author | Puneeth Chaganti <punchagan@fossee.in> |
Wed, 13 Oct 2010 12:52:11 +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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.. Objectives |
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.. ---------- |
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.. Clearly state the objectives of the LO (along with RBT level) |
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.. Prerequisites |
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.. ------------- |
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.. 1. Name of LO-1 |
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.. 2. Name of LO-2 |
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.. 3. Name of LO-3 |
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.. Author: Anoop Jacob Thomas <anoop@fossee.in> |
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Internal Reviewer : Puneeth |
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External Reviewer : |
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Checklist OK? : <put date stamp here, if OK> [2010-10-05] |
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=========================== |
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Getting started with Arrays |
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=========================== |
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.. #[Puneeth: Prerequisites and Objectives are missing. Fill them in] |
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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 into an |
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array and also why an array is preferred over lists. And array operations. |
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.. #[Puneeth: Fix the grammar above.] |
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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 have |
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heterogeneous data elements, that is, it can have only one type of data |
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type, either all integers, or strings, or float, and not a mix. |
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.. #[Puneeth: Use multiple short sentences, rather than one long sentence |
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I would've written something like this. |
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Unlike lists, arrays are homogeneous data structures. They can have only |
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type of data, ....] |
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Arrays are really fast in mathematical operations when compared to lists, |
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it is at least 80 to 100 times faster than lists. |
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.. #[Puneeth: For what size of an array is that the comparison? |
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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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.. #[Puneeth: 'I am assuming' doesn't sound right. Ask them to open if it |
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.. is not open?] |
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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 object |
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we passed to create an array. Now let us see how to create a two |
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dimensional array. Pause here and try to do it yourself before looking at |
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the solution. |
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.. #[Puneeth: I don't think this question can be solved by an average |
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.. viewer. Questions during the tutorial, should generally be to re-iterate |
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.. concepts learnt? ] |
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.. #[Puneeth: Also, you didn't even point out that we are converting a |
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.. list, using the ``array`` function. Bring the later section about |
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.. converting a list, here. A separate section is not necessary, IMHO.] |
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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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.. #[Puneeth: Again, you could explain a bit about the fact that we are |
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.. converting a list of lists.] |
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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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.. #[Puneeth: say, creating the same array as before. for some time I got |
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.. confused .] |
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And it created a single dimensional array of elements from 1 to 8. |
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print ar |
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.. #[Puneeth: be consistent with voice. say, we obtained... or something.] |
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And how can we make it a two dimensional array of order 2 by 4. Pause here |
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and try to do it yourself, try ``ar.tab`` and find a suitable method for |
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that. |
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{{{ switch to next slide, reshape() method }}} |
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We can use the function ``reshape()`` for that purpose and it can be done |
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as, |
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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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{{{ switch to next slide, creating array from list}}} |
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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 list, so |
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creating an array can be done so, first let us create a list ``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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.. #[Puneeth: s/order/shape or size ?] |
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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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.. #[Puneeth: s/object/method ?] |
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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 array, |
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it returned a tuple (4,). |
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.. #[Puneeth: first show a 2D array, so that it becomes easier to explain. |
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.. Also, the word ``tuple`` need not be mentioned. ] |
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{{{ switch to the next slide, unsolved exercise 2 }}} |
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Find out the shape of the other arrays that we have created. |
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.. #[Puneeth: solution missing.] |
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{{{ Array can have only a single type of data }}} |
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.. #[Puneeth: I guess, this whole section can be skipped. If you want to |
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.. keep this, just briefly mention that arrays are homogeneous in the |
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.. intro, don't explain it there.] |
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Now let us try to create a new array with a mix of elements and see what |
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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 only |
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homogeneous elements, but it didn't give an error. Let us check the values |
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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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{{{ switch to next slide, implicit type casting }}} |
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.. #[Puneeth: typecasting may be unnecessary. (Also too advanced?) an |
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.. average guy wouldn't use arrays with strings.] |
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.. #[Puneeth: You may want to mention that float is the default dtype.] |
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{{{ highlight all the array elements one by one using mouse movements }}} |
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all the elements have been implicitly type casted as string, though our |
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first three elements were integers. |
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.. #[Puneeth: when I type a4 it says some ``dtype`` etc. I don't understand |
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.. what it is, can you explain? ;)] |
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{{{ switch to the next slide, identity & zeros methods }}} |
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.. #[Puneeth: something needs to motivate this. why are we suddenly talking |
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.. of an identity matrix?] |
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An identity matrix is a square matrix in which all the diagonal elements |
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are one and rest of the elements zero. We can create an identity matrix |
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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 array |
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with all the diagonal elements as one and the rest of the elements as zero. |
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.. #[Puneeth: You say array here, matrix there -- it's a bit messed up. |
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.. Clarify, explicitly.] |
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``zeros()`` function accepts a tuple, which is the order of the array we |
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want to create, and it generates an array with all elements zero. |
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{{{ switch to the next slide, problem statement of solved exercise 1 }}} |
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Let us creates an array of the order four by five with all the elements |
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zero. We can do it using the method zeros, :: |
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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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:: |
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a |
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We can use all the mathematical operations with arrays, Now let us try 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 it |
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does not perform matrix multiplication. |
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{{{ switch to next slide, summary slide }}} |
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So this brings us to the end of this tutorial, in this tutorial we covered |
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basics of arrays, how to create an array, converting a list to an array, |
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basic array operations etc. |
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.. #[Puneeth: s/how to create an array/creating an array] |
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{{{ switch to next slide, thank you }}} |
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Thank you! |
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.. |
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Local Variables: |
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mode: rst |
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indent-tabs-mode: nil |
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sentence-end-double-space: nil |
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fill-column: 75 |
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End: |