Error messages in NetLogo when trying to use `with`

Question

I'm a new user of NetLogo (5.0.5) and I receive error messages when I try to do pretty basic things. In particular I get

Nothing named x has been defined

when I try to define/use various agentsets. For instance

show turtles with [color=red]

show turtles with [who<10]

ask turtles [set friends turtles with [color=red]]

generate this message (where friends is a turtle variable). However,

ask turtles [set friends turtles in-radius 7]

is ok. So it might have to do with 'with'. I'm not sure what do do here. I was following along Jose Vidal's excellent tutorial

https://www.youtube.com/watch?v=k5RMdrbJXpM&list=PLSx7bGPy9gbFCWOQ6bAb_4ASLIrblXhqP

and everything was working until i tried

show cats with [color=red]

which generated the above error-message. Can anyone help with this?

I haven't found anything about this anywhere but since I'm new there might be resources that do not show up on google (or here) that I've missed.

Answer 1

Unlike some other programming languages, NetLogo doesn't allow you to smash arithmetic and comparison operators up against identifiers; they have to be separated. So you need to write color = red, with the spaces, instead of color=red, and who < 10 instead of who<10. (If you smash them together, NetLogo thinks the whole thing is a single name for something.)

You can smash brackets [] and parentheses () up against other things without spaces, but not other punctuation.