for <==> while (an exception)

[VLD/VMB]

There is no problem; "for" is not entirely equivalent to the
corresponding "while" construct, as you have observed.
K&R didn't make this clear, although if you read about
"continue" in Appendix A, the difference becomes apparent.
Later commentators have usually been more careful in declaring
that the "for" loop is equivalent to the corresponding "while",
by adding a caveat about "continue".