Related
Combinations without repetitions look like this, when the number of elements to choose from (n) is 5 and elements chosen (r) is 3:
0 1 2
0 1 3
0 1 4
0 2 3
0 2 4
0 3 4
1 2 3
1 2 4
1 3 4
2 3 4
As n and r grows the amount of combinations gets large pretty quickly. For (n,r) = (200,4) the number of combinations is 64684950.
It is easy to iterate the list with r nested for-loops, where the initial iterating value of each for loop is greater than the current iterating value of the for loop in which it is nested, as in this jsfiddle example:
https://dotnetfiddle.net/wHWK5o
What I would like is a function that calculates only one combination based on its index. Something like this:
tuple combination(i,n,r) {
return [combination with index i, when the number of elements to choose from is n and elements chosen is r]
Does anyone know if this is doable?
You would first need to impose some sort of ordering on the set of all combinations available for a given n and r, such that a linear index makes sense. I suggest we agree to keep our combinations in increasing order (or, at least, the indices of the individual elements), as in your example. How then can we go from a linear index to a combination?
Let us first build some intuition for the problem. Suppose we have n = 5 (e.g. the set {0, 1, 2, 3, 4}) and r = 3. How many unique combinations are there in this case? The answer is of course 5-choose-3, which evaluates to 10. Since we will sort our combinations in increasing order, consider for a minute how many combinations remain once we have exhausted all those starting with 0. This must be 4-choose-3, or 4 in total. In such a case, if we are looking for the combination at index 7 initially, this implies we must subtract 10 - 4 = 6 and search for the combination at index 1 in the set {1, 2, 3, 4}. This process continues until we find a new index that is smaller than this offset.
Once this process concludes, we know the first digit. Then we only need to determine the remaining r - 1 digits! The algorithm thus takes shape as follows (in Python, but this should not be too difficult to translate),
from math import factorial
def choose(n, k):
return factorial(n) // (factorial(k) * factorial(n - k))
def combination_at_idx(idx, elems, r):
if len(elems) == r:
# We are looking for r elements in a list of size r - thus, we need
# each element.
return elems
if len(elems) == 0 or len(elems) < r:
return []
combinations = choose(len(elems), r) # total number of combinations
remains = choose(len(elems) - 1, r) # combinations after selection
offset = combinations - remains
if idx >= offset: # combination does not start with first element
return combination_at_idx(idx - offset, elems[1:], r)
# We now know the first element of the combination, but *not* yet the next
# r - 1 elements. These need to be computed as well, again recursively.
return [elems[0]] + combination_at_idx(idx, elems[1:], r - 1)
Test-driving this with your initial input,
N = 5
R = 3
for idx in range(choose(N, R)):
print(idx, combination_at_idx(idx, list(range(N)), R))
I find,
0 [0, 1, 2]
1 [0, 1, 3]
2 [0, 1, 4]
3 [0, 2, 3]
4 [0, 2, 4]
5 [0, 3, 4]
6 [1, 2, 3]
7 [1, 2, 4]
8 [1, 3, 4]
9 [2, 3, 4]
Where the linear index is zero-based.
Start with the first element of the result. The value of that element depends on the number of combinations you can get with smaller elements. For each such smaller first element, the number of combinations with first element k is n − k − 1 choose r − 1, with potentially some of-by-one corrections. So you would sum over a bunch of binomial coefficients. Wolfram Alpha can help you compute such a sum, but the result still has a binomial coefficient in it. Solving for the largest k such that the sum doesn't exceed your given index i is a computation you can't do with something as simple as e.g. a square root. You need a loop to test possible values, e.g. like this:
def first_naive(i, n, r):
"""Find first element and index of first combination with that first element.
Returns a tuple of value and index.
Example: first_naive(8, 5, 3) returns (1, 6) because the combination with
index 8 is [1, 3, 4] so it starts with 1, and because the first combination
that starts with 1 is [1, 2, 3] which has index 6.
