NetLogo中解决"that link is dead"错误及max-links列表含nobody问题

Let's break down what's going wrong with your code and fix both issues step by step.

Why You're Seeing These Problems #

1. "That link is dead" error: Your code is iterating over the max-links list and asking links to perform actions after some of them have already been deleted. Once a link runs die, any subsequent attempt to access its properties (like end1 or trust) will throw this error because the link no longer exists.

2. nobody in max-links: When a turtle has no outgoing links, max-one-of my-out-links [trust] returns nobody. Your original code didn't filter these out when building the link-set, so nobody gets added. The line you tried to fix this (if one-of max-links = nobody [...]) doesn't work because one-of picks a random element—you're not guaranteed to hit the nobody entry, and even if you do, regenerating max-links without filtering out turtles with no out-links will just re-add nobody.

Corrected Solution #

We'll restructure the logic to avoid modifying the list while iterating over it, and explicitly filter out invalid entries from max-links:

; Step 1: Generate a clean initial max-links (filter out turtles with no outgoing links)
set max-links sort-on [(- trust)] link-set [max-one-of my-out-links [trust]] of turtles with [any? my-out-links]

; Step 2: Find all reciprocal max-link pairs (A→B and B→A both in max-links)
let reciprocal-pairs []
foreach max-links [ current-link ->
  let reverse-link link [end2] of current-link [end1] of current-link
  ; Only add the pair if reverse exists, is in max-links, and we haven't already added the reverse pair
  if reverse-link != nobody and member? reverse-link max-links and not member? (list reverse-link current-link) reciprocal-pairs [
    set reciprocal-pairs lput (list current-link reverse-link) reciprocal-pairs
  ]
]

; Step 3: Process each reciprocal pair—delete the lower-weight link
foreach reciprocal-pairs [ pair ->
  let link1 first pair
  let link2 last pair
  ifelse [trust] of link1 < [trust] of link2 [
    ask link1 [die]
  ] [
    ask link2 [die]
  ]
]

; Step 4: Regenerate max-links to remove any entries for deleted links/turtles with no out-links
set max-links sort-on [(- trust)] link-set [max-one-of my-out-links [trust]] of turtles with [any? my-out-links]

show max-links

Key Improvements #

• No more dead link errors: We first identify all reciprocal pairs before deleting any links, so we never try to access a link that's already been removed.
• No nobody entries: Using turtles with [any? my-out-links] ensures we only include turtles that actually have outgoing links, so max-one-of never returns nobody.
• Avoid duplicate processing: We check that we haven't already added the reverse pair to reciprocal-pairs, so each pair is only handled once.

Quick Fix for Existing nobody Entries #

If you ever need to clean up nobody from a list without regenerating it, use filter:

set max-links filter [is-link? max-links] max-links

This removes any elements that aren't valid links (like nobody).

内容的提问来源于stack exchange，提问作者mohammad tahsiri