great

perhaps you can use signature.cbegin()

what's this??

you mean it evaluates to string each loop thus having O(n^2) time complexity

Nothing better than delegating on the standard libraries! ;)

Is there a point of the last two lines?

oh！

of course divisor is not zero

This one fails for an example from the text n=695, p=2

There is no ISO C 2020 standard. There is no reason to assume this valid syntax will be invalidated in the future.

No

This doesn't work when n==0

Is this valid ISO 2020 syntax in C?

I've never seen this syntax before and tried googling it with no success. Can someone briefly explain it to me??