"""
s1 = 0
for k in range(n):
s2 = s1 + choose(n - k - 1, r - 1)
if i < s2:
return k, s1
s1 = s2
You can reduce the O(n) loop iterations to O(log n) steps using bisection, which is particularly relevant for large n. In that case I find it easier to think about numbering items from the end of your list. In the case of n = 5 and r = 3 you get choose(2, 2)=1 combinations starting with 2, choose(3,2)=3 combinations starting with 1 and choose(4,2)=6 combinations starting with 0. So in the general choose(n,r) binomial coefficient you increase the n with each step, and keep the r. Taking into account that sum(choose(k,r) for k in range(r,n+1)) can be simplified to choose(n+1,r+1), you can eventually come up with bisection conditions like the following:
def first_bisect(i, n, r):
nCr = choose(n, r)
k1 = r - 1
s1 = nCr
k2 = n
s2 = 0
while k2 - k1 > 1:
k3 = (k1 + k2) // 2
s3 = nCr - choose(k3, r)
if s3 <= i:
k2, s2 = k3, s3
else:
k1, s1 = k3, s3
return n - k2, s2
Once you know the first element to be k, you also know the index of the first combination with that same first element (also returned from my function above). You can use the difference between that first index and your actual index as input to a recursive call. The recursive call would be for r − 1 elements chosen from n − k − 1. And you'd add k + 1 to each element from the recursive call, since the top level returns values starting at 0 while the next element has to be greater than k in order to avoid duplication.
def combination(i, n, r):
"""Compute combination with a given index.
Equivalent to list(itertools.combinations(range(n), r))[i].
Each combination is represented as a tuple of ascending elements, and
combinations are ordered lexicograplically.
Args:
i: zero-based index of the combination
n: number of possible values, will be taken from range(n)
r: number of elements in result list
"""
if r == 0:
return []
k, ik = first_bisect(i, n, r)
return tuple([k] + [j + k + 1 for j in combination(i - ik, n - k - 1, r - 1)])
I've got a complete working example, including an implementation of choose, more detailed doc strings and tests for some basic assumptions.
Let's say I have a vector of unique integers, for example [1, 2, 6, 4] (sorting doesn't really matter).
Given some n, I want to get all possible values of summing n elements of the set, including summing an element with itself. It is important that the list I get is exhaustive.
For example, for n = 1 I get the original set.
For n = 2 I should get all values of summing 1 with all other elements, 2 with all others etc. Some kind of memory is also required, in the sense that I have to know from which entries of the original set did the sum I am facing come from.
For a given, specific n, I know how to solve the problem. I want a concise way of being able to solve it for any n.
EDIT: This question is for Julia 0.7 and above...
This is a typical task where you can use a dictionary in a recursive function (I am annotating types for clarity):
function nsum!(x::Vector{Int}, n::Int, d=Dict{Int,Set{Vector{Int}}},
prefix::Vector{Int}=Int[])
if n == 1
for v in x
seq = [prefix; v]
s = sum(seq)
if haskey(d, s)
push!(d[s], sort!(seq))
else
d[s] = Set([sort!(seq)])
end
end
else
for v in x
nsum!(x, n-1, d, [prefix; v])
end
end
end
function genres(x::Vector{Int}, n::Int)
n < 1 && error("n must be positive")
d = Dict{Int, Set{Vector{Int}}}()
nsum!(x, n, d)
d
end
Now you can use it e.g.
