B$.$ Ask$-$the$-$user$-$to$-$quit

This pattern uses a Boolean variable to control the loop.You would use the ask$-$the$-$user$-$to$-$quit pattern when:

you don't know how many values there will be beforehand; but

there is no particular value that could mark the end of the sequence $($all possible inputs could be valid$).$

E$.$g$.$ calculating the total net worth of a group of people of an unknown size $-$ you can't use a sentinel$,$ because net worth can be positive $($more assets than liabilities$),$ zero, or negative $($more liabilities than assets$).$