Value from as step suprisingly becomes null in math step

[Danial Saleem]

So I have this query:

g.V().hasLabel('mylabel').count().as('a').V().count().math('_ - a')

which gives me this error:
The variable a for math() step must resolve to a Number - it is instead of type null with value null

even though I do set the value

[Stephen Mallette]

The error message isn't clear but basically you can't access a path after a reducing barrier like count():

gremlin> g.V().hasLabel('person').count().as('a').V().count().select('a')
gremlin> g.V().hasLabel('person').count().as('a').V().select('a')
==>4
==>4
==>4
==>4
==>4
==>4

If you need to bring a value across the barrier you typically have to keep it as a side-effect:

gremlin> g.V().hasLabel('person').count().aggregate('a').V().count().math('_ - a').by().by(unfold())
==>2.0

 

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[Danial Saleem]

thanks a lot. that did work for me.