--- a/arrays.txt Tue Apr 13 14:32:38 2010 +0530
+++ b/arrays.txt Wed Apr 14 12:20:26 2010 +0530
@@ -1,3 +1,208 @@
+Hello and welcome to the tutorial on Matrices.
+All matrices operations are done using arrays.
+We have already seen in previous session that how arrays are better suited for particular mathematical operations. In this session we are going to cover more details on Arrays, how to create/initialize them, how to manipulate and use them for solving given problem.
+
+First thing first, we start with creating a normal array by:
+a (equal to)= array([5, 8, 10, 13])
+
+and we have a as array. we can cross the content by
+a
+
+Here a is single dimension array, that is it has only one row. We can create multi-dimensional arrays by
+
+c = array([[11,12,13], [21,22,23], [31,32,33]])
+
+both c and a are arrays but with different dimensions or shape
+we can check shape of arrays by using shape attribute of arrays.
+a.shape
+c.shape
+
+some other handy array initialization methods are also available to make life easier.
+say we want to create a array of size 3x4 with all values initialized to be 1, we can use
+b = ones((3, 4))
+check value by
+b
+similarly, we already have a array, and we want to create one more array with same shape and initial values to be one, for that we will use
+d = ones_like(c)
+and d will be 3x3 array with all values 1
+
+Similarly there are functions like zeros and zeros_like which initialize array with all values being 0. One more useful function available is 'identity', it create identity matrix(array) of given order
+i = identity(3)
+i
+i = identity(5)
+i
+
+We have seen
+ Welcome to the Tutorial on arrays.
+
+ As mentioned in [the previous tutorial] arrays are much faster and
+ more efficient. In this tutorial we shall look at creating arrays,
+ accessing elements and changing them.
+
+ ---
+
+ Let's start with creating simple arrays. We've already seen how to
+ convert lists to arrays. Inputting a new array is similar to that.
+
+ In []:
+
+ Type /a/, to see what it is.
+
+ In []: a
+
+ We enter a multi-dimensional array this way -
+
+ In []: c = array([[11,12,13],
+ [21,22,23],
+ [31,32,33]])
+
+ To see what c is, we just type c in the prompt.
+
+ In []: c
+
+ To see the dimensions of the array c, we use c.shape
+ In []: c.shape
+
+ Now let us look at some special methods of creating an
+ array. There are various functions that allow us to create special
+ arrays.
+
+ The first one we shall look at is, /arange/. /arange/ is similar to
+ the range command, except that it returns an array and accepts
+ float arguments.
+
+ In []: a = arange(10)
+
+ In []: a
+ This is the array we just created.
+
+ In []: a.shape
+ Note that /a/ is one dimensional and has 10 elements, as expected.
+
+ We could also use a.shape to change the shape of the array a.
+ In []: a.shape = 2,5
+ Note that the total size of new array must be unchanged.
+
+ We type a, to see what it looks like
+ In []: a
+
+ ones command can be used to get an array with all the entries as
+ ones. We pass it the shape of the array that we require.
+
+ In []:
+
+ Look at b, by printing it out.
+ In []: b
+
+ To create an array with all entries as ones, with it's shape
+ similar to an already existing array, we use the ones_like
+ command.
+ In []: b = ones_like(a)
+
+ zeros and zeros_like are similar commands that can give you arrays
+ with all zeros. empty and empty_like give you empty arrays (arrays
+ with no initialization done.)
+
+ In []: b = zeros((3, 4))
+ In []: b = zeros_like(a)
+
+ The identity command can be used to obtain a square array with
+ ones on the main diagonal.
+
+ In []: identity(3)
+
+ To obtain a 2-D array, that is not necessarily square, eye command
+ can be used. Look at the documentation of eye (using eye?) for
+ more info.
+
+ ---
+
+ Now that we have learnt how to create arrays, let's move on to
+ accessing elements and changing them.
+
+ Let's work with the c, array which we had already created.
