For-Loop 带有负迭代器的“空”向量？

Question

Im trying to loop through the newest 10 Entries of a vector. The Size of the Vector can be anything between 0 and 100.

As easiest solution, i though starting the loop at vector.size() - 10; and then check inside if its not negative, i can access the vector object.

Sadly, it does not work (doesnt loop at all), but if i replace the same Code with hardcoded numbers, it works.

What im doing wrong, or where is the problem here?

for (int i = -10; i < 0; i++) {
    std::cout << "Normal Loop: i = " << i << std::endl;
}

This works, but looping over a vector doesnt:

std::vector<int> myVector;
for (int i = myVector.size() - 10; i < myVector.size(); i++) {
    std::cout << "Vector Loop: i = " << i << std::endl;
}

The first loop prints all 10 Numbers (From -10 to -1), but the second Loop doesnt print anything.

Since myVector.size() is 0 right now, it should be the same output as the first one

Answer 1

That's because std::vector::size() returns size_type as type, this type is unsigned. This means that i < myVector.size() compares two different types, int and unsigned int . The compiler will "promote" your int to an unsigned type. Because this int is negative and unsigned types can't hold negative values it will wrap around and you'll end up with a very big value and thus the loop condition is never met.

You could cast size() explicitly:

std::vector<int> myVector;
for (int i = myVector.size() - 10; i < static_cast<int>(myVector.size()); i++) {
    std::cout << "Vector Loop: i = " << i << std::endl;
}

Or for a cleaner approach try using rbegin() and loop until rend() or until you've looped 10 times, something like:

std::vector<int> myVector;
//fill vector..
auto itr = myVector.rbegin();
int count = 10;
while(itr != myVector.rend() && count-- > 0)
{
  //process.
  ++itr;
}