julia> genres([1, 2, 4, 6], 3)
Dict{Int64,Set{Array{Int64,1}}} with 14 entries:
16 => Set(Array{Int64,1}[[4, 6, 6]])
11 => Set(Array{Int64,1}[[1, 4, 6]])
7 => Set(Array{Int64,1}[[1, 2, 4]])
9 => Set(Array{Int64,1}[[1, 4, 4], [1, 2, 6]])
10 => Set(Array{Int64,1}[[2, 4, 4], [2, 2, 6]])
8 => Set(Array{Int64,1}[[2, 2, 4], [1, 1, 6]])
6 => Set(Array{Int64,1}[[2, 2, 2], [1, 1, 4]])
4 => Set(Array{Int64,1}[[1, 1, 2]])
3 => Set(Array{Int64,1}[[1, 1, 1]])
5 => Set(Array{Int64,1}[[1, 2, 2]])
13 => Set(Array{Int64,1}[[1, 6, 6]])
14 => Set(Array{Int64,1}[[4, 4, 6], [2, 6, 6]])
12 => Set(Array{Int64,1}[[4, 4, 4], [2, 4, 6]])
18 => Set(Array{Int64,1}[[6, 6, 6]])
EDIT: In the code I use sort! and Set to avoid duplicate entries (remove them if you want duplicates). Also you could keep track how far in the index on vector x in the loop you reached in outer recursive calls to avoid generating duplicates at all, which would speed up the procedure.
I want a concise way of being able to solve it for any n.
Here is a concise solution using IterTools.jl:
Julia 0.6
using IterTools
n = 3
summands = [1, 2, 6, 4]
myresult = map(x -> (sum(x), x), reduce((x1, x2) -> vcat(x1, collect(product(fill(summands, x2)...))), [], 1:n))
(IterTools.jl is required for product())
Julia 0.7
using Iterators
n = 3
summands = [1, 2, 6, 4]
map(x -> (sum(x), x), reduce((x1, x2) -> vcat(x1, vec(collect(product(fill(summands, x2)...)))), 1:n; init = Vector{Tuple{Int, NTuple{n, Int}}}[]))
(In Julia 0.7, the parameter position of the neutral element changed from 2nd to 3rd argument.)
How does this work?
Let's indent the one-liner (using the Julia 0.6 version, the idea is the same for the Julia 0.7 version):
map(
# Map the possible combinations of `1:n` entries of `summands` to a tuple containing their sum and the summands used.
x -> (sum(x), x),
# Generate all possible combinations of `1:n`summands of `summands`.
reduce(
# Concatenate previously generated combinations with the new ones
(x1, x2) -> vcat(
x1,
vec(
collect(
# Cartesian product of all arguments.
product(
# Use `summands` for `x2` arguments.
fill(
summands,
x2)...)))),
# Specify for what lengths we want to generate combinations.
1:n;
# Neutral element (empty array).
init = Vector{Tuple{Int, NTuple{n, Int}}}[]))
Julia 0.6
This is really just to get a free critique from the experts as to why my method is inferior to theirs!
using Combinatorics, BenchmarkTools
function nsum(a::Vector{Int}, n::Int)::Vector{Tuple{Int, Vector{Int}}}
r = Vector{Tuple{Int, Vector{Int}}}()
s = with_replacement_combinations(a, n)
for i in s
push!(r, (sum(i), i))
end
return sort!(r, by = x -> x[1])
end
#btime nsum([1, 2, 6, 4], 3)
It runs in circa 4.154 μs on my 1.8 GHz processor for n = 3. It produces a sorted array showing the sum (which may appear more than once) and how it is made up (which is unique to each instance of the sum).
Right now I have a code that can find the number of combinations of a sum of a value using numbers greater than zero and less than the value.
I need to alter the value in order to expand the combinations so that they include more than just the value.
For example:
The number 10 yields the results:
[1, 2, 3, 4], [1, 2, 7],
[1, 3, 6], [1, 4, 5],
[1, 9], [2, 3, 5], [2, 8],
[3, 7], [4, 6]
But I need to expand this to including any number that collapses to 1 as well. Because in essence, I need 100 = n in that the sum of the individual numbers within the digits = n. So in this case 100 = 1 because 100 --> 1+0+0 = 1
Therefore the number 1999 will also be a valid combination to list for value = 100 because 1999 = 1+9+9+9 = 28, and 28 = 2+8 = 10, and 10 = 1+0 = 1
Now I realize that this will yield an infinite series of combinations, so I will need to set limits to the range I want to acquire data for. This is the current code I am using to find my combinations.
def a(lst, target, with_replacement=False):
def _a(idx, l, r, t, w):
if t == sum(l): r.append(l)
elif t < sum(l): return
for u in range(idx, len(lst)):
_a(u if w else (u + 1), l + [lst[u]], r, t, w)
return r
return _a(0, [], [], target, with_replacement)
for val in range(100,101):
s = range(1, val)
solutions = a(s, val)
print(solutions)
print('Value:', val, "Combinations", len(solutions))
You seem to have multiple issues.
