docs/src/recipes/if-then-based-grouping.asciidoc

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[[if-then-based-grouping]]
== If-Then Based Grouping

Consider the following traversal over the "modern" toy graph:

[gremlin-groovy,modern]
----
g.V().hasLabel('person').groupCount().by('age')
----

The result is an age distribution that simply shows that every "person" in the graph is of a different age. In some
cases, this result is exactly what is needed, but sometimes a grouping may need to be transformed to provide a
different picture of the result. For example, perhaps a grouping on the value "age" would be better represented by
a domain concept such as "young", "old" and "very old".

[gremlin-groovy,modern]
----
g.V().hasLabel("person").groupCount().by(values("age").choose(
  is(lt(28)),constant("young"),
  choose(is(lt(30)),
         constant("old"),
         constant("very old"))))
----

Note that the `by` modulator has been altered from simply taking a string key of "age" to take a `Traversal`. That
inner `Traversal` utilizes `choose` which is like an `if-then-else` clause. The `choose` is nested and would look
like the following in Java:

[source,java]
----
if (age < 28) {
  return "young";
} else {
  if (age < 30) {
    return "old";
  } else {
    return "very old";
  }
}
----

The use of `choose` is a good intuitive choice for this `Traversal` as it is a natural mapping to `if-then-else`, but
there is another option to consider with `coalesce`:

[gremlin-groovy,modern]
----
g.V().hasLabel("person").
  groupCount().by(values("age").
  coalesce(is(lt(28)).constant("young"),
           is(lt(30)).constant("old"),
           constant("very old")))
----

The answer is the same, but this traversal removes the nested `choose`, which makes it easier to read.