为什么 Python for 循环不像 C for 循环那样工作? [英] Why Python for loop doesn't work like C for loop?

问题描述

C:

# include <stdio.h>
main()
{
    int i;
    for (i=0; i<10; i++)
    {
           if (i>5) 
           {
             i=i-1;     
             printf("%d",i);
           } 
    }
}

蟒蛇:

for i in range(10):
   if i>5: i=i-1
   print(i)

当我们编译 C 代码时，它会进入一个无限循环，永远打印 5，而在 Python 中则不会，为什么不呢?

When we compile C code, it goes into a infinite loop printing 5 forever, whereas in Python it doesn't, why not?

Python 输出为:

The Python output is:

0 1 2 3 4 5 5 6 7 8

0 1 2 3 4 5 5 6 7 8

推荐答案

在 Python 中，循环不会递增 i，而是从可迭代对象(在本例中为列表)中为其分配值.因此，在 for 循环内更改 i 不会混淆"循环，因为在下一次迭代中 i 将被简单地分配下一个值.

In Python, the loop does not increment i, instead it assigns it values from the iterable object (in this case, list). Therefore, changing i inside the for loop does not "confuse" the loop, since in the next iteration i will simply be assigned the next value.

在你提供的代码中，当i为6时，然后在循环中递减，改为5，然后打印出来.在下一次迭代中，Python 只需将其设置为列表 [0,1,2,3,4,5,6,7,8,9] 中的下一个值，即 7，并且很快.当没有更多值可取时，循环终止.

In the code you provided, when i is 6, it is then decremented in the loop so that it is changed to 5 and then printed. In the next iteration, Python simply sets it to the next value in the list [0,1,2,3,4,5,6,7,8,9], which is 7, and so on. The loop terminates when there are no more values to take.

当然，你提供的C循环中得到的效果在Python中还是可以实现的.由于每个 for 循环都是一个美化的 while 循环，从某种意义上说，它可以像这样转换:

Of course, the effect you get in the C loop you provided could still be achieved in Python. Since every for loop is a glorified while loop, in the sense that it could be converted like this:

for (init; condition; term) ...

相当于:

init
while(condition) {
    ...
    term
}

那么你的 for 无限循环可以用 Python 编写为:

Then your for infinite loop could be written in Python as:

i = 0
while i < 10:
    if i > 5:
        i -= 1
    print i
    i += 1

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