Python - sequence

sequence Sequence

sequence (sequence) is a set of ordered elements

(strictly speaking, it is a collection of objects, but since we have not introduced the concept of "object" yet, let's talk about elements for now)

A sequence can contain One or more elements, or no elements.

The basic data types we mentioned before can all be used as elements of a sequence. An element can also be another sequence, as well as other objects that we will introduce later.

There are two types of sequences: tuple (fixed value table; also translated as tuple) and list (table)

>>>s1 = (2, 1.3, 'love', 5.6, 9, 12, False)         # s1是一个tuple
>>>s2 = [True, 5, 'smile']                          # s2是一个list
>>>print s1,type(s1)
>>>print s2,type(s2)

The main difference between tuple and list is that once created, each element of tuple cannot be changed. Each element of the list can be changed.

A sequence as an element of another sequence

>>>s3 = [1,[3,4,5]]

Empty sequence

>>>s4 = []

Reference of elements

The subscripts of sequence elements start from 0:

>>>print s1[0]
>>>print s2[2]
>>>print s3[1][2]

Due to list The elements can be changed. You can assign a value to an element of the list:

>>>s2[1] = 3.0
>>>print s2

If you do this operation on a tuple, you will get an error message.

So, you can see that the reference of the sequence is implemented through s[], int is the subscript

Other reference methods

Scope reference: Basic style [lower limit:upper limit:step size]

>>>print s1[:5]             # 从开始到下标4 （下标5的元素 不包括在内）
>>>print s1[2:]             # 从下标2到最后
>>>print s1[0:5:2]          # 从下标0到下标4 (下标5不包括在内)，每隔2取一个元素 （下标为0，2，4的元素）
>>>print s1[2:0:-1]         # 从下标2到下标1

From As you can see above, when citing a range, if the upper limit is specified, the upper limit itself is not included.

Trail element reference

>>>print s1[-1]             # 序列最后一个元素
>>>print s1[-3]             # 序列倒数第三个元素

Similarly, if s1[0:-1], then the last element will not be referenced (again, not including the upper element itself)

Strings are tuples

Strings are a special kind of element and therefore can perform tuple-related operations.

>>>str = 'abcdef'
>>>print str[2:4]