PYTHON
LISTS
INTRODUCTION TO
LISTS
A list is a sequence of values (called elements) that can be any type.
Here are some examples of lists
[2,4,6,8,10]
[‘a’,’b’,’c’,’d’,’e’]
[‘bob’,23.0,145,[1,2]]
[‘hello’,’there’,’Bob’]
# This contains a string, float, int and
# another string. Each is an element
# We can put these in variables
nums = [3,2,5,4.5,3.0,1000]
names = [‘Bob’,’Sally’,’Tom’,’Harry]
empty = []
print names
#This is the empty string
 Check this out
>>>
['Bob', 'Sally', 'Tom', 'Harry']
>>>
LISTS ARE ORDERED
AND MUTABLE
#Unlike strings we can modify the individual elements of a
list.
numbers =[7,-2,3,4,5,6.0]
#list indices work like string
#indices
numbers[3]=10
print numbers
# we can print them
[7,-2,3,10,5,6.0]
#The in operator works here as well
>>>3 in numbers
True
TRAVERSING A LIST
for val in numbers:
7 -2 3 10 5 6.0
The length of the following list is
4.
s = [[1,2],3.0,’Harry’,[3,5,6,1,2]]
# Square each number in list
Of course the length of s[3] is 5
print val,
for I in range(len(numbers)):
numbers[i]=numbers[i]**2
print numbers
[49, 4, 9, 100, 25, 36.0]
The length of [] is 0
OPERATIONS ON LIST
+ works like it does on strings, i.e. it concatinates
[1,2,3]+ [4,5,6] becomes [1,2,3,4,5,6]
and
[1,2,3]*3 becomes [1,2,3,1,2,3,1,2,3]
What does 10*[0] give you?
[0,0,0,0,0,0,0,0,0,0]
LIST SLICES
t = [11,21,13,44,56,68]
print t[2,4]
[13,44]
# Remember: It doesn’t include slot 4!!
print t[3:]
[44,56,68]
# check this out
t[2:5]=[7,3,1] # You can update multiple elements
print t
[11,21,7,3,1,68]
LIST METHODS
# Append : adds a new element
# to the end
t=[2,4,6]
t.append(8)
print t
[2,4,6,8]
#extend: adds a list to end of list
t=[‘a’,b’,c’,d’]
t.extend([‘w’,’x’])
print t
[‘a’,b’,c’,d’,’w’,’x’]
#Sort: sorts the list in place
t = [4,3,5,2,7,1]
t.sort()
print t #t is modified!
[1,2,3,4,5,7]
ACCUMULATING A LIST
def add_them(t): # here t is a list of numbers
total =0
for x in t:
total += x
# same as total = total + x
return total
# Python already has something like this built in, called sum
total = sum(t)
#would return its sum
# of course you could write your own function to do anything
#you want to the elements of t
RETURNING A LIST
#What does the following do?
def do_it (s): # Here s is a list of strings
res =[]
for a in s:
res.append(s.capitalize())
return res
These guys traverse a list and
return parts of it in another list
#Returns a list of positive
#numbers
def get_pos(t):
res = []
for i in t:
if i>0:
res.append(i)
return res
DELETING FROM LIST
t=[6,3,7,8,1,9]
x=t.pop(3) #remove element in slot 3 and assign it to x
print t
[6,3,7,1,9]
#note : element in slot 3 ie 8 is now gone
---------------------------------------------------------------------------------------del t[3] #does the same thing as pop(3) but returns nothing
---------------------------------------------------------------------------------------t.remove(8) #use this to remove an 8 from the list
# it returns nothing
MATPLOTLIB
matplotlib.pyplot is a collection of command style functions
that make matplotlib work like MATLAB. Each pyplot function
makes some change to a figure: eg, create a figure, create a
plotting area in a figure, plot some lines in a plotting area,
decorate the plot with labels, etc....
import matplotlib.pyplot as plt
plt.plot([1,2,3,4])
plt.ylabel('some numbers')
plt.show()
from matplotlib.pyplot import *
plot([1,2,3,4])
ylabel('some numbers')
show()
Auto generates x values here!
PLOT
plot() is a versatile command, and will take an arbitrary
number of arguments. For example, to plot x versus y, you
can issue the command:
plt.plot([1,2,3,4], [1,4,9,16])
For every x, y pair of arguments, there is an optional third
argument which is the format string that indicates the color
and line type of the plot.
import matplotlib.pyplot as plt
plt.plot([1,2,3,4], [1,4,9,16], 'ro')
plt.axis([0, 6, 0, 20])
plt.show()
red circle
LINE AND TEXT PROPERTIES
import matplotlib.pyplot as plt
plt.plot([1,2,3,4], [1,4,9,16], linewidth=8.0 ) #thick line
plt.axis([0, 6, 0, 20]) #sets y axis values
plt.xlabel('This is the x axis', fontsize=14,color = 'red')
plt.ylabel('This is the y axis')
plt.title('Our Example Graph')
plt.show()
Text Properties
2d Line Properties
TWO GRAPHS
import matplotlib.pyplot as plt
# plot graphs
plt.plot([1,2,3,4,5], [1,4,9,16,25], linewidth = 4.0 , color='blue') #thick line
plt.plot([1,2,3,4,5],[1,2,3,4,5], linewidth = 4.0, color='red')
plt.axis([0, 6, 0, 30]) #sets y axis values
plt.xlabel('This is the x axis', fontsize = 14, color = 'green')
plt.ylabel('This is the y axis', fontsize = 14, color = 'green')
plt.title('Our Example with two Graphs', fontsize = 18, color = 'blue')
plt.show()
PROCESSING A LIST AND
GRAPHING USING MATPLOTLIB
import matplotlib.pyplot as plt
#Note x and y are lists
x= [-5,-4,-3,-2,-1,0,1,2,3,4,5]
y=[]
#build y from x , same size
for i in x:
y.append(i**2-2*i+3)
plt,plot(x,y)
plt.show()
Plot parameters are lists!
More points?
CSV FILES
Comma separated value files are very popular and it turns
out that python can easily process these guys. These are
quite simple involving nothing more than data separated by
commas.
Normally each line of a csv file has the same number of
values listed.
12,34,54,6,5
5,43,77,2,33
78,12,8,4,23
and so on.
We normally read each line of the file and
split it up using the commas as an
indication where to separate the values.
EXAMPLE
Suppose the past slides data is in file data.txt. The following
program will read in each line and print it out separated by
spaces (just to show it works)
file = open('data.txt','r')
for line in file:
nums = line.split(',') # nums is now a list of the values
for v in nums:
print v,
OUTPUT:
12 34 54 6 5
5 43 77 2 33
78 12 8 4 23