--- a/arrays.org Fri Apr 02 19:19:14 2010 +0530
+++ b/arrays.org Fri Apr 02 19:23:13 2010 +0530
@@ -1,16 +1,17 @@
* Arrays
*** Outline
***** Introduction
-******* Why do we want to do that?
-******* We shall use arrays (introduced before) for matrices
+******* 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
+******* Slicing??
+******* Striding??
***** {{Simple operations??}}
*** Script
Welcome to the Tutorial on arrays.
@@ -22,14 +23,18 @@
---
Let's start with creating simple arrays. We've already seen how to
- convert lists to arrays. Entering an array is similar to that.
+ convert lists to arrays. Inputting a new array is similar to that.
+
+ In []: a = array([5, 8, 10, 13])
- 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],
+ [21,22,23],
[31,32,33]])
To see what c is, we just type c in the prompt.
@@ -43,9 +48,9 @@
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 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. [is range covered?]
+ float arguments.
In []: a = arange(10)
@@ -53,33 +58,34 @@
This is the array we just created.
In []: a.shape
- Note that a is one dimensional and has 10 elements, as expected.
+ 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(number of elements) of new array must
- be unchanged.
+ Note that the total size of new array must be unchanged.
- We check re-shaped 'a' by
+ We type a, to see what it looks like
In []: a
- 'ones' function can be used to get an array with all the entries as
- 1s. We pass it the shape of the required array. For ex.
+ 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))
- b is 3(cross)4 array with all 1s
+ Look at b, by printing it out.
In []: b
- To create an array with all entries as ones, with a shape similar to
- an already existing array, we use the ones_like function.
+ 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.)
- {Do an up arrow and quickly show them a couple of examples?}
+ 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.
@@ -92,22 +98,20 @@
---
- Now that we have learnt how to create arrays, lets move on to
+ 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(second row
- third column), we say
+ 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.
- Additionally arrays provide a more convenient way to access the
- elements.
+ 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.
@@ -121,39 +125,57 @@
to access a single element.
In []: c[:,1], gives us the first column.
- The colon is a way to tell python to get all elements in that
- dimension from the array.
+ 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 a row of a
+ So, we could use a more explicit way to access the second row of
+ the array.
In []: c[1,:]
- We could use the colon to access specific portions of an array.
- In []: c[1,1:2]
- second column, from second row(1) till third(2) and excluding it
- In []: c[1:2,1]
- In []: c[1:2,1:2]
- ...
- [Oh, by the way this is termed as slicing. :)]
- {How many examples should we show here?}
+ 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 range command. As expected, the semi-colon notation also
- provides a way to specify a jump. This {concept/idea} is termed as
- Striding.
+ 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
+ say
In []: c[1::2,:]
In []: c[:,::2]
In []: c[::2,::2]
- {How many examples should we show here?}
---