我的 Python for 循环无限循环，但仅在多次运行时

Question

我正在尝试制作一个简单的算法，为您计算帕斯卡三角形中的一行，如图所示。

所以如果我想知道第五行的值，我只需在程序中输入“repeat = 5”，程序就会吐出“[1,5,10,10,5,1]”

而我正是使用 for 循环做到了这一点。 它可以工作......，如果你运行它一次，只要我for _ in range(value)第一个 for 循环就会无限循环。 我尝试过使用“While（条件）”和“While True”，但似乎没有任何效果。

start = [1] #This can be any of the "steps" of the pascal triangle
secondList = []
repeat = 2  # If repeat = 1 code runs fine

for _ in range(repeat):            # Without the for loop the code runs fine (But you have to manually update start.)
    print(f"First start: {start}") # Prints [1] for the first time, then [1,1]. Then it never prints again

    for idx,i in enumerate(start):

        if idx == 0:
            j = i + 0
            secondList.append(j)

        if idx == len(start)-1:
            j = i + 0
            secondList.append(j)

        else:
            j = i + start[idx+1]
            secondList.append(j)

        print(f"End of for loop, secondList: {secondList}") # Prints [1], then [1,1], then [1,1,1,2], then [1,1,1,2,2] and so on and so forth. The for loop should run once and then twice but it runs on forever.

    start = secondList # Sets the result as the start so that it can loop as many times as instructed.

print(secondList)

注意：没有循环的代码可以正常工作，例如，我设置start = [1,3,3,1] （这是第 3 行）和repeat = 1然后代码吐出[1,4,6,4,1] （这是下一行，第 4 行）

Answer 1

将secondList = []移动到for _ in range(repeat)之前for idx, i in... 。

您的代码中的问题实际上是由在第一次迭代结束时调用的start = secondList引起的。 在此之前， start和secondList指向不同的对象。 现在您将它们绑定到同一个 object。

在第二次迭代中，首先您没有清空临时的secondList 。 一个更严重的问题是secondList.append()还附加了一些东西到start ，因为它们现在是同一个对象！ 因此，每次迭代for idx, i in enumerate(start)时，列表start本身就会增长。 所以这个for循环永远不会结束。

所以我的补救措施是重新分配一个空列表给secondList ，以便（i）重新初始化secondList和（ii）切断 this 和start之间的链接。

start = [1]
repeat = 5
for _ in range(repeat):
    print(f"First start: {start}")
    secondList = []
    for idx,i in enumerate(start):
        if idx == 0:
            j = i + 0
            secondList.append(j)
        if idx == len(start)-1:
            j = i + 0
            secondList.append(j)
        else:
            j = i + start[idx+1]
            secondList.append(j)
        print(f"End of the (inner) for loop, secondList: {secondList}")
    start = secondList
print(secondList)

Output：

First start: [1]
End of the (inner) for loop, secondList: [1, 1]
First start: [1, 1]
End of the (inner) for loop, secondList: [1, 2]
End of the (inner) for loop, secondList: [1, 2, 1]
First start: [1, 2, 1]
End of the (inner) for loop, secondList: [1, 3]
End of the (inner) for loop, secondList: [1, 3, 3]
End of the (inner) for loop, secondList: [1, 3, 3, 1]
First start: [1, 3, 3, 1]
End of the (inner) for loop, secondList: [1, 4]
End of the (inner) for loop, secondList: [1, 4, 6]
End of the (inner) for loop, secondList: [1, 4, 6, 4]
End of the (inner) for loop, secondList: [1, 4, 6, 4, 1]
First start: [1, 4, 6, 4, 1]
End of the (inner) for loop, secondList: [1, 5]
End of the (inner) for loop, secondList: [1, 5, 10]
End of the (inner) for loop, secondList: [1, 5, 10, 10]
End of the (inner) for loop, secondList: [1, 5, 10, 10, 5]
End of the (inner) for loop, secondList: [1, 5, 10, 10, 5, 1]
[1, 5, 10, 10, 5, 1]