for 循环中的计数器没有变化

Question

when I put i=x in the given code, the i in the for loop doesn't get change.

s=input()
d={}
Str=""
for i in range(len(s)):
    if s[i] not in d:
        d[s[i]]=i
    else:
        x=d[s[i]]
        res=str("".join(d.keys()))
        if len(Str)<len(res):
            Str=res
        i=x
        d.clear()
print(Str)

Answer 1

The counter, i, does get changed to x, but only for a brief instance. Think of for i in range(n) being implemented like:

i = first value in range
while True:
    do something;
    i=x; # your assignment
    if no more values in range:
        break;
    else:
        i = next value in range;

So right after you assign x to i, you ultimately reach the bottom of the for loop and assign to i the next value in the range expression.

The above is not exactly how range is implemented, but you get the idea. You are expecting it to behave more like:

i = 0
while i < n:
    do something;
    i = x;
    i += 1

You could implement your loop that way.

Answer 2

i是计数器，所以如果你将它设置为循环内的一个值，它就不会正确计数。