To repeatedly add the decimal digits of an integer until you end with a single digit, you could use this code.
d = val
while d > 9:
d = sum(int(c) for c in str(d))
This acts in just the way you describe. However, there is an easier way. Repeatedly adding the decimal digits of a number is called casting out nines and results in the digital root of the number. This almost equals the remainder of the number when divided by nine, except that you want to get a result of 9 rather than 1. So easier and faster code is
d = val % 9
if d == 0:
d == 9
or perhaps the shorter but trickier
d = (val - 1) % 9 + 1
or the even-more-tricky
d = val % 9 or 9
To find all numbers that end up at 7 (for example, or any digit from 1 to 9) you just want all numbers with the remainder 7 when divided by 9. So start at 7 and keep adding 9 and you get all such values.
The approach you are using to find all partitions of 7 then arranging them into numbers is much more complicated and slower than necessary.
To find all numbers that end up at 16 (for example, or any integer greater than 9) your current approach may be best. It is difficult otherwise to avoid the numbers that directly add to 7 or to 25 without going through 16. If this is really what you mean, say so in your question and we can look at this situation further.
Using Elliptic curve factorization (in Python) I am able to find the PRIME factors of a 50 digit number in ~0.5 sec. Is there any way I can convert the prime factors to the factors of the number?
What I have realized by testing on small digits (496 and 28), multiplying the prime factors together in a specific order. Then multiplying those numbers almost gives me the factors, but it is not very flexible because I have only gotten the formula of what I need to multiply together from small list of prime factors (1,2,3,5).
Here's my version, which computes the element-wise products of the powerset of the set of prime factors, keeping only those products that are unique:
def divisors(n, fs=[]):
if fs == []: fs = factors(n)
divs = [1]
for f in fs:
temp = divs[:]
for d in divs:
if f * d not in temp:
temp.append(f*d)
divs = temp
return sorted(divs)
And here it is in action:
>>> factors(496)
[2, 2, 2, 2, 31]
>>> divisors(496)
[1, 2, 4, 8, 16, 31, 62, 124, 248, 496]
>>> factors(28)
[2, 2, 7]
>>> divisors(28)
[1, 2, 4, 7, 14, 28]
If the number is factored into powers of primes, like p^a q^b r^c then the possible factors of the number are all numbers of the form p^x q^y r^z for 0 ≤ x ≤ a, 0 ≤ y ≤ b, and 0 ≤ r ≤ z.
Since you can have different numbers of prime factors, this is a little programming problem. Have fun.
Maybe you're looking for the product of each (unique) set in the powerset. So for 18=2*3*3 you want the product for each set in {{},{2},{3},{3},{2,3},{2,3},{3,3},{2,3,3}}, giving you {1, 2, 3, 3, 6, 9, 18}.
You can use sympy module like this:
import sympy
sympy.ntheory.factorint(4960) #Factorization.
sympy.ntheory.divisors(4960) #List all divisors.
Is there a direct way of getting the Nth combination of an ordered set of all combinations of nCr?
Example: I have four elements: [6, 4, 2, 1]. All the possible combinations by taking three at a time would be:
[[6, 4, 2], [6, 4, 1], [6, 2, 1], [4, 2, 1]].
Is there an algorithm that would give me e.g. the 3rd answer, [6, 2, 1], in the ordered result set, without enumerating all the previous answers?
