NetLogo模型grow-grass函数报错：+运算期望数值却获布尔值true

Let's dig into this error you're seeing—it's a classic typo that's turning a numeric variable into a boolean, which breaks the grow-grass procedure. Here's what's going wrong and how to fix it:

1. The Root Cause: A Typo in setup-patches #

The error happens because one of your variables (grass-energy) is being assigned a boolean true/false value instead of a number. Look at this line in your setup-patches procedure:

set grass-energy grass- energy = 0

This line has two critical issues:

• An accidental space between grass- and energy creates invalid variable syntax, and NetLogo tries to interpret this as a boolean comparison (grass- energy = 0) instead of a value assignment.
• You’re using a comparison operator (=) instead of setting a value directly. Even if the variable name was correct, grass-energy = 0 returns true or false, not the number 0.

Fix this line to properly set grass-energy to 0 for brown patches:

set grass-energy 0

2. Missing Global Variable Declarations #

Your code references several global variables (like regrow-energy, green-patches, birth-energy) but doesn’t declare them. NetLogo may treat these as undefined, which could cause additional errors. Add a globals block at the top of your code to define all shared variables:

globals [
  regrow-energy
  green-patches
  grass-alive
  number
  birth-energy
  move-energy
  energy-from-grass
  reproduce-energy
  time-limit
  show-energy?
]

Make sure you set values for these variables (e.g., in your setup procedure) so they aren’t undefined when the model runs.

3. Minor Cleanup Fixes #

• You have two identical grow-grass procedures defined—remove one to avoid confusion.
• In setup-turtles, set energy energy + birth-energy won’t work for new turtles (they start with energy as undefined). Replace it with set energy birth-energy.

Corrected Full Code #

globals [
  regrow-energy
  green-patches
  grass-alive
  number
  birth-energy
  move-energy
  energy-from-grass
  reproduce-energy
  time-limit
  show-energy?
]

turtles-own [energy]
patches-own [grass-energy]

to setup
  clear-all
  setup-patches
  setup-turtles
  reset-ticks
end

to scatter
  clear-turtles
  setup-turtles
end

to go
  if ticks >= time-limit [ stop ]
  move-turtles
  check-patches
  eat-grass
  grow-grass
  reproduce
  check-death
  tick
end

to move-turtles
  ask turtles [
    ifelse move-left? [
      left random 45
      forward 1
      set energy energy - move-energy
    ] [
      right random 360
      forward 1
      set energy energy - move-energy
    ]
  ]
end

to eat-grass
  ask turtles [
    if pcolor = green [
      set energy energy + energy-from-grass
      set grass-energy grass-energy - energy-from-grass
    ]
    ifelse show-energy? [ set label energy ] [set label "" ]
  ]
end

to reproduce
  ask turtles [
    if energy >= reproduce-energy [
      set energy energy - birth-energy
      hatch 1 [ set energy birth-energy ]
    ]
  ]
end

to check-death
  ask turtles [ if energy <= 0 [ die ] ]
end

to check-patches
  ask patches with [count turtles-here >= 2] [
    ask one-of turtles-here [
      ask other turtles-here[set energy energy - (energy / 2)]
    ]
  ]
end

to grow-grass
  ask patches [ set grass-energy grass-energy + regrow-energy ]
end

to setup-patches
  ask n-of green-patches patches [
    set pcolor green
    set grass-energy grass-energy + grass-alive
  ]
  ask patches [
    if pcolor = black [
      set pcolor brown
      set grass-energy 0
    ]
  ]
end

to setup-turtles
  create-turtles number [ setxy random-xcor random-ycor ]
  ask turtles [
    set energy birth-energy
    set shape "cow"
    set size 1.5
  ]
end

内容的提问来源于stack exchange，提问作者Mason