A
list is an ordered set of values, where each value is identified by an index.
The values that make up a list are called its elements. Lists are similar to
strings, which are ordered sets of characters, except that the elements of a
list can have any type.
Lists are the most
versatile sequence type. The elements of a list can be any object, and lists
are mutable  they can be changed. Elements can be reassigned or removed, and
new elements can be inserted.
Creating Lists
To create a list, place a (possibly empty)
commaseparated list of objects or expressions in square brackets. For example,
the following initializes a list with three elements:
L1=
[10, 20, 30]
L2=["spam",
"bungee", "swallow"]
The
lists L1 contains 3 integer elements and the list L2 contains 3 strings.
The elements of lists can be any objects (even expressions,
functions or other lists!), and they need not all be the same type. The example
below, for instance, contains a list with different data tape.
L=[10,2.3,3+4j,'djd',3*4]
The following list contains a string, a float, an
integer, and another list. A list within another list is said to be nested.
L=["hello", 2.0, 5, [10, 20]]
The nested list can be used to represent a matrix.
A=[[1,2,3],[4,5,6],[7,8,9]]
will
create a nested list A( which can be used as a matrix)
print
A[0] will print [1,2,3]
print
A[1][2] will print 6.
The
following command will create empty lists
L1=list()
# using built in list function
L2=[]
The
range command generate a list of integers
>>range(1,5)
[1,2,3,4]
Using other lists also we can create a new list
L2=L1[:]
# here L2 is created as copy of L1
L2=L1[2:6]
# here L2 is created with 2,3,4 and 5 elements of L1.
Python supports a concept called "list comprehensions". It can be used to
construct lists in a very natural, easy way, like a mathematician is used to
do.
This will create a list of even numbers less than 100.
[ i for i in
range(100) if i%2==0]
More Examples of List Comprehensions
>>> S = [x**2 for x in range(10)]
>>> V = [2**i for i in range(13)] >>> M = [x for x in S if x % 2 == 0] >>> print S; print V; print M

Accessing List Elements
To access the i'th element of a list, use the list name
followed by the index in square brackets.
Note that lists in Python are zeroindexed, so the
first element is at index 0, the second is at index 1, and so on. Negative
indices wrap around, so 1 is the last element, 2 is the penultimate element,
etc
Example:
L
= [10, 20, [60, 70, 80], 30]
>>>L[2] >>>L[1] >>>L[1] >>>L[3] >>>L[5/2] >>>L[2][2]
[60,70,80] [20] [30] [20] [60,70,80] 80
Note: We can use an expression for index. But only integer values
are permitted for index.
Slice operator works on list also. We know
that a slice of a list is its sublist.
>>>L[1:4] >>>L[2:] >>>L[:3] >>>L[::2]
[20,[60,70,80],30] [[60,70,80],30] [10,20,[60,70,80]] [10,[60,70,80]]
List Traversal
We can also access
elements of a list by using looping constructs while and for. For is more
efficient. If we are using while we should also know the length of the list,
which can be found by the sequence function len().
Example:
i=0 for i in L:
while i <len(L): print i,
print
L[i],
i+=1
List
Operations
The + operation
concatenate lists
>>>A=[10,20,30]
>>>B=[40,50,60,70,80]
>>>A+B
[10,20,30,
40,50,60,70,80]
The * operator repeats a
list given number of times.
>>>
[1, 2, 3] * 3
[1, 2, 3, 1, 2, 3, 1, 2, 3]
Slicing
operator[::] can be used to extract elements from a list
>>>B=[40,50,60,70,80]
>>>B[2:5]
[60,70,80]
Membership
of an element in a list can be tested with in
and not in
>>>B=[40,50,60,70,80]
>>>40
in B
True
>>>100
in B
False
>>>100
not in B
True
Changing/Modifying
List elements
The
list is a mutable sequence. It means that we can change the list elements.
To
change the i’th element simply assign a new value to the i’th index
>>>L=[10,20,30,40]
>>>L[2]=100
# will set the 3^{rd} element to 100 in the list L.
>>>L
[10,20,100,40]
>>>L[1]=200
# will change the last element to 200.
>>>L
[10,20,100,200]
We
can also use slicing to change more than one value using another list
>>>L[2:4]=[111,222]
>>>L
[10,20,111,222]
Adding
Elements to the List
One
way to add elements to the list are by using concatenation (+ operator).We can
concatenate an element or a list to the beginning or at the end.
>>>L=[20,30,40]
>>>L+[50]
[20,30,40,50]
>>>[10]+L
[10,20,30,40]
We
can also use the append() method to
add elements at the end of the list.
