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Calculating combinatons


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Until now I always refered to a certain website if I needed to calculate the number of k-element combinations within n-element set.

Today I wrote a macro that does the calculation for me. It is still hot, so there are still errors due occasional division by 0 which I am gonna deal with later, and few other shortcomings. But in general it works ok.

The website does the calculaton somehow for any REASONABLE n and k values while my macro will fail if it needs to calculate i.e. 25! (factorial of 25) as a part of calculating the combinations number. The website is probably (for sure) using a different algorithm to do the calculations than the one I used in my script. So, can anyone tell how does calculating combinations number look like in i.e VBA or Java? I mean, is there a native procedure (or something like this) one could use, and only enter the values of n and k, or it requires writing the whole code calculating the number of combinations step by step(in a way I did in the macro below). I know the website calculator is better at avoiding situation when an integer would receive a number larger than it can store. I just wonder whether the programmer deserves the credit for it or it is just an upside of the programming language used.

 

 

The macro attached prompts for the values of n and k and provides the result.

combinations calculator.mex

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It failed my first few simple tests:

n = 3 k = 2

n = 3 k = 1

n = 4 k = 1

n = 6 k = 5

 

I assume it fails with k = 1 for all n. And whenever n-k = 1.

 

I'd simply get the macro to open Excel and insert n and k into the function COMBIN().

 

 

--

Terry, East Grinstead, UK

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I assume it fails with k = 1 for all n. And whenever n-k = 1.

You're right. Due the algorithm I used, while calculating combinations for k=1 and for n-k=1 division by 0 occurs.

As for k=1 - it is easy to fix, cause when k=1 the combination number equals n. As for n-k=1 I know alright why the division by 0 occurs and I will deal with this later.

 

I'd simply get the macro to open Excel and insert n and k into the function COMBIN().

In the future I will need to calculate combinations on different users' machines. Dynamically. The need of extra software used in the process is not really an option. The macro I wrote works just fine within the range of numbers I will need it to work. It won't work for ie. n=400 and k=390 but that's ok, I'm not gonna need to calculate such numbers, ever.

But since the macro is written, I was just wondering how I could make the macro more varsatile, make it work with greater range of numbers. It would require a different algorithm... or I was hoping that there might exist some native procedure similar to Excel's function COMBIN() in VBA or Java or whatever language I could insert into ME script.

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