def unrank(self, r):
"""
Returns the subset of s that has rank k.
EXAMPLES::
sage: Subsets(3,2).unrank(0)
{1, 2}
sage: Subsets([2,4,5],2).unrank(0)
{2, 4}
"""
lset = __builtin__.list(self.s)
n = len(lset)
if self.k not in range(len(self.s) + 1):
return None
elif r >= self.cardinality() or r < 0:
return None
else:
return Set([lset[i] for i in choose_nk.from_rank(r, n, self.k)])
def unrank(self, r):
"""
Returns the subset of s that has rank k.
EXAMPLES::
sage: Subsets(3,2).unrank(0)
{1, 2}
sage: Subsets([2,4,5],2).unrank(0)
{2, 4}
"""
lset = __builtin__.list(self.s)
n = len(lset)
if self.k not in range(len(self.s)+1):
return None
elif r >= self.cardinality() or r < 0:
return None
else:
return Set([lset[i] for i in choose_nk.from_rank(r, n, self.k)])
def unrank(self, r):
"""
Returns the subset of s that has rank k.
EXAMPLES::
sage: Subsets(3).unrank(0)
{}
sage: Subsets([2,4,5]).unrank(1)
{2}
"""
lset = __builtin__.list(self.s)
n = len(lset)
if r >= self.cardinality() or r < 0:
return None
else:
for k in range(n + 1):
bin = binomial(n, k)
if r >= bin:
r = r - bin
else:
return Set([lset[i] for i in choose_nk.from_rank(r, n, k)])
def unrank(self, r):
"""
Returns the subset of s that has rank k.
EXAMPLES::
sage: Subsets(3).unrank(0)
{}
sage: Subsets([2,4,5]).unrank(1)
{2}
"""
lset = __builtin__.list(self.s)
n = len(lset)
if r >= self.cardinality() or r < 0:
return None
else:
for k in range(n+1):
bin = binomial(n,k)
if r >= bin:
r = r - bin
else:
return Set([lset[i] for i in choose_nk.from_rank(r, n, k)])