#### IfcLoop

Natural language names

##### Semantic definitions at the entity

Entity definition

Definition from ISO/CD 10303-42:1992: A loop is a topological entity constructed from a single vertex, or by stringing together connected (oriented) edges, or linear segments beginning and ending at the same vertex. It is typically used to bound a face lying on a surface. A loop has dimensionality of 0 or 1. The domain of a 0-dimensional loop is a single point. The domain of a 1-dimensional loop is a connected, oriented curve, but need not to be manifold. As the loop is a circle, the location of its beginning/ending point is arbitrary. The domain of the loop includes its bounds, an 0 ≤ Ξ < ∞.

A loop is represented by a single vertex, or by an ordered collection of oriented edges, or by an ordered collection of points. A loop is a graph, so M and the graph genus Gl may be determined by the graph traversal algorithm. Since M = 1, the Euler equation (1) reduces in this case to

where V and El are the number of unique vertices and oriented edges in the loop and Gl is the genus of the loop.

> NOTE: Corresponding STEP entity: loop, the following subtypes have been incorporated into IFC: poly_loop as IfcPolyLoop, vertex_loop as IfcVertexLoop, edge_loop as IfcEdgeLoop. Please refer to ISO/IS 10303-42:1994, p. 136 for the final definition of the formal standard.

> HISTORY: New Entity in IFC Release 2.x.

Informal propositions:

1. A loop has a finite extent.
2. A loop describes a closed (topological) curve with coincident start and end vertices.

##### Inherited definitions from supertypes

Entity inheritance
Attribute inheritance

# | Attribute | Type | Cardinality | Description | A
--- | --- | --- | --- | --- | ---
IfcRepresentationItem
IfcTopologicalRepresentationItem
IfcLoop

##### Formal representations

XSD Specification
`<xs:element name="IfcLoop" type="ifc:IfcLoop" substitutionGroup="ifc:IfcTopologicalRepresentationItem" nillable="true"/>
<xs:complexType name="IfcLoop">
<xs:complexContent>
<xs:extension base="ifc:IfcTopologicalRepresentationItem"/>
</xs:complexContent>
</xs:complexType>
`
EXPRESS Specification
`ENTITY IfcLoop
SUPERTYPE OF(IfcPolyLoop)
SUBTYPE OF (IfcTopologicalRepresentationItem);
END_ENTITY;

EXPRESS-G diagram

`

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