正数和负数索引在Python中是什么意思？

通过索引，我们可以访问Python序列数据类型中的项目。字符串、列表、元组和范围对象都是序列数据类型。在本教程中，我们将详细介绍索引。

什么是列表索引？

任何编程语言中的线性数据结构都是围绕索引构建的。每台机器的默认索引从0开始，一直到n-1。在这种情况下，n表示相应数据结构中的总项目数。类型包括

• Positive indexing − Increases from 0 to 1.

• Negative indexing − each traversal moves from tail to head, starting with the last element.

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These facilitate our ability to access the many components of this data structure. Let's examine the procedures in the next part.

Tuple Indexing

类似于我们如何访问列表和字符串中的元素，我们也可以访问元组中的元素。因此，索引和切片是我们访问元素所需的唯一方法。此外，索引是直观的，从索引零开始，就像列表一样。此外，我们在方括号中放置的数字表示元组的索引。让我们看一些使用元组索引来检索元组元素的示例。

Example 1

示例1

tup1 = (10, 3, 4, 22, 1)
# for accessing the first element of the tuple
print(tup1[0])

Output

10

Example 2

tup1 = (10, 3, 4, 22, 1)
# accessing the third element of the tuple
print(tup1[2])

Output

4

Example 3

示例 3

tup1 = (10, 3, 4, 22, 1)
print(tup1[10])
# gives an error as the index is only up to 4

Output

IndexError: tuple index out of range

Example 4

tup1 = (10, 3, 4, 22, 1)
print(tup1[1+3])
# the expression inside the square brackets results in an integer index 4. Hence, we get the element at the 4th index.

Output

1

Zero-Based Indexing in Python

In Python, positive zero-based indexing is the fundamental method for accessing iterable items.

As a result, an index starting at 0 may refer to any element in the iterable.

第一个以零为基准的索引元素的索引为0，第二个为1，依此类推。

Example 5

myList = [1, 2, 3, 4, 5]
# NORMAL INDEXING
print(myList[0])
print(myList[1]) 
print(myList[2])
# NEGATIVE INDEXING (STARTS FROM THE LAST ELEMENT IN THE LIST)
print(myList[-1])
print(myList[-2])
print(myList[-3])
print(myList[-3:])

Output

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

负索引

If we wish to print the components starting at the end, we may also use negative indexes. Additionally, by specifying the index number with a negative sign, we may carry out negative tuple indexing (-). As a result, this suggests that the compiler starts thinking about the elements in reverse order, beginning with the element that comes last in the tuple.

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Example 3

示例 3

tup = (22, 3, 45, 4, 2.4, 2, 56, 890, 1)
print(tup[-4])

Output

2

Now, we can use negative indexes in slicing also.

Example 4

tup = (22, 3, 45, 4, 2.4, 2, 56, 890, 1)
print(tup[-4:-1])

Output

(2, 56, 890)

Tuple index() 方法

The tuple index() function aids in locating an element's index or location within a tuple. Essentially, this function serves two purposes −

Giving the tuple's first instance of each element.

If the indicated element is absent from the tuple, throwing an error.

Syntax

tuple.index(value)

示例5：使用负索引删除元素

使用pop()函数并将-1作为参数传递给它，我们可以删除该列表的最后一个元素，并获得一个新列表。

my_list = [45, 5, 33, 1, -9, 8, 76]
my_list.pop(-1)
print(my_list)

Output

[45, 5, 33, 1, -9, 8]

Example 6: Finding the index of an element

tup = (22, 3, 45, 4, 2.4, 2, 56, 890, 1)
print(tup.index(45))
print(tup.index(890))
#prints the index of elements 45 and 890

Output

2
7

在Python列表中使用负索引的优势

• 需要更少的代码行数，并且可以在一行中完成反转。

• Simplifies difficult processes.

• 在要求很少复杂性的情况下快速运行

Conclusion

总之，Python允许从零开始进行正向索引和从-1开始进行负向索引。在Python中，负向索引表示索引过程从可迭代对象的末尾开始。最后一个元素位于索引-1，倒数第二个元素位于索引-2，依此类推。负向索引在Python计算机语言中支持数组，但在大多数其他编程语言中不支持。这意味着索引值-1提供了数组的最后一个元素，-2提供了倒数第二个元素。负向索引开始的地方是数组的末尾。

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