The difference between list and set in Python

list:

literally means a collection. In Python, elements in a List are represented by square brackets []. You can define a List like this:

L = [12, 'China', 19.998]

You can see that the types of elements are not required to be the same. Of course, you can also define an empty List:

L = []

The List in Python is ordered, so if you want to access the List, you must obviously access it through the serial number, just like the subscript of the array, it is the same as the subscript Starting from 0:

>>> print L[0]12

Do not cross the boundary, otherwise an error will be reported

>>> print L[3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

List can also be accessed in reverse order, and the serial number is represented by a subscript such as "the x-th from the bottom", such as -1 This subscript represents the penultimate element:

>>> L = [12, 'China', 19.998]
>>> print L[-1]19.998

-4 is obviously out of bounds, as follows:

>>> print L[-4]

Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    print L[-4]
IndexError: list index out of range
>>>

List is added to the end through the built-in append() method, and insert () method is added to the specified position (the subscript starts from 0):

>>> L = [12, 'China', 19.998]
>>> L.append('Jack')
>>> print L
[12, 'China', 19.998, 'Jack']
>>> L.insert(1, 3.14)
>>> print L
[12, 3.14, 'China', 19.998, 'Jack']
>>>

Note that there are several methods in python that are similar to append, but the effects are completely different. When using them, you need to choose the correct method according to actual needs

1. append() Appends a new element to the end of the list. The list only occupies one index position and adds it to the original list.

2. extend() Appends a list to the end of the list and adds the elements in the list to the end of the list. Each element of is appended, and

is added to the original list. For example, list1=[1, 2, 3] .list2=[4, 5, 6]

list1.append(list2 ) The result is [1, 2, 3, [4, 5, 6]]

The result of list1.extend(list2) is [1, 2, 3, 4, 5, 6]

3. Using the number directly seems to have the same effect as using extend(), but it actually generates a new list to store the sum of the two lists. It can only be used to add the two lists.

4. = The effect is the same as extend(). It adds a new element to the original list and adds it to the original list.

Delete the last tail element through pop(). You can also specify a parameter to delete the specified position:

>>> L.pop()
'Jack'
>>> print L
[12, 3.14, 'China', 19.998]
>>> L.pop(0)
>>> print L
[3.14, 'China', 19.998]

You can also copy and replace through subscripts

>>> L[1] = 'America'
>>> print L
[3.14, 'America', 19.998]

set:

set is also a set of numbers, unordered, and the content cannot be repeated. By calling set( ) method creation:

>>> s = set(['A', 'B', 'C'])

The meaning of accessing a set is just to check whether an element is in the set. Pay attention to case sensitivity:

>>> print 'A' in s
True
>>> print 'D' in s
False

Also traverse through for:

s = set([('Adam', 95), ('Lisa', 85), ('Bart', 59)])

for x in s:
    print x[0],':',x[1]

>>>
Lisa : 85
Adam : 95
Bart : 59

Add and delete elements through add and remove (keep them non-repeating). When adding elements, use the add() method of set

>>> s = set([1, 2, 3])
>>> s.add(4)
>>> print s
set([1, 2, 3, 4])

If the added element already exists in the set, add( ) will not report an error, but it will not be added:

>>> s = set([1, 2, 3])
>>> s.add(3)
>>> print s
set([1, 2, 3])

When deleting elements in the set, use the remove() method of the set:

>>> s = set([1, 2, 3, 4])
>>> s.remove(4)
>>> print s
set([1, 2, 3])

If the deleted element does not exist in the set, remove() will report an error:

>>> s = set([1, 2, 3])
>>> s.remove(4)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 4

So if we want to determine whether an element meets some different conditions, using set is the best choice. The following example:

months = set(['Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec',])
x1 = 'Feb'
x2 = 'Sun'

if x1 in months:
    print 'x1: ok'
else:
    print 'x1: error'

if x2 in months:
    print 'x2: ok'
else:
    print 'x2: error'

>>>
x1: ok
x2: error

In addition, the calculation efficiency of set is higher than that of list.

For more Python related technical articles, please visit the Python Tutorial column to learn!

The above is the detailed content of The difference between list and set in Python. For more information, please follow other related articles on the PHP Chinese website!