>>>L=[10,20,30]
>>>L.append(40)
>>>L.append(50)
>>>L
[10,20,30,40,50]
We
can also use insert() method to
insert element at a particular position.
>>>L.insert(0,5)
# will insert 5 before index 0
>>>L
[5,10,20,30,40,50]
>>>L.insert(1,100)
>>>L
[5,10,20,30,40,100,50]
The
list can be extended with more elements by extend()
method
>>>L=[10,20,30]
>>>L.extend([40,50,60])
>>>L
[10,20,30,40,50,60]
Deleting
Elements from a List
pop() method
can be used to take the last element from a list. When the list is empty it
will return index error.
>>>L=[10,20,30]
>>>L.pop()
30
>>>L
[10,20]
The
method remove() will remove the
specified element from the list. If the element is not present in the list, it
will return value error.
>>>L=[10,20,30]
>>>L.remove(20)
>>>L
[10,30]
The
del() function can also be used to
delete an element from a list, if you don’t know the element to delete. It uses
the index to delete the element.
>>>L=[10,20,30,40]
>>>del(L[1])
>>>del(L[1])
>>>L
[10,30]
We
can use slicing with del() to delete
more elements
>>>L=[10,20,30,40]
>>>del(L[1:3])
>>>L
[10,40]
>>>L=[10,20,30,40]
>>>del(L[1:3:1])
>>>L
[10,20]
Searching
and Sorting
The
index() method can be used to find
the position of an element in a list. If the element is not present it will
return a value error.
Note:
we can use ‘in’ to check whether an element is present in the list or not.
>>>L=[10,20,30,40]
>>>L.index(30)
2
The
count() method returns number of
occurrences of an element in a list.
>>>L=[10,20,30,30,40]
>>>L.count(30)
2
>>>L.count(100)
0
The
sort() function can be used to sort
the list in ascending or descending order
>>>
L=[10, 30, 40]
>>>
L.sort()
>>>L
[30,
10, 40]
>>>
L.sort(reverse=True)
>>>
L
[40,
10, 30]
When
you sort a list containing strings, ASCII ordering is used
>>>
L=["binu","anu","biju"]
>>>
L.sort()
>>>
L
['anu',
'biju', 'binu']
>>>
L.sort(reverse=True)
>>>
L
['binu',
'biju', 'anu']
We
can also specify a function as key so that the list will be sorted based on
that function. The following example sort the list elements based on the length
of the string.
>>>
L=['binu','anu','bimal']
>>>
L.sort(key=len)
>>>
L
['anu',
'binu', 'bimal']
>>>
L=['binu','anu','bimal']
>>>
L.sort(key=len,reverse=True)
>>>
L
['bimal',
'binu', 'anu']
It
is noted that when you use sort() method the original list is modified. We can
use sorted() method to create a new sorted list without changing the original.
L=['bimal',
'binu', 'anu']
>>>
sorted(L)
['anu',
'bimal', 'binu']
>>>
sorted(L,reverse=True)
['binu',
'bimal', 'anu']
>>>
sorted(L,reverse=True,key=len)
['bimal',
'binu', 'anu']
Other
useful functions
len()
function will return the number of elements in the list.
>>>
L=[10,20,30,40]
>>>
len(L)
4
min()
and max() will return the minimum
and maximum element
>>>
L=[10,20,30,40]
>>>
min(L)
10
>>>
max(L)
40
sum()
function will return the sum of the elements.
>>>
L=[10,20,30,40]
>>>
sum(L)
100
We
can find average of the list elements L by sum(L)/len(L)
>>>
sum(L)/len(L)
25
Strings
and Lists
We
can use split method in the string module to split a string and create a list
of words.
Now
you can process these list using the list functions.
>>>
s="this is a test"
>>>
w=s.split(' ')
>>>w
['this',
'is', 'a', 'test']
>>>
"_".join(w) # words are joined with ‘_’ character.
'this_is_a_test'
Aliasing
and Cloning List
When
we assign one list to another variable it creates an alias of the list
object.It means that it will not create a separate list object.
>>>a=[10,20,30]
>>>b=a
# this will create an alias
We
can test the object id using the function id()
to test that they refer to the same object.
>>>
id(a)
35033696
>>>
id(b) # note that the ids are same
35033696
>>>a[2]=100
>>>b
[10,20,100]
If
you want to clone a list , ie; if we need a copy of the list, slicing can be
used. This will create a new object. Note that the ids of the objects are
different.
