error when cout vector size minus a number bigger than the size ( vector.size()-n )

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I have a vector a={1,2}, so a.size()=2;

I found that:

cout<<(a.size()-3);

will give me a weird number (e.g. 18446744073709551615).

why is it not -1?

if I do:

cout<<(a.size-2);

or

int s = a.size-3;
cout<<s;

everything is normal (-1).

vector size c++
1 Answers

size() returns a size_t, which is an unsigned type. When performing a calculation that's supposed to return a negative number, it underflows, and you get the huge value you observed. When you explicitly assign the result to a (signed) int, you treat the result as a signed number, and get the expected result of -1.

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