Loop Pattern

Looping is essential for traversing to an arbitrary depth. The loop step allows you to loop over a particular set of steps in the pipeline. For the examples to follow, the Grateful Dead graph discussed in Defining a More Complex Property Graph is used.

g = new TinkerGraph()
GraphMLReader.inputGraph(g, new FileInputStream('data/graph-example-2.xml'))

Lets start off with a simple example. You can make an explicit pipeline to walk a particular path through the graph (outE.inV).

gremlin> g.v(89).outE.inV.paths
==>[v[89], e[7021][89-followed_by->83], v[83]]
==>[v[89], e[7022][89-followed_by->21], v[21]]
==>[v[89], e[7023][89-followed_by->206], v[206]]
==>[v[89], e[7006][89-followed_by->127], v[127]]
...

You can also loop for an arbitrary depth (i.e. any arbitrary number of loops through the looped section of the pipeline). In the example below, the loop step is used for this. The argument to loop states how many steps back to loop over (i.e. outE.inV). The provided closure says to continue to loop while the number of loops that have occurred is less than 3. Thus, what is emitted from loop is the vertices 2 steps away from vertex 89 (Dark Star). Finally, the paths step is used to emit the paths of length 2 emanating from vertex 89.

gremlin> g.v(89).outE.inV.loop(2){it.loops < 3}.paths
==>[v[89], e[7021][89-followed_by->83], v[83], e[1411][83-followed_by->13], v[13]]
==>[v[89], e[7021][89-followed_by->83], v[83], e[1410][83-followed_by->12], v[12]]
==>[v[89], e[7021][89-followed_by->83], v[83], e[1415][83-followed_by->114], v[114]]
==>[v[89], e[7021][89-followed_by->83], v[83], e[1414][83-followed_by->15], v[15]]
...

Note the the expression above is equivalent to the “unrolled” version below.

gremlin> g.v(89).outE.inV.outE.inV.paths        
==>[v[89], e[7021][89-followed_by->83], v[83], e[1411][83-followed_by->13], v[13]]
==>[v[89], e[7021][89-followed_by->83], v[83], e[1410][83-followed_by->12], v[12]]
==>[v[89], e[7021][89-followed_by->83], v[83], e[1415][83-followed_by->114], v[114]]
==>[v[89], e[7021][89-followed_by->83], v[83], e[1414][83-followed_by->15], v[15]]
...

In fact:

gremlin> g.v(89).outE.inV.loop(2){it.loops < 3} == g.v(89).outE.inV.outE.inV            
==>true

The it component of the loop step closure has three properties that are accessible. These properties can be used to reason about when to break out of the loop.

1. it.object: the object moving through the pipeline.
2. it.path: the current path of the object.
3. it.loops: the number of times the traverse has looped through the loop section of the pipeline.

Finally, use GremlinPipeline.toString() to see how loop wraps a section of the pipeline.

gremlin> println g.v(89).outE.inV.paths              
[VertexEdgePipe<OUT_EDGES>, EdgeVertexPipe<IN_VERTEX>, PathPipe]
==>null
gremlin> println g.v(89).outE.inV.loop(2){it.loops < 3}.paths
[LoopPipe<[VertexEdgePipe<OUT_EDGES>, EdgeVertexPipe<IN_VERTEX>]>, PathPipe]
==>null