# HG changeset patch
# User Santosh G. Vattam <vattam.santosh@gmail.com>
# Date 1269956855 -19800
# Node ID e5194abc864f32f9dc0b6979676f6637db8bcf9d
# Parent  9794cc4144987cdb1729fbef8b2c6c365e49446f# Parent  76fe6a48386f2b7bd4958ce4d360b2e35f1144ab
Branches merged.

diff -r 76fe6a48386f -r e5194abc864f statistics.txt
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/statistics.txt	Tue Mar 30 19:17:35 2010 +0530
@@ -0,0 +1,77 @@
+Hello welcome to the tutorial on statistics and dictionaries in Python.
+
+In the previous tutorial we saw the `for' loop and lists. Here we shall look into
+calculating mean for the same pendulum experiment and then move on to calculate
+the mean, median and standard deviation for a very large data set.
+
+Let's start with calculating the mean acceleration due to gravity based on the data from pendulum.txt.
+
+We first create an empty list `g_list' to which we shall append the values of `g'.
+In []: g_list = []
+
+For each pair of `L' and `t' values in the file `pendulum.txt' we calculate the 
+value of `g' and append it to the list `g_list'
+In []: for line in open('pendulum.txt'):
+  ....     point = line.split()
+  ....     L = float(point[0])
+  ....     t = float(point[1])
+  ....     g = 4 * pi * pi * L / (t * t)
+  ....     g_list.append(g)
+
+We proceed to calculate the mean of the value of `g' from the list `g_list'. 
+Here we shall show three ways of calculating the mean. 
+Firstly, we calculate the sum `total' of the values in `g_list'.
+In []: total = 0
+In []: for g in g_list:
+ ....:     total += g
+ ....:
+
+Once we have the total we calculate by dividing the `total' by the length of `g_list'
+
+In []: g_mean = total / len(g_list)
+In []: print 'Mean: ', g_mean
+
+The second method is slightly simpler. Python provides a built-in function called "sum()" that computes the sum of all the elements in a list. 
+In []: g_mean = sum(g_list) / len(g_list)
+In []: print 'Mean: ', g_mean
+
+The third method is the simplest. Python provides a built-in function `mean' that
+calculates the mean of all the elements in a list.
+In []: g_mean = mean(g_list)
+In []: print 'Mean: ', g_mean
+
+Python provides support for dictionaries. Dictionaries are key value pairs. Lists are indexed by integers while dictionaries are indexed by strings. For example:
+In []: d = {'png' : 'image',
+      'txt' : 'text', 
+      'py' : 'python'} 
+is a dictionary. The first element in the pair is called the `key' and the second 
+is called the `value'. The key always has to be a string while the value can be 
+of any type.
+
+Dictionaries are indexed using their keys as shown
+In []: d['txt']
+Out[]: 'text'
+
+In []: d['png']
+Out[]: 'image'
+
+The dictionaries can be searched for the presence of a certain key by typing
+In []: 'py' in d
+Out[]: True
+
+In []: 'jpg' in d
+Out[]: False
+Please note the values cannot be searched in a dictionaries.
+
+In []: d.keys()
+Out[]: ['py', 'txt', 'png']
+is used to obtain the list of all keys in a dictionary
+
+In []: d.values()
+Out[]: ['python', 'text', 'image']
+is used to obtain the list of all values in a dictionary
+
+In []: d
+Out[]: {'png': 'image', 'py': 'python', 'txt': 'text'}
+Please observe that dictionaries do not preserve the order in which the items
+were entered. The order of the elements in a dictionary should not be relied upon.