Note you can generate the sequence by recursively generating all combinations with the first element, then all combinations without. In both recursive cases, you drop the first element to get all combinations from n-1 elements. In Python:
def combination(l, r):
if r == 0:
yield []
elif len(l) == r:
yield l
else:
for c in (combination(l[1:], r-1)):
yield l[0:1]+c
for c in (combination(l[1:], r)):
yield c
Any time you're generating a sequence by making a choice like this, you can recursively generate the kth element by counting how many elements a choice generates and comparing the count to k. If k is less than the count, you make that choice. Otherwise, subtract the count and repeat for the other possible choices you could make at that point. If there are always b choices, you can view this as generating a number in base b. The technique still works if the number of choices varies. In pseudocode (when all choices are always available):
kth(k, choicePoints)
if choicePoints is empty
return empty list
for each choice in head of choicePoints:
if k < size of choice
return choice and kth(k, tail of choicePoints)
else
k -= size of choice
signal exception: k is out-of-bounds
This gives you a 0-based index. If you want 1-based, change the comparison to k <= size of choice.
The tricky part (and what is unspecified in the pseudocode) is that the size of a choice depends on previous choices. Note the pseudocode can be used to solve a more general case than the problem.
For this specific problem, there are two choices (b= 2) and the size of the 1st choice (i.e. including the 1st element) is given by n-1Cr-1. Here's one implementation (which requires a suitable nCr):
def kthCombination(k, l, r):
if r == 0:
return []
elif len(l) == r:
return l
else:
i=nCr(len(l)-1, r-1)
if k < i:
return l[0:1] + kthCombination(k, l[1:], r-1)
else:
return kthCombination(k-i, l[1:], r)
If you reverse the order of the choices, you reverse the order of the sequence.
def reverseKthCombination(k, l, r):
if r == 0:
return []
elif len(l) == r:
return l
else:
i=nCr(len(l)-1, r)
if k < i:
return reverseKthCombination(k, l[1:], r)
else:
return l[0:1] + reverseKthCombination(k-i, l[1:], r-1)
Putting it to use:
>>> l = [6, 4, 2, 1]
>>> [kthCombination(k, [6, 4, 2, 1], 3) for k in range(nCr(len(l), 3)) ]
[[6, 4, 2], [6, 4, 1], [6, 2, 1], [4, 2, 1]]
>>> powOf2s=[2**i for i in range(4,-1,-1)]
>>> [sum(kthCombination(k, powOf2s, 3)) for k in range(nCr(len(powOf2s), 3))]
[28, 26, 25, 22, 21, 19, 14, 13, 11, 7]
>>> [sum(reverseKthCombination(k, powOf2s, 3)) for k in range(nCr(len(powOf2s), 3))]
[7, 11, 13, 14, 19, 21, 22, 25, 26, 28]
TLDR? Just scroll to the very bottom for my final solution.
I stumbled across this question while I was looking for methods to both get the index a specified combination would be located at if it were in a lexicographically sorted list and vice versa, for a choice of objects from some potentially very large set of objects and couldn't find much on the latter (the inverse of your problem is not so elusive).
Since I also solved (what I thought was) your exact problem before I thought I'd post my solutions to both here.
** EDIT: My requirement is what your requirement was too - I saw the answers and thought recursion was fine. Well now, after six long years you have it; just scroll down.**
For your requirement as (I thought it was) posed in the question this will do the job just fine:
def iterCombinations(n, k):
if k==1:
for i in range(n):
yield [i]
return
result = []
for a in range(k-1, n):
for e in iterCombinations(n, k-1):
if e[-1] == a:
break
yield e + [a]
You can then lookup the item in a collection ordered in the descending order (or use some equivalent compare methodology), so for the case in question:
>>> itemsDescending = [6,4,2,1]
>>> for c in iterCombinations(4, 3):
... [itemsDescending[i] for i in c]
...
[6, 4, 2]
[6, 4, 1]
[6, 2, 1]
[4, 2, 1]
This is also possible straight out of the box in Python, however:
>>> import itertools
>>> for c in itertools.combinations(itemsDescending, 3):
... c
...
