Operations on Python Lists

Operations on Python Lists

It is possible to perform operations like addition, multiplication and searching on Python Lists. 

Addition Operation on Python Lists

Addition operation on Lists is possible using + operator:

>>> list1 = [1, 2] 
>>> list1
[1, 2]

>>> list2 = [3, 4] 
>>> list2
[3, 4]

>>> list1 + list2
[1, 2, 3, 4]

Multiplication Operation on Python Lists

Multiplication operation on Lists is possible using * operator:

>>> mylist = [1, 2] 
>>> mylist
[1, 2]

>>> mylist * 3 
[1, 2, 1, 2, 1, 2]

Note: The + and * operations do not modify the list. The original list will remain same even after use of + and * symbols on it. 

Search operation in Python Lists

in keyword can be used to search an element in the given list.

>>> insurance = ['sbi', 'icici', 'hdfc', 'hsbc'] 
>>> insurance
['sbi', 'icici', 'hdfc', 'hsbc']

>>> 'sbi' in insurance 
True

>>> 'seas' in insurance 
False

Assignment operation to Slices

Use of assignment operator to slices is possible. It can even change the size of the list or clear it entirely:

>>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']

>>> letters

['a', 'b', 'c', 'd', 'e', 'f', 'g']

# replace some values of lists

>>> letters[2:5] = ['C', 'D', 'E']

>>> letters

['a', 'b', 'C', 'D', 'E', 'f', 'g']

# remove elements from list

>>> letters[2:5] = []

>>> letters

['a', 'b', 'f', 'g']

# clear the list by replacing all the elements with an empty list

>>> letters[:] = []

>>> letters

[]

Built-in Functions for Python Lists

Built-in Functions for Python Lists

len( ) function with List

len ( ) finds the number of items or elements in a list. Following code will return the length of the list - insurance. 

>>> insurance = ['sbi', 'icici', 'hdfc', 'hsbc'] 
>>> print(len(insurance))
4

min() and max() functions with Lists

Mininum and Maximum value can be identified using min() and max() function from the lists as shown below:

>>> list1 = [1, 2, 5, 10, 8] 
>>> print(min(list1))
1

>>> print(max(list1))
10

Built-in Methods for Python Lists

Python Lists - Built-in Methods

Let's learn some of the top built-in methods available for Python - lists.

Using append() with List

append() function can be used to add new items at the end of the list:

>>> cubes = [1, 8, 27, 64, 125]

>>> cubes

[1, 8, 27, 64, 125]

# append 216 at the end

>>> cubes.append(216)

# append cube of 7 at the end

>>> cubes.append(7 ** 3)

>>> cubes

[1, 8, 27, 64, 125, 216, 343]

Using insert() with List

insert() function will add element to a list at specified position(index number). Check the following example:

>>> list1 = ['a', 'b', 'c']
>>> list1.insert(2, 'kp')
>>> list1
['a', 'b', 'kp', 'c']

Using extend() with List

Python extend() function combines two lists. Check the following example:

>>> myList1 = ['a', 'b', 'c']
>>> myList2 = [1, 2, 3]
>>> myList1.extend(myList2)
>>> print(myList1)
['a', 'b', 'c', 1, 2, 3]

Using pop() and remove() with List

pop() function will get the value of an item and remove it from the given list. pop() function needs index number of a list as a parameter. Check example of pop():

>>> list1 = ['a', 'b', 'c', 'd']

>>> list1

['a', 'b', 'c', 'd']

>>> list1.pop(2)

'c'

>>> list1

['a', 'b', 'd']

Using remove() with List

remove() function deletes an item from a list. remove() function needs actual value of an item as a parameter. Check following example of remove():

>>> list1 = ['a', 'b', 'c', 'd']

>>> list1.remove('d')

>>> list1

['a', 'b', 'c']

Using reverse() with List 

Reverse the elements of a given list.

>>> list1 = ['a', 'b', 'c', 'd']

>>> list1

['a', 'b', 'c', 'd']

>>> list1.reverse()

>>> list1

['d', 'c', 'b', 'a'] 

Using sort() with List 

Sort the elements of a given list alphabetically or numerically.

>>> list1 = ['c', 'b', 'a', 'd']

>>> list1

['c', 'b', 'a', 'd']

>>> list1.sort()

>>> list1

['a', 'b', 'c', 'd']

Python Lists

In Python, list is one of the basic data structure useful to store multiple data items. 

• Python list is the compound data types which can be used to group values. 
• List is a kind of collections in Python. 
• The list can be written as a list of comma-separated values (items) between square brackets. 
• Lists might contain items with different data types. Let’s use Python’s List.

Creating a Python's List

Following example will create a list of characters, numbers and words.

# List of characters

>>> myList = ['a', 'b', 'c', 'd', 'e']
>>> myList
['a', 'b', 'c', 'd', 'e']

# List of numbers

>>> squares = [1, 4, 9, 16, 25]
>>> squares
[1, 4, 9, 16, 25]

# List of words

>>> insurance = ['sbi', 'hdfc', 'icici', 'lic']

>>> insurance

['sbi', 'hdfc', 'icici', 'lic']

Accessing values from a List

Elements of List can be used by referring it's index number. For example, to print first value from list - insurance, use following code:

>>> insurance[0]

'sbi'

>>> print(insurance[0])

sbi

Like strings, lists can also be indexed and sliced:

# index 0 returns the first item
>>> squares = [1, 4, 9, 16, 25]
>>> squares[0]
1

# index -1 returns the last item
>>> squares[-1]
25

# slicing returns a new list
>>> squares[-4:]
[4, 9, 16, 25]

All slice operations return a new list containing the requested elements. This means that the following slice returns a new (shallow) copy of the list:

