Python Lists

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The most basic data structure in Python is the sequence. Each element of a sequence is assigned a number - its position or index. The first index is zero, the second index is one, and so forth.

Python has six built-in types of sequences, but the most common ones are lists and tuples, which we would see in this tutorial.

There are certain things you can do with all sequence types. These operations include indexing, slicing, adding, multiplying, and checking for membership. In addition, Python has built-in functions for finding the length of a sequence and for finding its largest and smallest elements.

Python Lists:

The list is a most versatile datatype available in Python which can be written as a list of comma-separated values (items) between square brackets. Good thing about a list is that items in a list need not all have the same type.

Creating a list is as simple as putting different comma-separated values between squere brackets. For example:

list1 = ['physics', 'chemistry', 1997, 2000];
list2 = [1, 2, 3, 4, 5 ];
list3 = ["a", "b", "c", "d"];

Like string indices, list indices start at 0, and lists can be sliced, concatenated and so on.

Accessing Values in Lists:

To access values in lists, use the square brackets for slicing along with the index or indices to obtain value available at that index. Following is a simple example:

#!/usr/bin/python

list1 = ['physics', 'chemistry', 1997, 2000];
list2 = [1, 2, 3, 4, 5, 6, 7 ];

print "list1[0]: ", list1[0]
print "list2[1:5]: ", list2[1:5]

When the above code is executed, it produces the following result:

list1[0]:  physics
list2[1:5]:  [2, 3, 4, 5]

Updating Lists:

You can update single or multiple elements of lists by giving the slice on the left-hand side of the assignment operator, and you can add to elements in a list with the append() method. Following is a simple example:

#!/usr/bin/python

list = ['physics', 'chemistry', 1997, 2000];

print "Value available at index 2 : "
print list[2];
list[2] = 2001;
print "New value available at index 2 : "
print list[2];

Note: append() method is discussed in subsequent section.

When the above code is executed, it produces the following result:

Value available at index 2 :
1997
New value available at index 2 :
2001

Delete List Elements:

To remove a list element, you can use either the del statement if you know exactly which element(s) you are deleting or the remove() method if you do not know. Following is a simple example:

#!/usr/bin/python

list1 = ['physics', 'chemistry', 1997, 2000];

print list1;
del list1[2];
print "After deleting value at index 2 : "
print list1;

When the above code is executed, it produces following result:

['physics', 'chemistry', 1997, 2000]
After deleting value at index 2 :
['physics', 'chemistry', 2000]

Note: remove() method is discussed in subsequent section.

Basic List Operations:

Lists respond to the + and * operators much like strings; they mean concatenation and repetition here too, except that the result is a new list, not a string.

In fact, lists respond to all of the general sequence operations we used on strings in the prior chapter.

Python ExpressionResults Description
len([1, 2, 3])3Length
[1, 2, 3] + [4, 5, 6][1, 2, 3, 4, 5, 6]Concatenation
['Hi!'] * 4['Hi!', 'Hi!', 'Hi!', 'Hi!']Repetition
3 in [1, 2, 3]TrueMembership
for x in [1, 2, 3]: print x,1 2 3Iteration

Indexing, Slicing, and Matrixes:

Because lists are sequences, indexing and slicing work the same way for lists as they do for strings.

Assuming following input:

L = ['spam', 'Spam', 'SPAM!']

 

Python ExpressionResults Description
L[2]'SPAM!'Offsets start at zero
L[-2]'Spam'Negative: count from the right
L[1:]['Spam', 'SPAM!']Slicing fetches sections

Built-in List Functions & Methods:

Python includes the following list functions:

SNFunction with Description
1cmp(list1, list2)
Compares elements of both lists.
2len(list)
Gives the total length of the list.
3max(list)
Returns item from the list with max value.
4min(list)
Returns item from the list with min value.
5list(seq)
Converts a tuple into list.

Python includes following list methods

SNMethods with Description
1list.append(obj)
Appends object obj to list
2list.count(obj)
Returns count of how many times obj occurs in list
3list.extend(seq)
Appends the contents of seq to list
4list.index(obj)
Returns the lowest index in list that obj appears
5list.insert(index, obj)
Inserts object obj into list at offset index
6list.pop(obj=list[-1])
Removes and returns last object or obj from list
7list.remove(obj)
Removes object obj from list
8list.reverse()
Reverses objects of list in place
9list.sort([func])
Sorts objects of list, use compare func if given


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