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| andreas at ag.or.at |
 Posted: Fri Dec 18, 1998 6:20 pm |
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Why does the following function not work as intended? It seems like the
test for N being even doesn't work :(
pow(N,0) -> io:write({pow,N,0}), io:nl(), 1;
pow(N,2) -> N * N;
pow(N,1) -> N;
pow(N,M) when N rem 2 == 0 ->
io:write({powNeven,N,M}), io:nl(),
X = pow(N,M div 2), X * X.
28> test:pow(10,3).
{powNeven,10,3}
100
Andreas
--
Win95: n., A huge annoying boot virus that causes random spontaneous system
crashes, usually just before saving a massive project. Easily cured by
UNIX. See also MS-DOS, IBM-DOS, DR-DOS, Win 3.x, Win98.
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| mbjk at eecs.umich.edu |
| Posted: Fri Dec 18, 1998 6:35 pm |
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Andreas Kostyrka <andreas_at_ag.or.at> wrote:
> Why does the following function not work as intended? It seems like the
> test for N being even doesn't work :(
Well, it does! But you probably want to test that M is even, not that
N is even.
Also, you will need to handle the case that M is odd.
So the resulting code could look like:
pow(N,0) -> io:write({pow,N,0}), io:nl(), 1;
pow(N,1) -> N;
pow(N,M) when M rem 2 == 0 ->
io:write({powNeven,N,M}), io:nl(),
X = pow(N,M div 2), X * X;
pow(N,M) -> % M is odd, > 1
X = pow(N,M-1),
X*N.
I also deleted the clause:
> pow(N,2) -> N * N;
... since that case is taken care of when M rem 2 == 0.
/martin
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| joe at erix.ericsson.se |
| Posted: Mon Dec 21, 1998 9:07 am |
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> So the resulting code could look like:
>
> pow(N,0) -> io:write({pow,N,0}), io:nl(), 1;
> pow(N,1) -> N;
> pow(N,M) when M rem 2 == 0 ->
> io:write({powNeven,N,M}), io:nl(),
> X = pow(N,M div 2), X * X;
> pow(N,M) -> % M is odd, > 1
> X = pow(N,M-1),
> X*N.
>
If you look in http://www.erlang.org/examples/examples-2.0.html
And in http://www.erlang.org/examples/examples-2.0.tgz
You'll find 3 modules:
lin.erl - this contains some simple functions for linear
algebra. (including pow/2)
primes.erl - Contains routines for generating large primes.
rsa_key.erl - Shows how a public key crypto system could have been
implemented in Erlang.
--
Joe Armstrong Computer Science Laboratory +46 8 719 9452
AT2/ETX/DN/SU Ericsson Telecom SE-126 25 Stockholm Sweden
joe_at_cslab.ericsson.se http://www.ericsson.se/cslab/~joe
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