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Re: How to do Combinations/Permutations in BlueJ

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On 11/8/12 9:10 AM, Eric Sosman wrote:
> On 11/7/2012 11:53 PM, lebaz95@gmail.com wrote:
>> On Wednesday, November 7, 2012 8:28:19 PM UTC-6, leb...@gmail.com wrote:
>>> I would like to create a program that will do problems like
>>> (xa+yb)^z. But I would need to do things like (5!/3!(5-3)!) How would
>>> I get this done?
>>
>>               rslt = 1;
>>            for(i = e; i > 0; i --)
>>               {
>>                   rslt *= i;
>>               }
>>
>> I asked my brother and he helped me. e in this program is a user input
>> so you may replace it as you see fit.
>
>      A warning: If `e' is greater than 12, this calculation will
> produce values too large for an `int':
>
>       479001600 = 12!
>      2147483647 = Integer.MAX_VALUE
>      6227020800 = 13!
>
> You can go somewhat higher by making `rslt' a `long':
>
>       2432902008176640000 = 20!
>       9223372036854775807 = Long.MAX_VALUE
>      51090942171709440000 = 21!
>
> ... but for anything over 20 even `long' will not suffice.  You
> should make sure `e' is 20 or smaller (12 or smaller for `int'),
> or take a look at the BigInteger class.
>

Or, since you are dividing by factorials, you can factor them out to 
start with.

5!/3!(5-3)! =
   5*4*3*2*1/(3*2*1)(2*1) =
   (5*4)/(2*1) * (3*2*1)/(3*2*1) =
   5*4/2

The general formula being n!/r!(n-r)!

I believe it is possible to keep the results in the range of integers, 
if the final result is.

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How to do Combinations/Permutations in BlueJ lebaz95@gmail.com - 2012-11-07 18:28 -0800
  Re: How to do Combinations/Permutations in BlueJ lebaz95@gmail.com - 2012-11-07 20:53 -0800
    Re: How to do Combinations/Permutations in BlueJ markspace <-@.> - 2012-11-07 23:03 -0800
      Re: How to do Combinations/Permutations in BlueJ Gene Wirchenko <genew@ocis.net> - 2012-11-08 11:16 -0800
    Re: How to do Combinations/Permutations in BlueJ Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-11-08 09:10 -0500
      Re: How to do Combinations/Permutations in BlueJ Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-11-08 10:48 -0500
        Re: How to do Combinations/Permutations in BlueJ Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-11-08 13:18 -0500
          Re: How to do Combinations/Permutations in BlueJ Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-11-08 23:14 -0500
  Re: How to do Combinations/Permutations in BlueJ Arne Vajhøj <arne@vajhoej.dk> - 2012-11-08 15:52 -0500

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