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Is there a way to reverse a list in Gremlin?

Gremlin queries often work with or produce lists. Sometimes it is desirable to be able to reverse a list. Currently Gremlin does not have a reverse step so you cannot do something like:

g.inject(['A','B','C','D']).reverse()

Nor is there a reverse option for order so you cannot do something like

g.inject(['A','B','C','D']).order(local).by(reverse)

Is there a way to reverse a list in Gremlin today without having to fall back on using closures?

Solution

  • It is possible to do this today using just existing Gremlin steps. The example below takes advantage of the index step to give each element of a list an index number. For example:

    gremlin> g.inject(['A','B','C','D']).index()
==>[[A,0],[B,1],[C,2],[D,3]]

    Given that building block, we can use those index values and order the list.

    gremlin>  g.inject(['A','B','C','D']).index().
......1>    unfold().
......2>    order().
......3>      by(tail(local,1),desc)
==>[D,3]
==>[C,2]
==>[B,1]
==>[A,0]

    The last step is to return the reordered list with the index values removed.

    gremlin>  g.inject(['A','B','C','D']).index().
......1>    unfold().
......2>    order().
......3>      by(tail(local,1),desc).
......4>    limit(local,1).
......5>    fold()  
==>[D,C,B,A]