I'm looking for the (hopefully built-in) function in Julia that calculates the number of combinations
I could obviously implement my own using factorials, but I'm almost certain somebody has already worried about this.
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1Minor note: the binomial coefficient counts *combinations*, not permutations. – Alex Riley Apr 30 '18 at 13:09
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1You're absolutely right, and the difference is important because you are counting permutations to be the same. I'll edit. – rodrigolece Apr 30 '18 at 13:15
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Chances are you're looking for the binomial function that returns the binomial coefficient. It's currently in base
Here are some simple examples:
julia> binomial(2,1)
2
julia> binomial(3,2)
3
If you want to see the actual combinations, then you can use the Combinatorics package's combinations(a,n) function. This gives you an iterable with all the possible combinations of length n of array a.
julia> using Combinatorics
julia> collect(combinations(1:3,2))
3-element Array{Array{Int64,1},1}:
[1, 2]
[1, 3]
[2, 3]
Julia Learner
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niczky12
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Be aware to use BigInt if you want to take binomial of "big" numbers like 200
julia> binomial(3,2)
3
julia> binomial(300,200)
ERROR: OverflowError: binomial(300, 200) overflows
Stacktrace:
[1] binomial(::Int64, ::Int64) at ./intfuncs.jl:876
[2] top-level scope at none:0
julia> binomial(BigInt(300),BigInt(200))
4158251463258564744783383526326405580280466005743648708663033657304756328324008620
Steven Siew
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2`julia> binomial(big"300", 200) 4158251463258564744783383526326405580280466005743648708663033657304756328324008620` also works. – Julia Learner Sep 21 '18 at 23:00