Error when unwrapping world: "OF expected input to be a turtle agentset or patch agentset or turtle or patch but got NOBODY instead."

[Unruh Lee]

I was experimenting with making the world not wrapped, and upon setup and go, this error appears: 

"OF expected input to be a turtle agentset or patch agentset or turtle or patch but got NOBODY instead." 

I'm guessing this might have to do with the following code: 

to turn-towards-resource  ;; turtle procedure
  set heading 0
  let best-amount 0
  let best-direction []
  ifelse [ cacao-crop = false ] of patch-ahead 1 [ ; if the patch ahead is not an owned cacao farm, let the first direction tested be the "best direction" so far.
    set best-direction [0]
    set best-amount resource-ahead
  ]....

Not sure how to deal with this. When the world is not wrapped, is it necessary to make an "if clause" to the effect "if any? patch-ahead..."?

Full code of model attached in case that's handy. 

Thanks!

Lee

[Charles Staelin]

That is likely the problem you are encountering and, yes, you do need to check for their being a patch ahead when you are working in an unwrapped world.  Because NetLogo evaluates conditionals in order, you could use

ifelse any? patch-ahead 1 AND [ cacao-crop = false ] of patch-ahead 1 [ 

BTW, you could also just use [NOT cacao-crop] instead of  [ cacao-crop = false ] , but that is just a matter of style.

Charles

[Unruh Lee]

Thank you, Dale and Charles!

Lee

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[Dale Frakes]

I believe that will happen with a turtle at the edge that's also facing that edge and then trying to get patch-ahead.

You might check for that condition and have those turtles rotate 180 or check if patch-ahead is NOBODY before trying to use it.

Dale

--
Dale Frakes
E-mail: dfr...@gmail.com ** Web: http://www.dalefrakes.com

"that white light is made of colors, that color measures light waves,
that transparent air reflects light...? It does no harm to the romance
of a sunset to know a little about it." -Carl Sagan

[James Steiner]

Using CAN-MOVE? N  is the best way to casually see if a movement would take a turtle off the edge of the world, or to look ahead to see if there is a valid patch at that distance, as it avoids any use of patch-ahead N which could throw an error in some circumstances. 

You can also use

IS-PATCH? (Expression that might return "nobody")

Combined with PATCH-AT-HEADING-AND-DISTANCE, this is good for a turtle to look in a direction other than its heading. 

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