I am trying to write a recursive version of Power(x,n) that works by
breaking n down into halves(where half of n=n/2), squaring
Power(x,n/2), and multiplying by x again if n was odd, and to find a
suitable base case to stop the recursion. Can someone give me an
example of this?
Thanks!
Nov 13 '05
64 7331
James Hu <jx*@despammed. com> wrote in message news:<Se******* *************@c omcast.com>... On 2003-10-29, Tim Woodall <go****@woodall .me.uk> wrote: Richard Heathfield <do******@addre ss.co.uk.invali d> wrote in message news:<bn******* ***@hercules.bt internet.com>.. . I'm surprised there is much if any difference:
long mpow(long x, long e) { long r=1;
while(e) { if(e&1) r*=x; x*=x; e>>=1; } return r; }
Its fairly trivial to remove one multiplication at the cost of n if tests (where n is the number of bits in e) and one multiplication can become an assignment but other than that I don't think fewer multiplications are possible using the square and multiply idiom.
double ulpow (double x, unsigned long n) { double t = 1;
if (n & 1) t = x; while (n >>= 1) { x *= x; if (n & 1) t *= x; } return t; }
Am I missing something? Where did we add an additional test per iteration?
You need more tests that that. but an extra
while(!(n&1)) {
n >>= 1;
x*=x;
}
might be sufficient to cover the cases where the least significant
bit of n isn't 1. But you need an extra test for n==0 otherwise my
while loop is infinite. but you can then drop your first if as it will
always be true.
I think I was probably wrong about needing all the extra test but I
was thinking of trying to do it all in one loop where you need a
if(t==1) t=x else t*=x;
Tim. go****@woodall. me.uk (Tim Woodall) wrote in message news:<fa******* *************** ****@posting.go ogle.com>... James Hu <jx*@despammed. com> wrote in message news:<Se******* *************@c omcast.com>... double ulpow (double x, unsigned long n) { double t = 1;
if (n & 1) t = x; while (n >>= 1) { x *= x; if (n & 1) t *= x; } return t; }
You need more tests that that. [...]
Tim, I seem to be unable to find a case that causes the
above routine to misbehave. Could you give me a hint?
Thanks,
-- James
James Hu wrote: go****@woodall. me.uk (Tim Woodall) wrote in message news:<fa******* *************** ****@posting.go ogle.com>... James Hu <jx*@despammed. com> wrote in message news:<Se******* *************@c omcast.com>... double ulpow (double x, unsigned long n) { double t = 1;
if (n & 1) t = x; while (n >>= 1) { x *= x; if (n & 1) t *= x; } return t; }
You need more tests that that. [...]
Tim, I seem to be unable to find a case that causes the above routine to misbehave. Could you give me a hint?
This code avoids multiplying by unity if `n' is odd,
but still performs the "wasted" multiplication if `n' is
even. Consider the case for `n' equal to 2:
t = 1
if (n & 1) [false]
while (n >>= 1) [n <- 1, true]
x *= x
if (n & 1) [true]
t *= x [t <- 1 * x, wasted]
while (n >>= 1) [n <- 0, false]
The most convenient way to avoid the wastage may be
to use two loops in sequence. Just typed in, untested:
if (n == 0)
return 1;
while (!(n & 1)) {
x *= x;
n >>= 1;
}
t = x;
while (n >>= 1) {
x *= x;
if (n & 1)
t *= x;
}
-- Er*********@sun .com
Eric Sosman <Er*********@su n.com> wrote in message news:<3F******* ********@sun.co m>... James Hu wrote: Tim, I seem to be unable to find a case that causes the above routine to misbehave. Could you give me a hint?
This code avoids multiplying by unity if `n' is odd, but still performs the "wasted" multiplication if `n' is even.
I see now. Thanks!
-- James
Richard Heathfield wrote: Well, I was indeed comparing different algorithms. My mistake was to label them "recursive" and "iterative" . It was in fact the difference between square-and-multiply and multiply-n-times that I was attempting to highlight, and I made a complete pig's breakfast of it by attaching informal labels ("recursive" and "iterative" ) to these algorithms instead of spelling out precisely what I mean. Silly of me.
When tail-recursion optimization can be performed on a recursive
algorithm, it can be written iteratively.
AFAIU, in such cases, the iterative version will perform better than
the recursive version. This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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