Introduction to Python basic learning list

2. Introduction to lists

One of the built-in data types in Python is list: list.

List is an ordered collection.

A list consists of a series of elements arranged in a specific order. Represented by [ ]. The data types of the elements in the

list can also be different, for example:

##>>> L = ['Apple', 123, True]

2.1 Index list

starts from 0 instead of 1. When the index exceeds the range, Python will report an IndexError error, so make sure the index does not go out of range.

If you want to get the last element, in addition to calculating the index position, you can also use -1 as the index to get the last element directly.

2.2 Modify, add and delete list elements

2.2.1 Add elements

Add with append()

2.2.2 Insert elements

Insert

insert() Specify the index and value of the new element.

##>>> classmates.insert(1, 'Jack')2.2.3 Delete elements

>>> classmates

['Michael', 'Jack', 'Bob', 'Tracy', 'Adam']

1.1. To delete del, you need to know the

index

position del ABC[0]1.2. To delete the element at the end of the list, use the pop() method.

1.3. To delete the element at the specified position, use the pop(i) method, where i is the index position.

1.4. Use the pop() method to delete elements (pop), and then use the delete value.

1.5. Delete elements according to

value

remove()2.2.4 Modify elements

To replace an element with another element, you can Directly assign the value to the corresponding index position.

classmates[1] = 'Sarah'##2.3 Organization List

Method sort() sorts the list permanently, alphabetically.

Function sorted() temporarily sorts the list.

Print the list backwards reverse().

Determine the length of the list len().

2.3.1 The list contains another list

The list element can also be another list, such as:

>>> s = ['python', 'java', ['asp', 'php'], 'scheme']

To get 'php', you can write p[1] or s[2][1], so s can be regarded as a two-dimensional array.

3. Operation list

3.1 Traverse the entire list

Use a for loop

for a in A:Don’t miss the colon

                                                                                                                        Pay attention to the indentation

Summary after the end of the print () loop

3.2 Create a list of values ​​

1. Function range(), range(1 ,5) Only 4 numbers can be printed but not 5

2. Use the function list() to convert range() into a list list(range(1,5))

3. * * 2 hearts represent the square

4. Perform simple statistical calculations on the list of numbers min() minimum value; max() maximum value; sum() total

5. List parsing Combine the for loop and the code to create new elements into one line, and automatically append the new elements:

　[value**2 for value in range(1,11)]

3.3 Use part of the list

3.3.1 Slice

For operations that often take a specified index range, it is very cumbersome to use loops. Therefore, Python provides the slice (Slice) operator to simplify the operation. .

1. [ : ] The starting point of the slice starts from 0, [: 4] means 0 to 3, [-3:] means the last 3

2. [2 : ] means From the 3rd to the end

3. Even if you don’t write anything, just write [ : ] to copy a list as it is.

4. [ : : step size]; [ : : -1] reverse the characters

5. The string 'xxx' can also be regarded as a list, each element Just one character. Therefore, strings can also be sliced, but the result is still a string.

6. Number of times: str(n) == str(n)[ : :-1]

3.4 Tuple

Another ordered list is called a tuple :tuple.

Tuples are very similar to lists, but tuples cannot be modified once initialized. Python calls values ​​that cannot be modified immutable, and immutable lists are called tuples. Because tuples are immutable, the code is safer. If possible, use tuple instead of list.

3.4.1 Define tuple

1. Add elements in (), separated by commas, accessed the same as list. But the value cannot be modified. But it can be assigned a value. The numbers can be directly (1,2,3,4), and the characters need to be quoted ('a', 'b', 'c')

2. When Python displays a tuple with only one element, it also A comma will be added to prevent you from misunderstanding it as a parentheses in the sense of mathematical calculations.

　t = (1,)

3. The so-called "invariance" of tuple means that the pointer of each element of tuple will never change. That is, if it points to 'a', it cannot be changed to point to 'b'. If it points to a list, it cannot be changed to point to other objects. But the list pointed to can itself be changing! How to create a tuple whose content remains unchanged? Then it must be ensured that each element of the tuple itself cannot be changed.

3.5 Set code format

1. PEP8

2. Indent format, 4 spaces

3. Line length should not exceed 79 characters

4. Use blank lines to organize the structure

>>> len(s)

4

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