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digitalmars.D.learn - BigInt Bug or just me?

reply "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
Apr 21 2012
parent reply 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
Apr 21 2012
parent reply "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.
Apr 21 2012
parent 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
Apr 21 2012