Python Programming Essentials - M12 - Lists

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Python Lists and references
Pavan Verma
@YinYangPavan
Python Programming Essentials

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Python lists
 List is an ordered collections of items
 Item could be any Python data type (strings, number, other
lists, tuple, dictionary)
 Similar to arrays in (C / JAVA / C++)
 Lists are the simplest data structure in Python
colors = [‘red’, ‘blue’, ‘green’]
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List creation
Use square brackets [] to create a list
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>>> colors = ['red', 'blue', 'green']
>>> colors
Output:
['red', 'blue', 'green']

Access List Elements
list[index]
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>>> colors
>>> colors[0]
'red'
>>> colors[1]
'blue'
>>> colors[2]
'green'

Negative Indexes
list[-index]
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>>> colors
>>> colors[-1]
'green'
>>> colors[-2]
'blue'
>>> colors[-3]
'red'

List operations
 length
 append
 insert
 remove
 delete
 pop
 Slicing
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Length of a list
Use len()function to get the length of list
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>>> len(colors)
Output:
3

Append an element
 Use append() to add an element to the list
 append adds the element at the end of a list
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>>> colors.append('orange')
>>> colors
['red', 'blue', 'green', 'orange']

Insert an element at an index
 list.insert(index, value)
 Use insert to insert an element at a particular index
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>>> colors ['red', 'blue', 'green', 'orange']
>>> colors.insert(1, 'black')
>>> colors
['red', 'black', 'blue', 'green', 'orange']

Remove an element from list
remove(x) removes the first item from the list whose
value is x
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>>> colors
['red', 'black', 'blue', 'green', 'orange']
>>> colors.remove('black')
>>> colors

Remove an element from list (2)
If you try to remove an element which is not there in the list a
ValueError is raised
11
>>> colors
>>> colors.remove('yellow')
Traceback (most recent call last): File
"<pyshell#24>", line 1, in <module>
colors.remove('yellow')
ValueError: list.remove(x): x not in list

Delete an element from list
del list[index]
Use del operator to delete an element from a list
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>>> colors
>>> del colors[1]
>>> colors
['red', 'green', 'orange']

Sorting a list
 sort()sorts the list in place
 In place means that sort works on the original
list rather than giving back a new list
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>>> colors
['red', 'green', 'orange']
>>> colors.sort()
>>> colors
['green', 'orange', 'red']

Reverse a list
 reverse()reverses the list in place
 In place means that reverse()works on the
original list rather than giving back a new list
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>>> colors
['green', 'orange', 'red']
>>> colors.reverse()
>>> colors
['red', 'orange', 'green']

Using list as stack
 A stack is a list with just two operations:
• Push
• Pop
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Stack – push
40
30
20
10
16
50
40
30
20
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stack.append(50)
>>> stack = [10, 20, 30, 40]
>>> stack.append(50)
>>> stack
[10, 20, 30, 40, 50]

Stack – pop
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50
40
30
20
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stack.pop( )
>>> stack
[10, 20, 30, 40, 50]
>>> stack.pop()
50
>>> stack
[10, 20, 30, 40]
40
30
20
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List slicing
 Slicing: Extracting parts of list
 Syntax:
list[start:end]
list[start:]
list[end:]
list[:]
 start inclusive and excluding end
 Slicing returns a new list
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List slicing (2)
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>>> colors = ['yellow', 'red', 'blue', 'green', 'black']
>>> colors[0:]
['yellow', 'red', 'blue', 'green', 'black']
>>> colors[:4]
['yellow', 'red', 'blue', 'green']
>>> colors[1:3]
['red', 'blue']
>>> colors[:]
['yellow', 'red', 'blue', 'green', 'black']

Summary
 What is a list
 List creation
 Access elements of a list
 Basic List operations
 Sorting and reversing a list
 Using list as stack
 Slicing
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Resources
 Tutorial on lists
http://www.tutorialspoint.com/Python/Python_lists.htm
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Python Programming Essentials - M12 - Lists

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