return a number with the first X bits set

+2 Q:

what is a more efficient way to accomplish this in clojure:

(defn ones
  ([n] (ones n 1 1))
  ([n i res]
    (if (< i n)
      (recur n (inc i) (bit-set res i))
      res)))

preferably it should still "do the right thing" when it comes to numerical type.

+6 A:

Why not take 2^(X-1) (by setting only the Xth bit) and then subtract 1?

mquander 2009-07-14 18:24:09

That gives you the last X bits. ~(2^(width of type - X) - 1) will do it.

Matt Kane 2009-07-14 18:27:55

Oh, I see, I mistakenly thought by "first" he meant "least significant."

mquander 2009-07-14 18:30:00

I do want the least significan bits. (ones 4) => 15

Arthur Ulfeldt 2009-07-14 18:53:51

Will exponentiation really be faster than a sequence of bit-shifts?

Hank Gay 2009-07-14 18:58:06

the code for this is (defn ones [x] (- (bit-set 0 x) 1) thanks!

Arthur Ulfeldt 2009-07-14 19:01:19

@Hank, yes, exponentiation of 2 can usually be assumed to be translated to bitshifts by the compiler.

Svante 2009-07-14 19:02:24

Doh! It didn't register that the base was 2, even though that's obviously the only way for this to work.

Hank Gay 2009-07-14 19:11:51

ansaurus