python: for循环时列表长度改变，会发生什么？

for循环时列表长度改变，会发生什么？

以以下两种形式为例：

# 第一种形式
for i in li:
# 第二种形式
for i in range(len(li))

先说结论：

• 无论是哪种遍历，都是严格按照下标进行，li[0]之后必然是li1
• 形式1(for i in li)会不断检查长度确定遍历终点，到达终点停止。
• 形式2(for i in range(len(li)))的长度会在遍历一开始确定,假如数组长度变长，遍历次数不会增多，假如数组长度变短，可能诱发访问越界错误。

import numpy
li = []
for i in range(5):
    li.append(10-i)
for i in li:
    print(i)
    li.pop(0)
    print(li)

10
[9, 8, 7, 6]
8
[8, 7, 6]
6
[7, 6]

列表变小，循环次数也变少

li2 = []
for i in range(5):
    li2.append(i)
for i in li2:
    li2.append(i+5)
    if i>20:
        break
    print(li2)

[0, 1, 2, 3, 4, 5]
[0, 1, 2, 3, 4, 5, 6]
[0, 1, 2, 3, 4, 5, 6, 7]
[0, 1, 2, 3, 4, 5, 6, 7, 8]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25]

列表变大，循环次数也变大，（假如没有break，这段代码会无限循环）

li = []
for i in range(5):
    li.append(10-i)
for i in range(len(li)):
    print(li[i])
    li.pop(0)
    print(li)

10
[9, 8, 7, 6]
8
[8, 7, 6]
6
[7, 6]



---------------------------------------------------------------------------

IndexError                                Traceback (most recent call last)

<ipython-input-4-b4c48012bfa9> in <module>
      3     li.append(10-i)
      4 for i in range(len(li)):
----> 5     print(li[i])
      6     li.pop(0)
      7     print(li)


IndexError: list index out of range

列表变小，访问越界错误

li = []
for i in range(5):
    li.append(i)
for i in range(len(li)):
    print(li[i])
    li.append(i+5)
    print(li)

0
[0, 1, 2, 3, 4, 5]
1
[0, 1, 2, 3, 4, 5, 6]
2
[0, 1, 2, 3, 4, 5, 6, 7]
3
[0, 1, 2, 3, 4, 5, 6, 7, 8]
4
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

列表变大：循环次数不变

最后

以上就是诚心小天鹅为你收集整理的python: for循环时列表长度改变，会发生什么？的全部内容，希望文章能够帮你解决python: for循环时列表长度改变，会发生什么？所遇到的程序开发问题。

如果觉得靠谱客网站的内容还不错，欢迎将靠谱客网站推荐给程序员好友。