(6, 4, 2)
(6, 4, 1)
(6, 2, 1)
(4, 2, 1)
Here is what I did for my requirement (and really for yours!) of a non-recursive algorithm that does not create or traverse the ordered list for either direction, but rather uses a simple but effective non-recursive implementation of nCr, choose(n, k):
def choose(n, k):
'''Returns the number of ways to choose k items from n items'''
reflect = n - k
if k > reflect:
if k > n:
return 0
k = reflect
if k == 0:
return 1
for nMinusIPlus1, i in zip(range(n - 1, n - k, -1), range(2, k + 1)):
n = n * nMinusIPlus1 // i
return n
To get the combination at some (zero-based) index in a forward sorted list:
def iterCombination(index, n, k):
'''Yields the items of the single combination that would be at the provided
(0-based) index in a lexicographically sorted list of combinations of choices
of k items from n items [0,n), given the combinations were sorted in
descending order. Yields in descending order.
'''
if index < 0 or index >= choose(n, k):
return
n -= 1
for i in range(k):
while choose(n, k) > index:
n -= 1
yield n
index -= choose(n, k)
n -= 1
k -= 1
To get the (zero-based) index at which some combination would reside in a reverse ordered list:
def indexOfCombination(combination):
'''Returns the (0-based) index the given combination would have if it were in
a reverse-lexicographically sorted list of combinations of choices of
len(combination) items from any possible number of items (given the
combination's length and maximum value)
- combination must already be in descending order,
and it's items drawn from the set [0,n).
'''
result = 0
for i, a in enumerate(combination):
result += choose(a, i + 1)
return result
It's overkill for your example (but I realise now that that was just an example); this is how that would go for each index in turn:
def exampleUseCase(itemsDescending=[6,4,2,1], k=3):
n = len(itemsDescending)
print("index -> combination -> and back again:")
for i in range(choose(n, k)):
c = [itemsDescending[j] for j in iterCombination(i, n, k)][-1::-1]
index = indexOfCombination([itemsDescending.index(v) for v in c])
print("{0} -> {1} -> {2}".format(i, c, index))
>>> exampleUseCase()
index -> combination -> and back again:
0 -> [6, 4, 2] -> 0
1 -> [6, 4, 1] -> 1
2 -> [6, 2, 1] -> 2
3 -> [4, 2, 1] -> 3
This can find the index given some long list or return the combination at some astronomical index in the blink of an eye, for example:
>>> choose(2016, 37)
9617597205504126094112265433349923026485628526002095715212972063686138242753600
>>> list(iterCombination(_-1, 2016, 37))
[2015, 2014, 2013, 2012, 2011, 2010, 2009, 2008, 2007, 2006, 2005, 2004, 2003,
2002, 2001, 2000, 1999, 1998, 1997, 1996, 1995, 1994, 1993, 1992, 1991, 1990, 1989,
1988, 1987, 1986, 1985, 1984, 1983, 1982, 1981, 1980, 1979]
or, since that was the very last one and could be fast due to the reflection in choose(n, k), here's one from right in the middle and it seems just as fast...
>>> choose(2016, 37)//2
4808798602752063047056132716674961513242814263001047857606486031843069121376800
>>> list(iterCombination(_, 2016, 37))
[1978, 1973, 1921, 1908, 1825, 1775, 1747, 1635, 1613, 1598, 1529, 1528, 1521,
1445, 1393, 1251, 1247, 1229, 1204, 1198, 922, 901, 794, 699, 685, 633, 619, 598,
469, 456, 374, 368, 357, 219, 149, 93, 71]
This last example pauses for thought for a split second, but wouldn't you?
