不了解Python中的嵌套For循环

Question

我不明白下面代码的第二个for循环中的i是如何工作的。

di = [96, 15, 33, 87]
for i in range(len(di)):
    total = di[i]
    for j in range(i+1,len(di)):
        print(i)
0
0
0
1
1
2

为什么输出为0,0,0,1,1,2。我在第二个for循环中如何受到第一个循环的影响？有继承吗？请原谅这里的新手。

Answer 1

len(di)是4.所以循环

for i in range(len(di)):

将重复4次。由于range的工作方式（从下限开始，默认为0，如果没有指定，则低于上限1），i将在第一次重复时为0，在第二次重复时为1，依此类推。要计算range(x, y)生成多少个对象，在这种情况下，for i in range(x, y)将重复多少次，你可以简单地做number of repetitions = y - x。所以在这种情况下：len(di) - 0 (default lower bound) = 4。

循环

for j in range(i+1, len(di)):
    print(i)

将重复print(i)命令len(di) - (i + 1)次。请记住，i由外循环定义。所以，在第一次循环期间

for i in range(len(di)):

i等于0，因此print(i)命令将被执行4 - (0+1) = 3次 - 它将打印i(=0) 3次。在第二个循环中，iequals 1，因此它将被打印2次，依此类推。所以这里发生了什么，格式化为代码以提高可读性：

First outer loop: 
i = 0
total = di[i] = di[0] = 96
--> first inner loop of first outer loop:
    j = i + 1 = 1
    i is printed -> prints 0

    second inner loop of first outer loop:
    j = j+1 = 2
    i is printed -> prints 0 again

    third inner loop of first outer loop:
    j = j+1 = 3 --> since len(di) = 4, the upper bound of range(i+1, len(di)) is reached, so this is the last Repetition
    i is printed -> prints 0 again

Second outer loop:
i = 1
total = di[1] = 15
--> first inner loop of second outer loop:
    j = i+1 = 2
    i is printed -> prints 1

    second inner loop of second outer loop:
    j = i+1 = 3  -> upper bound of range reached, last repetition
    i is printed -> prints 1 again

Third outer loop:
i = 2
total = di[2] = 33
--> first inner loop of third outer loop:
    j = i+1 = 3  -> upper bound of range is reached, only Repetition
    i is printed -> prints 2

Fourth (and final) outer loop:
i = 3 -> upper bound of range(len(di)) reached, last Repetition
total = di[3] = 87
since j = i+1 = 4, the inner loop does not get executed at all (both bounds of range are equal), so 3 doesn't get printed

end of code.

Answer 2

在编程语言中，变量可用于范围内。当您使用新变量开始循环时，它将可供使用，直到您结束它。

当你开始学习python的过程时，最好的做法之一是阅读官方文档。 https://docs.python.org/3/tutorial/controlflow.html

Answer 3

为了帮助您理解，请尝试以下方法：

di = [96, 15, 33, 87]
for i in range(len(di)):
    print("first loop, i =", i)
    total = di[i]
    for j in range(i+1,len(di)):
        print("second loop, j =", j)
        print("second loop, i =", i)

两个循环中的i是相同的。每次外循环运行时，它都会运行内循环，直到“for”语句完成。