>>>
b=a[::]
>>>
b
[10,
20, 30]
>>>
a
[10,
20, 30]
>>>
id(a)
35033696
>>>
id(b)
35034096
Now you can try the following
programs using Lists
1.Read list of numbers and check whether the given element in
found in the list. If found display the positions else display the message
element is not found.( index() method only find the first occurrence. use a
loop)
2.Count the number of occurrences of an element in a list. ( use
count() method or use a loop)
3.Read a
list and remove the first and last element .Create a new list( use del(), pop()
and slicing)
4. Write program to read list of names and print the names in
sorted order.(use sort())
5. Read list of numbers and print the list in reverse sorted
order.(use sort())
6. Program to find the sum of all even numbers in a group of ‘n’
numbers entered by the user. (university question)
7. Write a Python program to count the number of –ve terms and
zeros in a given set of n numbers (university question)
8. Write a Python program to input a list of ‘n’ numbers.
Calculate and display the average of ‘n’ numbers. Also display cube of each
value in the list.(university question)
9.Read a list of ‘n’ numbers and find the smallest and largest
among them.(use min() and max())
10.Find the second largest and second smallest element from a
list.( use sort().min(),max())
11.Read list of numbers and find the median.(refer example)
12.Read list of numbers and find
the standard deviation.
13.Find the duplicate elements from a list.( use count())
14.Read a string and print the words in alphabetical order ( use
split() and sort())
15.Read a string and print each words and its length.Also print number
of words.(use split() and len())
16.Read a string and remove all occurrence of a word ( use split()
and join())
17.Read a string and replace all occurrence of a word with another
(use split() and join())
18.Read a string and sort the words according to the length of the
words.
19.Read list of names and print the sorted list in uppercase
letters.
20.Remove all duplicate elements from a list.( refer example)
21.Read a string and remove all duplicate words.
22.Read a list and remove the duplicate elements completely. ( refer example)
23.Read a list and create a new list from this after removing all occurrences of a given element.
24.Remove the smallest and largest element from the list.
25.Students marks of an exam is stored as a list. Create two list of marks from this.( passed and failed, less than 50% is failed). Also find the number of passed and failed students.Average marks of passed students,Top scorer and lowest mark scorer.
22.Read a list and remove the duplicate elements completely. ( refer example)
23.Read a list and create a new list from this after removing all occurrences of a given element.
24.Remove the smallest and largest element from the list.
25.Students marks of an exam is stored as a list. Create two list of marks from this.( passed and failed, less than 50% is failed). Also find the number of passed and failed students.Average marks of passed students,Top scorer and lowest mark scorer.
26.write a python program to input a sentence and find the number of words in the sentence and print the words in uppercase.Also print the number of question marks(?), periods(.) and commas(,) present in the string. ( university question)
27.Write a python program to input a list of 'n' numbers.Calculate and display the sum of cubes each value in the list. ( university question)
27.Write a python program to input a list of 'n' numbers.Calculate and display the sum of cubes each value in the list. ( university question)
Sample Programs
Program to read list of names and sort the list in alphabetical
order.( university question)
n=input("Enter the number of names....")
names=[]
print "Enter {} names".format(n)
for i in range(n):
nam=raw_input()
names.append(nam)
names.sort()
print "names in alphabetical order"
for nam in names:
print nam
Program to find the sum of all even numbers in a group of n
numbers entered by the user. (university question)
n=input("Enter the number of elements...")
print "Enter the {} elements".format(n)
l=[] # creating an empty list
for i in range(n):
x=input()
l.append(x)
s=0
for i in range(n):
if l[i]%2==0:
s=s+l[i]
print "Sum of all even numbers",s
Note:you can also enter a list by using square brackets.
Eg: L=input(“Enter a list..”)
[10,20,30] # give the input like a list.
This will create a list L.
Program to read a string and remove the given words from the
string.
s=raw_input("Enter the string....")
wr=raw_input("Enter the word to remove....")
wrds=s.split(" ")
ns=""
for w in wrds:
if w!=wr:
ns=ns+" "+w
print "new string...",ns
Program to read list of numbers and find the median
#We can find median by sorting the list and then take the middle element.
If the list contains even number of elements take the average of the two middle
elements.
n=input("Enter how many numbers....")
print "Enter {} numbers....".format(n)
ls=[]
for i in range(n):
x=input()
ls.append(x)
ls.sort()
print ls
mid=n/2
if n%2==1:
print "Median",ls[mid]
else:
print
"median",(ls[mid]+ls[mid1])/2.0
Program to remove all duplicate elements from a list
lst=[]
n=input("enter how many numbers..")
print "Enter elements..."
for i in range(n):
x=input()
lst.append(x)
nlst=[]
for x in lst:
if x not in nlst:
nlst.append(x)
print "new list after removing duplicates"
print nlst
The simple way to remove duplicate elements from a list is to
convert into a set and then convert back to list
lst=[]
n=input("enter how many numbers..")
print "Enter elements..."
for i in range(n):
x=input()
lst.append(x)
nlst=list(set(lst))
print "new list after removing duplicates"
print nlst
This
program will completely remove duplicate elements without keeping any copy.
l=input('Enter
list with duplicates...')
nl=list(set(l))
# list with only one copy
nlwd=[]
# new list without duplicates
for i
in nl:
if l.count(i)==1: #non duplicate element
nlwd.append(i)
print
"New list after removing duplicates completely..."
print
nlwd
Matrix Implementation
using Lists
We
can implement matrix operation using list. Matrix operation can be implemented
using
nested list. List inside another list is called nested list.