+
+ In []: c
+
+ Let's say we want to access the element 23 in c, we say
+
+ In []: c[1][2]
+ Note that this is similar to accessing an element inside a list of
+ lists. Also, note that counting again starts from 0.
+
+ But arrays provide a more convenient way to access the elements.
+ In []: c[1, 2]
+
+ Now, we can also change the element using a simple assignment.
+ In []: c[1, 2] = -23
+
+ Let's look at accessing more than one elements at a time. We begin
+ with accessing rows.
+ In []: c[1] gives us the second row. (counting starts from 0)
+
+ To get a column, we use a syntax that is similar to the one used
+ to access a single element.
+ In []: c[:,1], gives us the first column.
+
+ The colon specifies that we wish to obtain all elements in that
+ dimension from the array.
+
+ So, we could use a more explicit way to access the second row of
+ the array.
+ In []: c[1,:]
+
+ The colon can be used to access specific portions of the array,
+ similar to the way we do with lists.
+ In []: c[1,1:3]
+ Observe that we get the second and third columns from the second
+ row. As with lists, the number after the colon is excluded when
+ slicing a portion of the array.
+
+ In []: c[1:3,1]
+ Now, we get the second and third rows from the first column.
+
+ In []: c[1:3,1:3]
+ We get the second and third rows and the second and third
+ columns.
+
+ The numbers before and after the colons are optional. If the
+ number before the colon is omitted, it is assumed to be zero by
+ default. If the element after the colon is omitted, it is assumed
+ to be until the end.
+
+ In []: c[1:, 1:]
+ This is essentially similar to the previous example. We are using
+ the default value i.e, the end, instead of specifying 3,
+ explicitly.
+
+ In []: c[:2, :2]
+ We have omitted specifying the zero before the colon, explicitly.
+
+ ---
+
+ You may have observed the similarity of the semi-colon notation to
+ the notation used in lists. As expected, the semi-colon notation
+ also provides a way to specify a jump. This {concept/idea} is
+ termed as Striding.
+
+ To get every alternate row of c, starting from the first one, we say
+ In []: c[::2,:]
+
+ To get every alternate row of c, starting from the second one, we
+ say
+ In []: c[1::2,:]
+
+
+ In []: c[:,::2]
+ In []: c[::2,::2]
+
+ ---
+
+ We come to the end of this tutorial on arrays. In this tutorial,
+ you've learnt how to create arrays and access, change elements.
+
+ Thank you.
+
Hello friends and welcome to the second tutorial in the series of spoken tutorials on Python for Scientific computing.
In the previous tutorial we learnt about arrays and we told you that numpy arrays are faster and more efficient . In this tutorial we shall look at creating arrays, accessing elements and changing them.
@@ -5,7 +210,7 @@
Let's start with creating simple arrays. We've already seen how to convert lists to arrays. Inputting a new array is similarto that.
-On your Ipython terminal type a = array open parenthesis and then open square brackets 5,8,10,13 ,close square brackets and close parenthesis . This create an array a . You can see what a is by typing a on the terminal .
+On your Ipython terminal type a = array open parenthesis and then open square brackets 5,8,10,13 close square brackets and close parenthesis . This create an array a . You can see what a is by typing a on the terminal .
Now we will try to create a multi-dimensional array type in your ipython terminal
c= array open parenthesis , then open square brackets 11,12,13 close square bracket 'comma' start square bracket 21 , 22 ,23close square bracket 'comma' open 31,32,33 close square bracket another close square bracket which closes the first sqaure bracket and parenthesis which closes the first parenthesis . Now to see the dimensions of the array c we will do c.shape . We can see that c is a 3 by 3 matrix .
@@ -15,13 +220,4 @@
Ones can be use to get all entries as ones . We can pass it the shape of the array as required .
type b=ones open parenthesis , another open parenthesis , 3,4 , close second parenthesis and close first parenthesis . Look at b , by printing it out .
To create an array with all entries as ones, with it's shape similar to an already existing array, we use the ones_like
-command. type b= ones_like in parenthesis a .
-
-
-
-
-
-
-
-
-
+command. type b= ones_like in parenthesis a .