• "Tyro[17]" <nospam home.com> Apr 21 2012
• Jordi Sayol <g.sayol yahoo.es> Apr 21 2012
• Jordi Sayol <g.sayol yahoo.es> Apr 21 2012
• "Tyro[17]" <nospam home.com> Apr 21 2012

"Tyro[17]" <nospam home.com> writes:

Why does the following implementation of the binomial coefficient 
yield two different answers?

import std.stdio, std.bigint;

void main()
{
     // Correct result when using long
     writeln("(40  20) = ", binomial(40L, 20L));

     // 2 times the expected result when using BigInt
     writeln("(40  20) = ", binomial(BigInt(40), BigInt(20))/2);
}

T binomial(T)(T n, T k)
{
     T iter(T n, T k, T i, T prev)
     {
         if (i >= k) return prev;
         return iter(n, k, i+1, ((n-i)*prev)/(i+1));
     }

     if (k > (n-1)) return iter(n, k, cast(T)0, cast(T)1);
     return iter(n, (n-k), cast(T)0, cast(T)1);
}

Additionally, why is there no implicit conversion from integer to 
BigInt?
Surely now precision will be lost when performing this 
conversion. All
those casts are butt ugly if you ask me. I believe one should be 
able to
assign any integral value to BigInt and reasonably expect that it 
be
implicitly converted.

Thanks,
Andrew

Jordi Sayol <g.sayol yahoo.es> writes:

Al 21/04/12 16:07, En/na Tyro[17] ha escrit:
 Why does the following implementation of the binomial coefficient yield two
different answers?
 


Only happens when compiling to 32-bit.

32-bit:
(40  20) = 137846528820
(40  20) = 68923264410

64-bit:
(40  20) = 137846528820
(40  20) = 137846528820

-- 
Jordi Sayol

Jordi Sayol <g.sayol yahoo.es> writes:

Al 21/04/12 16:42, En/na Tyro[17] ha escrit:
 Actually, in that case it only happens when compiling to 64-bit.
 
 Note: comb1(BigInt(40), BigInt(20))/2; The the BigInt version is
 being divided by two (on MAC OS X) in order to yield the
 correct result.
 


You are right, sorry.

My test was done in a Linux 64-bit.

-- 
Jordi Sayol

"Tyro[17]" <nospam home.com> writes:

On Saturday, 21 April 2012 at 14:30:49 UTC, Jordi Sayol wrote:
 Al 21/04/12 16:07, En/na Tyro[17] ha escrit:
 Why does the following implementation of the binomial 
 coefficient yield two different answers?
 


 Only happens when compiling to 32-bit.

 32-bit:
 (40  20) = 137846528820
 (40  20) = 68923264410

 64-bit:
 (40  20) = 137846528820
 (40  20) = 137846528820


Actually, in that case it only happens when compiling to 64-bit.

Note: comb1(BigInt(40), BigInt(20))/2; The the BigInt version is
being divided by two (on MAC OS X) in order to yield the
correct result.