>>> import random
>>> rSet = set(random.randint(0, 10000000) for i in range(900))
>>> len(rSet)
900
>>> rList = sorted(rSet, reverse=True)
>>> combinations.indexOfCombination(rList)
61536587905102303838316048492163850175478325236595592744487336325506086930974887
88085020093159925576117511028315621934208381981476407812702689774826510322023536
58905845549371069786639595263444239118366962232872361362581506476113967993096033
00541202874946853699568596881200225925266331936183173583581021914595163799417151
30442624813775945054888304722079206982972852037480516813527237183254850056012217
59834465303543702263588008387352235149083914737690225710105023486226582087736870
38383323140972279867697434315252036074490127510158752080225274972225311906715033
86851377357968649982293794242170046400174118714525559851836064661141086690326842
25236658978135989907667078625869419802333512020715700514133380517628637151215549
05922388534567108671308819960483147825031620798631811671493891643972220604919591
22785587505280326638477135315176731640100473359830821781905546117103137944239120
34912084544221250309244925308316352643060056100719194985568284049903555621750881
39419639825279398618630525081169688672242833238889454445237928356800414839702024
66807635358129606994342005075585962080795273287472139515994244684088406544976674
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83654290108678890682359278892580855226717964180806265176337132759167920384512456
91624558534942279041452960272707049107641475225516294235268581475735143470692000
78400891862852130481822509803019636619427631175355448729708451565341764545325720
79277290914349746541071731127111532099038538549697091038496002102703737347343739
96398832832674081286904287066696046621691978697914823322322650123025472624927566
99891468668052668317066769517155581261265629289158798073055495539590686279250097
27295943276536772955923599217742543093669565147228386873469711200278811335649924
13587219640724942441913695193417732608127949738209466313175361161142601108707568
19470026889319648128790363676253707359290547393198350533094409863254710237344552
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30411365119180944456819762167312738395140461035991994771968906979578667047734952
21981545694935313345331923300019842406900689401417602004228459137311983483386802
30352489602769346000257761959413965109940729263098747702427952104316612809425394
85037536245288888254374135695390839718978818689595231708490351927063849922772653
26064826999661128817511630298712833048667406916285156973335575847429111697259113
53969532522640227276562651123634766230804871160471143157687290382053412295542343
14022687833967461351170188107671919648640149202504369991478703293224727284508796
06843631262345918398240286430644564444566815901074110609701319038586170760771099
41252989796265436701638358088345892387619172572763571929093224171759199798290520
71975442996399826830220944004118266689537930602427572308646745061258472912222347
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52555927020553995188323691320225317096340687798498057634440618188905647503384292
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6245509252811753667386001506195
However going back from that index to the combination of 900-choose-10,000,000 that it represents with the previous implementation would be very slow (since it simply subtracts one from n at each iteration).
For such large lists of combinations we can instead do a binary search of the space, and the overhead we add means it will only be a little slower for small lists of combinations:
def iterCombination(index, n, k):
'''Yields the items of the single combination that would be at the provided
(0-based) index in a lexicographically sorted list of combinations of choices
of k items from n items [0,n), given the combinations were sorted in
descending order. Yields in descending order.
'''
if index < 0 or n < k or n < 1 or k < 1 or choose(n, k) <= index:
return
for i in range(k, 0, -1):
d = (n - i) // 2 or 1
n -= d
while 1:
nCi = choose(n, i)
while nCi > index:
d = d // 2 or 1
n -= d
nCi = choose(n, i)
if d == 1:
break
n += d
d //= 2
n -= d
yield n
index -= nCi
From this one may notice that all the calls to choose have terms that cancel, if we cancel everything out we end up with a much faster implementation and what is, I think...
The optimal function for this problem
def iterCombination(index, n, k):
'''Yields the items of the single combination that would be at the provided
(0-based) index in a lexicographically sorted list of combinations of choices
of k items from n items [0,n), given the combinations were sorted in
descending order. Yields in descending order.
'''
nCk = 1
for nMinusI, iPlus1 in zip(range(n, n - k, -1), range(1, k + 1)):
nCk *= nMinusI
nCk //= iPlus1
curIndex = nCk
for k in range(k, 0, -1):
nCk *= k
nCk //= n
while curIndex - nCk > index:
curIndex -= nCk
nCk *= (n - k)
nCk -= nCk % k
n -= 1
nCk //= n
n -= 1
yield n
A final reminder that for the use case of the question one would do something like this:
def combinationAt(index, itemsDescending, k):
return [itemsDescending[i] for i in
list(iterCombination(index, len(itemsDescending), k))[-1::-1]]
>>> itemsDescending = [6,4,2,1]
>>> numberOfItemsBeingChosen = 3
>>> zeroBasedIndexWanted = 1
>>> combinationAt(zeroBasedIndexWanted, itemsDescending, numberOfItemsBeingChosen)
[6, 4, 1]
One way to do it would be by using properties of bits. This still requires some enumeration, but you wouldn't have to enumerate every set.
