Take this line of code for example:
for(int i=n-1; ~i; --i)
My question is what does the ~i mean. What I already know is the for loops needs the following:
for (type var = ; condition; something to do with var) {
code here
}
But how is ~i a condition?
All I know is ~i returns the bitwise complement of i, or flip the bits.
In the 2-compliment representation of integers this loop
for(int i=n-1; ~i; --i)
is equivalent to
for(int i=n-1; i != -1; --i)
In c++, this loop
for (int i=n-1; ~i; --i)
will terminate when all the bits of the underlying representation of i are 1s.
In c++-20 the underlying representation of an int is required to be two's complement. The only int with all bits set to 1 in this representation, is -1. So the loop effectively becomes
for (int i=n-1; i != -1; --i)
as @Vlad pointed out in their answer.
Note that implementations pretty much always use two's complement representation, but c++ did not require it before c++20.