# Totally Ordered Finite Sets¶

AUTHORS:

• Stepan Starosta (2012): Initial version
class sage.sets.totally_ordered_finite_set.TotallyOrderedFiniteSet(elements, facade=True)

Totally ordered finite set.

This is a finite enumerated set assuming that the elements are ordered based upon their rank (i.e. their position in the set).

INPUT:

• elements – A list of elements in the set
• facade – (default: True) if True, a facade is used; it should be set to False if the elements do not inherit from Element or if you want a funny order. See examples for more details.

EXAMPLES:

sage: S = TotallyOrderedFiniteSet([1,2,3])
sage: S
{1, 2, 3}
sage: S.cardinality()
3


By default, totally ordered finite set behaves as a facade:

sage: S(1).parent()
Integer Ring


It makes comparison fails when it is not the standard order:

sage: T1 = TotallyOrderedFiniteSet([3,2,5,1])
sage: T1(3) < T1(1)
False
sage: T2 = TotallyOrderedFiniteSet([3,var('x')])
sage: T2(3) < T2(var('x'))
3 < x


To make the above example work, you should set the argument facade to False in the constructor. In that case, the elements of the set have a dedicated class:

sage: A = TotallyOrderedFiniteSet([3,2,0,'a',7,(0,0),1], facade=False)
sage: A
{3, 2, 0, 'a', 7, (0, 0), 1}
sage: x = A.an_element()
sage: x
3
sage: x.parent()
{3, 2, 0, 'a', 7, (0, 0), 1}
sage: A(3) < A(2)
True
sage: A('a') < A(7)
True
sage: A(3) > A(2)
False
sage: A(1) < A(3)
False
sage: A(3) == A(3)
True


But then, the equality comparison is always False with elements outside of the set:

sage: A(1) == 1
False
sage: 1 == A(1)
False
sage: 'a' == A('a')
False
sage: A('a') == 'a'
False


Since trac ticket #16280, totally ordered sets support elements that do not inherit from sage.structure.element.Element, whether they are facade or not:

sage: S = TotallyOrderedFiniteSet(['a','b'])
sage: S('a')
'a'
sage: S = TotallyOrderedFiniteSet(['a','b'], facade = False)
sage: S('a')
'a'


Multiple elements are automatically deleted:

sage: TotallyOrderedFiniteSet([1,1,2,1,2,2,5,4])
{1, 2, 5, 4}

Element
le(x, y)

Return True if $$x \le y$$ for the order of self.

EXAMPLES:

sage: T = TotallyOrderedFiniteSet([1,3,2], facade=False)
sage: T1, T3, T2 = T.list()
sage: T.le(T1,T3)
True
sage: T.le(T3,T2)
True

class sage.sets.totally_ordered_finite_set.TotallyOrderedFiniteSetElement(parent, data)

Element of a finite totally ordered set.

EXAMPLES:

sage: S = TotallyOrderedFiniteSet([2,7], facade=False)
sage: x = S(2)
sage: print(x)
2
sage: x.parent()
{2, 7}