Maths symbol

Maths symbol

Postby Hypernova » Thu Jun 27, 2013 10:47 am

I'm not a mathematician and I don't understand how to interpret the three dots between multiplication symbols; x...x in this formula for equating possible combinations:

Image

Can somebody tell me how to interpret it please?
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Re: Maths symbol

Postby micseydel » Thu Jun 27, 2013 11:06 am

Moved to the bar, because this is not a Python question.

They're ellipses, implying that there is more in between but that the pattern should be recognizable. So the whole thing is the product of all the parts with denominators (k, k-1, k-2, k-3, ..., 3, 2, 1) and the numerator has its own pattern.
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Simple math code

Postby Hypernova » Fri Jun 28, 2013 6:35 pm

Where am I going wrong here? If n=4 and k=2, I can only get 12 where 6 is the solution. I'm using this equation to find the number of possible combinations ;
Image

Here's my code:
Code: Select all
def choose(n,k):
   a=((n-k+2)/2)
   c=((n-k+1)/1)
   b=n/k
   for i in range(1,k-1):
      b=b*((n-i)/(k-i))
   return (int(b*a*c))

Running choose(4, 2) … Error: choose(4, 2) has wrong value 12, expected 6


BTW, this is a graded exercise for learning python and I included the error report.

Edit: Sorry, I didn't realize this had been moved
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Re: Maths symbol

Postby micseydel » Sat Jun 29, 2013 4:50 am

Don't bother with a and c here. Just initialize a variable to 1, and then multiply it by each part of your product. My function body is 4 lines, including initialization, loop header, loop body and return statement. Try simplifying your code, and if you still have a question after doing so, post your simplified code.

When I wrote this function, I took advantage of the associative property, as a hint for simpler code.
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Re: Maths symbol

Postby Hypernova » Sat Jun 29, 2013 6:22 pm

Yeah you're right. I realized I was getting double the required amount because my code was wrong, and I was getting 36 when i corrected it, so I then thought I was multiplying 6 by 6 and getting 36 and kinda guessed that I was doing the same thing twice and multiplying it together. So I removed the a and b part and pretty much instantly solved it. Thanks.
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