division by 7 efficiently ???

[garrickp at gmail.com]

On Feb 1, 8:25 pm, "Krypto" <krypto.wiz... at gmail.com> wrote:
> The correct answer as told to me by a person is
> (N>>3) + ((N-7*(N>>3))>>3)
> The above term always gives division by 7

Does anybody else notice that this breaks the spirit of the problem
(regardless of it's accuracy)? 'N-7' uses the subtraction operator,
and is thus an invalid solution for the original question.

Build a recursive function, which uses two arbitrary numbers, say 1
and 100. Check each, times 7, and make sure that your target number,
N, is between them. Increase or decrease your arbitrary numbers as
appropriate. Now pick a random number between those two numbers, and
check it. Figure out which two the answer is between, and then check a
random number in that subset. Continue this, and you will drill down
to the correct answer, by using only *, +, >, and <.

I'll bet money that since this was a programming interview, that it
wasn't a check of your knowledge of obscure formulas, but rather a
check of your lateral thinking and knowledge of programming.

~G