>>> squares[:]

[1, 4, 9, 16, 25]

Lists also support operations like concatenation:

>>> squares + [36, 49, 64, 81, 100]

[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

Lists are a mutable type; hence it is possible to change content of list. Check the following examples:

# something's wrong in following code; cube of 4 is not 65; let's change it.

>>> cubes = [1, 8, 27, 65, 125]

>>> cubes[3] = 64  # replace the wrong value

>>> cubes

[1, 8, 27, 64, 125]

Read more Python Lists Handouts

• Example 1 : 3 ways to store numbers in a python list 
• Built-in Methods for Python Lists (append, insert, pop, remove, etc.)
• Built-in Functions for Python Lists (cmp, len, max, min, etc.)
• Operations on Python Lists (addition, multiplication)

* * * * *

Sub-string in Python using Slicing

Python - Sub-string Function using Slicing

Index number of a string can be used to obtain individual characters from a string. Slicing can be used to obtain a sub-string from the given string. Check the following examples:

>>> str = 'PythonPracticals'

#Code to refer position 0 (included) to 3 (excluded)

>>> str[0:3]
'Pyt'

# position 0 (included) to 6 (excluded)

>>> str[0:6]

'Python'

# position 4 (included) to 6 (excluded)

>>> str[4:6]

'on'

In slicing, the starting index number is included and ending index number is excluded in the result. 

If we omits the first index number in slicing, it will be considered as 0. An omitted second index will be considered as the size of the string being sliced. Observe following interesting string slicing examples:

>>> str = 'PythonPracticals'

# prints from beginning to index 6(excluded)
>>> str[:6]
'Python'

# prints from beginning to index 16(excluded)
>>> str[:16]
'PythonPracticals'

# prints from index -4 to end of string
>>> str[-4:]
'cals'

>>> str[:6] + str[6:]
'PythonPracticals'

>>> str[6:] + str[:6]
'PracticalsPython'

For non-negative indices, the length of a slice(substring) is the difference of the indices. If both are within proper boundary (limits), the length of str[1:4] is 3.

Attempting to use out of range index (too large/small) will result in an error as shown below:

>>> str = 'PythonPracticals'

>>> str[101]  # the str has only 16 characters
Traceback (most recent call last):
  File "< pyshell#1>", line 1, in <module>
IndexError: string index out of range

Out of range slice indexes are handled properly by Python interpreter. Check following example:

>>> str = 'PythonPracticals'

>>> str[6:30]
'Practicals'

Python strings cannot be changed — they are immutable. Therefore, assigning value to a specific index position will result in an error.

>>> str[0] = 'K'

Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
       str[0]=’K’
TypeError: 'str' object does not support item assignment

If you want to modify your string, you should create a new one:

>>> 'K' + str[1:]
'KythonPracticals'

String Length using in-built function len()

Built-in function len() returns the length of a given string:

>>> str = 'PythonPracticals'
>>> len(str)
16

>>> str1 = 'K. Patel'
>>> len(str1)
8

>>> print(len(str+str1))

24

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String and Index number in Python

Python - String and Index number

Strings can be used in Python using index (subscript).

Each character of a String is assigned an index number from left to right starting with number 0. For example, for a string “Python”, the index numbers are:

 +-----------------------+

 | P | y | t | h | o | n |

 +-----------------------+

   0   1   2   3   4   5  

0 to 5 are respective index number assigned to individual character of a string Python.

 +-----------------------+

 | P | y | t | h | o | n |

 +-----------------------+

  -6  -5  -4  -3  -2  -1  

-1 to -6 are the corresponding negative indices of a string “Python”.

The first character of a string is indexed as 0. Observe the following examples to understand string index numbers:

>>> str = 'PythonPracticals'

# prints character at position 0
>>> str[0]  
'P'

# prints character at position 1
>>> str[1]  
'y'

# prints character at position 2
>>> str[2]  
't'

# prints character at position 5
>>> str[5]
'n'

In Python string, index may also have negative numbers. To refer string from the end to first character of a string, negative index is useful. Check the following examples:

>>> str = 'PythonPracticals'

# prints last character
>>> str[-1]  
's'

# prints second-last character
>>> str[-2]
'l'

# third character from end of string
>>> str[-3]
'a'

Note that since -0 is considered as 0, negative index starts with -1 value.

* * * * *

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Python String Merge (Concatenation)

String Merge (String concatenation)

String concatenation means the operation of joining two strings. It is also known as string merge. In Python programming, two strings can be merged (concatenated) using the + operator.

>>> 'Python' + 'Practicals'

'PythonPracticals'

#Multiple strings are automatically merged

>>> 'Python''Practicals'  
'PythonPracticals'

>>> 'Ahmedabad''Gujarat''India'  
'AhmedabadGujaratIndia'

This works only with two literals; a variable and a string can’t be merged directly. Look at the example given below:

>>> str = 'Python'
>>> str 'Practicals'

SyntaxError: invalid syntax

>>> (str*2) 'Practicals'

SyntaxError: invalid syntax

To concatenate a variable and a string literal, use + operator as shown below:

>>> str = 'Python'
>>> str + 'Practicals'

PythonPracticals

# expression + a string is valid

>>> (str*2) + 'Practicals'
'PythonPythonPracticals'

Multiplying String with * Operator

String can be repeated using * operator in Python. Check the following example:

>>> 3 * 'Python '

'Python Python Python '

>>> 2 * 'Practicals '

'Practicals Practicals '

* * * * *

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