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-* Arrays
-*** Outline
-***** Introduction
-******* What do we want to do
-********* We shall use arrays (mentioned before) which
-********* shall be used for matrices in future
-******* Arsenal Required
-********* working knowledge of lists
-***** Initializing matrices
-******* Entering element-wise
-******* using special functions
-***** Accessing and Changing elements
-******* Slicing??
-******* Striding??
-***** {{Simple operations??}}
-*** Script
- 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 []: a = array([5, 8, 10, 13])
-
- 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 []: b = ones((3, 4))
-
- 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.
-
-*** Notes