Consider 2x2 matrix A A=[[1,2],[3,4]]
We can write a nested for loop to iterate and print the list in
matrix form
for i in range(2):
for j in range(2):
print A[i][j],”
“,
print
Nested for loop can also be used to read the mxn matrix elements
as a list.Here nested list is initialized with 0 after reading the number of
rows (m)and number of columns(n) before reading the elements.
The following
program will read a m x n matrix as list and print it in the matrix form.
m=input("Enter
the number of rows..")
n=input("Enter
the number of columns..")
A=[[0
for c in range(n)]for r in range(m)] #initializing a nested list
print
"Enter the elements of each row line by line.."
for
r in range(m):
for c in range(n):
A[r][c]=input()
print
"The Matrix is....."
for
r in range(m):
for c in range(n):
print A[r][c]," ",
print
The
following program will find the sum of two matrix A+B=C
m=input("Enter the number of
rows..")
n=input("Enter the number of
columns..")
A=[[0 for c in range(n)]for r in
range(m)]
B=[[0 for c in range(n)]for r in
range(m)]
C=[[0 for c in range(n)]for r in
range(m)]
print "Enter the elements of each
row of matrix A line by line.."
for r in range(m):
for c in range(n):
A[r][c]=input()
print "Enter the elements of
each row of matrix B line by line.."
for r in range(m):
for c in range(n):
B[r][c]=input()
for r in range(m):
for c in range(n):
C[r][c]=A[r][c]+B[r][c]
print "The Matrix A
is....."
for r in range(m):
for c in range(n):
print A[r][c]," ",
print
print "The Matrix B
is....."
for r in range(m):
for c in range(n):
print B[r][c]," ",
print
print "The sum C=A+B...."
for r in range(m):
for c in range(n):
print C[r][c]," ",
print
The
above program is written efficiently using functions
def
readmatrix(X):
for r in range(m):
for c in range(n):
X[r][c]=input()
def
printmatrix(X):
for r in range(m):
for c in range(n):
print X[r][c]," ",
print
def
findsum(A,B,C):
for r in range(m):
for c in range(n):
C[r][c]=A[r][c]+B[r][c]
m=input("Enter
the number of rows..")
n=input("Enter
the number of columns..")
A=[[0
for c in range(n)]for r in range(m)]
B=[[0
for c in range(n)]for r in range(m)]
C=[[0
for c in range(n)]for r in range(m)]
print
"Enter the elements of each row of matrix A line by line.."
readmatrix(A)
print
"Enter the elements of each row of matrix B line by line.."
readmatrix(B)
findsum(A,B,C)
print
"The Matrix A is....."
printmatrix(A)
print
"The Matrix B is....."
printmatrix(B)
print
"The sum C=A+B...."
printmatrix(C)
The following program will multiply
two square matrix A*B=C
def
readmatrix(X):
for r in range(m):
for c in range(n):
X[r][c]=input()
def
printmatrix(X):
for r in range(m):
for c in range(n):
print X[r][c]," ",
print
def
findprod(A,B,C):
for i in range(m):
for j in range(n):
for k in range(n):
C[i][j]=C[i][j]+A[i][k]*B[k][j]
m=input("Enter
the number of rows..")
n=input("Enter
the number of columns..")
A=[[0
for c in range(n)]for r in range(m)]
B=[[0
for c in range(n)]for r in range(m)]
C=[[0
for c in range(n)]for r in range(m)]
print
"Enter the elements of each row of matrix A line by line.."
readmatrix(A)
print
"Enter the elements of each row of matrix B line by line.."
readmatrix(B)
findprod(A,B,C)
print
"The Matrix A is....."
printmatrix(A)
print
"The Matrix B is....."
printmatrix(B)
print
"The Product C=A*B...."
printmatrix(C)
Try the following matrix programs
using nested lists
1.Find
the sum of two matrices.
2.Find
the difference of two matrices.
3.Find
the product of two matrices.
4.Find
the transpose of a matrix.
5.Print the diagonal elements and find the
trace(sum of diagonal elements).
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