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[[looping]]
== Looping
One common use case when working with Gremlin is to perform complex looping statements using the `repeat()` step.  While many of the common patterns for looping within traversals are discussed in the documentation there are several more complex patterns that include additional steps which are also commonly used.  This section attempts to demonstrate how to use some of these more complex measurements.

image:tree-lca.png[width=230,float=right] The following examples will use this graph depicted here:

[gremlin-groovy]
----
g.addV().property(id, 'A').as('a').
  addV().property(id, 'B').as('b').
  addV().property(id, 'C').as('c').
  addV().property(id, 'D').as('d').
  addV().property(id, 'E').as('e').
  addV().property(id, 'F').as('f').
  addV().property(id, 'G').as('g').
  addE('hasParent').from('a').to('b').
  addE('hasParent').from('b').to('c').
  addE('hasParent').from('d').to('c').
  addE('hasParent').from('c').to('e').
  addE('hasParent').from('e').to('f').
  addE('hasParent').from('g').to('f').iterate()
----

=== Conditional Looping with Max Depth

One common situation encountered when writing `repeat()` loop is the need to exit the loop when either a 
specific condition is met or a maximum depth is reached.  Given that graph above the following traversal 
demonstrates how to accomplish this type of complex exit condition.  In this example the traversal will 
start at vertex A and loop through all outgoing edges until it reaches vertex C or until it has 
completed 3 loops.

[gremlin-groovy,existing]
----
g.V('A').
  repeat(out().simplePath()).
    until(hasId('C').or().loops().is(3))
----

In the above case the traversal processes the `out().simplePath()` step once to move from vertex A to 
vertex B, a second time to move from vertex B to vertex C, and then exits the traversal because it 
satisfies the `hasId('C')` condition since it's reached vertex C.  If we however change that exit condition 
to be vertex G, we will see that this traversal now will continue to process thorough vertex X and will 
exit based on reaching the maximum number of loops (3).

[gremlin-groovy,existing]
----
g.V('A').
  repeat(out().simplePath()).
    until(hasId('G').or().loops().is(3))
----

The key portion of the above traversal is the `until()` step.  This step is capable of accepting a `Traversal` 
object which continues the `repeat()` loop traversing until the evaluated output of the traversal is true, when
the traverser exits the `repeat()` loop.  This methodology allows complex logical conditions 
to be used as the exit criteria.

=== Emitting Loop Depth

Another common situation encountered when using `repeat()` loops is the desire to emit not only the value 
of the traverser at each step, but also to emit the depth of that element in the repeat loop.  Below are 
several different recipes for accomplishing this task based on what the desired output:

If the desired output is to get each vertex and its associated depth this can be accomplished using this traversal.

[gremlin-groovy,existing]
----
g.withSack(1).V('A').
  repeat(both().simplePath().
    sack(assign).by(loops())
  ).emit().
  project('vertex', 'depth').
    by().
    by(sack())
----

However, if the desired output is to get a single result per level containing all the vertices at that level 
this can be accomplished with this traversal.

[gremlin-groovy,existing]
----
g.V('A').
  repeat(both().simplePath().group('x').
    by(loops())
  ).emit().times(3).
  cap('x')
----

=== Optional Loop Depth

A third common use of `repeat()` loops is to loop through a series of steps a certain number of times or
until there are no additional edges to traverse.  The examples below demonstrate how this loop termination can
be accomplished.

The first method, is an extension of our "Conditional Looping with Max Depth" pattern above where the condition
in the `until()` checking for a vertex with a degree of zero.

[gremlin-groovy,existing]
----
g.V('A').
  repeat(out().simplePath()).
    until(outE().count().is(0).or().loops().is(5))
----

Another way to accomplish this requirement is to use the `optional()` step.  Here use the `optional()` step
within a `repeat().times()` loop.  In the example below we start at vertex A and traverse all `out()` edges
up to a maximum of 5 times.

[gremlin-groovy,existing]
----
g.V('A').repeat(optional(out())).times(5)
----

Each of these patterns can be combined with any variety of other steps to satisfy more complex traversal requirements.  
For example, we can first use a more traditional `repeat()` loop to start at vertex A, traverse all `both()`
edges two times, and then traverse `out()` edges up to five times.

[gremlin-groovy,existing]
----
g.V('A').repeat(both()).times(2).repeat(optional(out())).times(5)
----
