Python如何从嵌套内层for循环中递增外层for循环的变量？

Hey there! Let me walk you through what's going on with your code and how to fix it.

First, let's recap your code and the unexpected behavior you ran into:

Your code:

x = [0,1,2,3,4,5,6,7,8,9,10]
for i in x:
    print(x[i])
    for j in x:
        i += 1

You expected incrementing i inside the inner for j loop would make the outer loop skip ahead, but instead you ended up with output like i:0; x[i]:0 i:1; x[i]:1 ... i:5; x[i]:5 — not what you wanted at all.

The root cause #

Here's the key thing about Python's for...in loops: the loop variable (i in your case) gets reassigned on every iteration directly from the iterable you're looping over (which is x here). When you do i +=1 inside the inner loop, you're only changing the value of that i variable right then. But as soon as the inner loop finishes and the outer loop starts its next iteration, Python will grab the next element from x and assign it to i — completely overwriting the changes you made inside the inner loop.

In short: your inner loop's increments are being ignored by the outer loop, because the outer loop doesn't care about the current value of i — it just pulls the next item from x each time.

The fix #

If you want full control over how i increments (including letting the inner loop alter it), swap the outer for loop for a while loop. With while, you manage the i variable yourself, so any changes you make to it (even inside the inner loop) will stick.

Here's how that would look:

x = [0,1,2,3,4,5,6,7,8,9,10]
i = 0
while i < len(x):
    print(f"i:{i}; x[i]:{x[i]}")
    for j in x:
        i += 1
        # Add a check to avoid index errors if i goes beyond the list length
        if i >= len(x):
            break

Now, every time you increment i in the inner loop, the outer while loop will use that updated value to check if it should keep running. This way, your inner loop's changes to i actually affect the outer loop's behavior, just like you intended.

内容的提问来源于stack exchange，提问作者Nilesh Ingle