For your example, you have 4 numbers in your set. So if you were generating all the possible combinations of 4 numbers, you could enumerate them as follows:
{6, 4, 2, 1}
0000 - {(no numbers in set)}
0001 - {1}
0010 - {2}
0011 - {2, 1}
...
1111 - {6, 4, 2, 1}
See how each "bit" corresponds to "whether that number is in your set"? We see here that there are 16 possibilities (2^4).
So now we can go through and find all of the possibilities that have only 3 bits turned on. This will tell us all of the combinations of "3" that exist:
0111 - {4, 2, 1}
1011 - {6, 2, 1}
1101 - {6, 4, 1}
1110 - {6, 4, 2}
And lets rewrite each of our binary values as decimal values:
0111 = 7
1011 = 11
1101 = 13
1110 = 14
Now that we've done that - well, you said you wanted the "3rd" enumeration. So lets look at the 3rd largest number: 11. Which has the bit pattern 1011. Which corresponds to... {6, 2, 1}
Cool!
Basically, you can use the same concept for any set. So now all we've done is translate the problem from "enumerating all the sets" to "enumerating all of the integers". This might be a lot easier for your problem.
From the Python 3.6 itertools recipes:
def nth_combination(iterable, r, index):
'Equivalent to list(combinations(iterable, r))[index]'
pool = tuple(iterable)
n = len(pool)
if r < 0 or r > n:
raise ValueError
c = 1
k = min(r, n-r)
for i in range(1, k+1):
c = c * (n - k + i) // i
if index < 0:
index += c
if index < 0 or index >= c:
raise IndexError
result = []
while r:
c, n, r = c*r//n, n-1, r-1
while index >= c:
index -= c
c, n = c*(n-r)//n, n-1
result.append(pool[-1-n])
return tuple(result)
In practice:
iterable, r, index = [6, 4, 2, 1], 3, 2
nth_combination(iterable, r, index)
# (6, 2, 1)
Alternatively, as mentioned in the docstring:
import itertools as it
list(it.combinations(iterable, r))[index]
# (6, 2, 1)
See also more_itertools - a third party library that implements this recipe for you. Install via:
> pip install more_itertools
just a rough sketch:
arrange your numbers into upper triangular matrix of tuples:
A(n-1,n-1)
Aij = [i+1, j-1]
if you traverse matrix row first, you will get combinations in increasing order for two elements. To generalize to three elements, think of your matrix rows as another triangular matrix, rather than a vector. It kind of creates a corner of a cube.
At least this is how I have would approach the problem
let me clarify this, you do not have to store the matrix, you will need to compute index.
Let me work out to dimensional example, which you in principle could expand to 20 dimensions(bookkeeping may be atrocious).
ij = (i*i + i)/2 + j // ij is also the combination number
(i,j) = decompose(ij) // from ij one can recover i,j components
I = i // actual first index
J = j + 1 // actual second index
this two-dimensional example works for any number n, and you dont have to tabulate permutations.
Yes there a direct way of getting the Nth combination of an ordered set of all combinations of nCr? Say you need to generate 0th, 3rd, 6th.. combinations of a given set. You can generate it directly without generating combinations in between using JNuberTools. You can even generate next billionth combination (if your set size is large)
Here is the code example:
JNumberTools.combinationsOf(list)
.uniqueNth(8,1000_000_000) //skip to billionth combination of size 8
.forEach(System.out::println);
The maven dependency for JNumberTools is :
<dependency>
<groupId>io.github.deepeshpatel</groupId>
<artifactId>jnumbertools</artifactId>
<version>1.0.0</version>
</dependency>