for循环运行long long int，但不运行unsigned long long int

Question

I am running the following for loop

for(unsigned long long int i = N-1; i >= 0; i--){
    cin>>L[i];
}

when the program reaches this code segment, it stops responding. But when i remove the unsigned , like this

for(long long int i = N-1; i >= 0; i--){
    cin>>L[i];
}

it works fine. Why is this happening?

Answer 1

When i is unsigned, i >= 0 is always true, and so your loop condition is always satisfied.

When i is 0, i-- causes i to be equal to std::numeric_limits<unsigned long long>::max() , which varies by system, but a typical value would be 9223372036854775807. Your loop counter would then start counting down from there.