+identity(3)
+As you can see the identity function returned a three by three square matrix
+with all the diagonal elements as ones and the rest of the elements as zeros.
+.. #[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 that we
+want to create, and it generates an array with all elements as zeros.
+{{{ 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, learned how to create an array, saw how to convert a list
+to an array, and basic array operations etc.
+.. #[Puneeth: s/how to create an array/creating an array]
+{{{ switch to next slide, thank you }}}
+Thank you!
+..
+Local Variables:
+mode: rst
+indent-tabs-mode: nil
+sentence-end-double-space: nil
+fill-column: 75
+End: