Python Learning Notes (1)-List

What is a list?

A list consists of a series of elements arranged in a particular order. The list looks like this: [1,2,3,4,1]. In a list, you can consist of 0 or more elements, separated by commas, with the same value element allowing multiple occurrences

Create a list using [] or list ()

For example, create an empty list

Empty_list = [] or empty_list = List ()

Use list () to convert other data types into lists

List of strings:

>>> list (' Cat ')

[' C ', ' a ', ' t ']

Tuple list:

>>> num = (all in all)

>>> list (num)

[1, 2, 3]

Get an element using [offset]

Starting from 0, you can also use a negative number and a tail of-1

>>> a = [2,3,4,5]

>>> A[0]

2

>>> A[-1]

5

Use the position of the index list element

>>> A

[2, 3, 4, ' A ', ' B ', ' C ']

>>> A.index (' a ')

3

A indexerror error occurs when the specified offset exceeds the list range.

Using slices to get a sub-list

[First:end: (STEP)]

>>> a = [1,2,3,4,5,6]

>>> A[::1]

[1, 2, 3, 4, 5, 6]

>>> A[::2]

[1, 3, 5]

>>> A[::-2]

[6, 4, 2]

modifying elements using [offset]

>>> a = [1,2,3,4,5,6]

>>> a[0]=10

>>> A

[10, 2, 3, 4, 5, 6]

Add an element at the end of the list (append)

>>> a = [+ +]

>>> A.append (4)

>>> A

[1, 2, 3, 4]

Add an element at the specified position in the list (insert)

>>> A

[1, 2, 3, 4]

>>> A.insert (1, ' happy ')

>>> A

[1, ' Happy ', 2, 3, 4]

One list merged into another list (extend)

>>> a = [1,2,3,4]

>>> B = [' A ', ' B ', ' C ']

>>> A.extend (b)

>>> A

[1, 2, 3, 4, ' A ', ' B ', ' C ']

Delete a location element

Delete any location element using the DEL statement

>>> A

[1, 2, 3, 4, ' A ', ' B ', ' C ']

>>> del A[0]

>>> A

[2, 3, 4, ' A ', ' B ', ' C ']

Using method pop to delete an element

The list is like a stack, and deleting the element at the end of the list is equivalent to ejecting the top element

>>> A

[2, 3, 4, ' A ', ' B ', ' C ']

>>> A.pop ()

C

>>> A

[2, 3, 4, ' A ', ' B ']

>>> A.pop (2)

4

>>> A

[2, 3, ' A ', ' B ']

Remove an element by value (remove)

>>> A

[2, 3, ' A ', ' B ']

>>> A.remove (2)

>>> A

[3, ' A ', ' B ']

Working with lists

List for saving variables

>>> [1,2,3,4,5]

List for looping

>>> a = [0,1,2,3,4]

>>> for I in a: #等价于for I in rang (5)

Print (i)

0

1

2

3

4

In and not in operators

You can determine whether a value is in the list, return a Boolean value

>>> 1 in [0,1,2,3,4]

True

>>> 5 Not in [0,1,2,3,4]

True

>>> 1 not in [0,1,2,3,4]

False

>>> 5 in [0,1,2,3,4]

False

Multiple assignment techniques

>>> One,two,three = [+]

>>> One

1

>>>

2

>>> Three

3

The number and list of variables must be strictly equal, otherwise python will ValueError

Common methods

Get the list length using Len ()

>>> A

[0, 1, 2, 3, 4]

>>> Len (a)

5

Use COUNT () to record the number of occurrences of a specific value

>>> a = [1,2,3,4,3,5,1,1,3,1,3]

>>> A.count (1)

4

>>> A.count (3)

4

Rearrange elements using sort ()

Python provides two functions

The list method sort () reads the original list and changes the contents of the original list;

The general function sorted () returns a copy of the ordered list and the contents of the original list are unchanged.

>>> a = [5,4,3,2,1]

>>> A.sort ()

>>> A

[1, 2, 3, 4, 5]

>>> a = [5,4,3,2,1]

>>> Sorted (a)

[1, 2, 3, 4, 5]

>>> A

[5, 4, 3, 2, 1]

Sort () Method note

1. sort the list on the spot, do not write a = A.sort () such a code that attempts to record the return value
2. You cannot sort a list that has both numeric and string values, causing TypeError Error
3. when sorting strings, use the ASCII character Order "

Convert to a string using join ()

Join () is the inverse process of split ()

>>> a = [' Hello ', ' world ']

>>> b = ', '. Join (a)

>>> b

' Hello,world '

>>> C = b.split (', ')

>>> C

[' Hello ', ' world ']

Use = assignment, use copy () to assign a value

If you use = To assign multiple variables, changing any of them will cause the values of other variables to be modified as well.

>>> a = [+ +]

>>> B = A

>>> b[0] = ' a '

>>> A

[' A ', 2, 3]

>>> b

[' A ', 2, 3]

>>> a[2] = ' B '

>>> b

[' A ', 2, ' B ']

>>> A

[' A ', 2, ' B ']

Because A and B point to the same object, whether we modify the contents of the list either through a or through B, the result will be on the other side.

There are 3 ways we can copy the value of one list to another new list:

1. b = A.copy ()
2. c = List (a)
3. D = a[:]

b, C, D are all copies of a, changing any one variable will not change the other variables

Python Learning Notes (1)-List