diff -r 25b4e962b55e -r c7f0069d698a getting-started-with-arrays/script.rst --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/getting-started-with-arrays/script.rst Sat Oct 09 03:56:06 2010 +0530 @@ -0,0 +1,242 @@ +.. 4.1 LO: getting started with arrays (2) [anoop] +.. ------------------------------------------------ +.. * why arrays +.. + speed - simply say +.. + array level operations +.. * creating arrays +.. + direct data +.. + list conversion +.. + homogeneous +.. + builtins - identitiy, zeros, +.. * array operations +.. + =+ - * /= + +=========================== +Getting started with Arrays +=========================== + +{{{ 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. + +{{{ 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. + +Arrays are really fast in mathematical operations when compared to +lists, it is at least 80 to 100 times faster than lists. + +{{{ 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. + +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. + +This is how we create two dimensional arrays. +:: + + a2 = array([[1,2,3,4],[5,6,7,8]]) + +Let us see an easy method of creating an array with elements 1 to 8. +:: + + ar = arange(1,9) + +And it created a single dimensional array of elements from 1 to 8. +:: + + print ar + +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. + +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) + +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). + +{{{ 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, +:: + + a1.shape + +``a1.shape`` object is a tuple, and since a1 is a single dimensional +array, it returned a tuple (4,). + +{{{ switch to the next slide, unsolved exercise 2 }}} + +Find out the shape of the other two arrays that we have created. + +{{{ Array can have only a single type of data }}} + +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, + +{{{ 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. + +{{{ switch to the next slide, identity & zeros methods }}} + +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. + +``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 the 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, recap 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. + +{{{ switch to next slide, thank you }}} + +Thank you! + +.. Author: Anoop Jacob Thomas + Reviewer 1: + Reviewer 2: + External reviewer: