python列表迭代期间的元素修改

Question

任何人都可以向“元素”解释以下代码在迭代过程中究竟发生了什么？

array = [2,3,4]

for element in array:

    element = 3

print(array)

>>>[2, 3, 4]

输出是 [2, 3, 4] 而不是 [3, 3, 3]

我是否正确理解在使用“for element in l”语法时，我们只能引用但不能修改此处发生的数组的每个元素？

PS我看到了一个名为“为什么你不应该像'for element in array'那样迭代”的问题，但是我找不到那个问题，所以我是这样问的。似乎我发现了以这种方式迭代的缺点之一。如果可能，请将我重定向到上述问题。

Answer 1

解释在上面的示例中，不可能对循环中的变量元素进行任何更改。代码要获得预期的输出，请尝试以下操作：array = [2,3,4]for i in range(len(array)):    array[i] = 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Answer 2

解释在上面的示例中，不可能对循环中的变量元素进行任何更改。代码要获得预期的输出，请尝试以下操作：array = [2,3,4]for i in range(len(array)):    array[i] = 3print(array)

Answer 3

当迭代开始元素变量已经在数组中具有当前值时。当您为其分配 3 时，它将包含它，直到下一次迭代时它将再次采用数组的当前值，依此类推。要获得 [3, 3, 3] 您需要按照以下方式进行操作：array = [2,3,4]for i in range(0,len(array)):    array[i]=3print(array)

Answer 4

那是因为elementis 是for循环范围内的局部变量。运行这个片段。我希望它可以不言自明（我用e代替element），我也用来enumerate获取索引：array = [2,3,4]for i, e in enumerate(array):    print('i =', i, 'e =', e)    e = 100    print('e = 100-->','e =', e, 'but array[i]=',array[i])    array[i] = e    print('array[i] = e --> array[i]=',array[i])    print('-'*10)print(array) #=> [100,100,100]快速解释e和i是局部变量，它们array在每次迭代时接收元素的值和 的索引。在循环内部，您可以更改 的值，e但不会影响array. 要更改其中的值array，需要通过索引 ( array[i])访问它。