PYTHON
LISTS
CHAPTER 10
FROM
THINK PYTHON
HOW TO THINK LIKE A COMPUTER SCIENTIST
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
PROCESSING A LIST AND
GRAPHING USING MATPLOTLIB
from pylab import *
x= [-5,-4,-3,-2,-1,0,1,2,3,4,5]
y=[]
#build y from x
for i in x:
y.append(i**2-2*i+3)
print y
plot(x,y)
show()
Plots parameters are lists!