Скачать или смотреть How to Check cheiro Values in NetLogo Patches Efficiently

Discover how to correctly access and evaluate patch variables in a NetLogo program to avoid common pitfalls.
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This video is based on the question https://stackoverflow.com/q/64727873/ asked by the user 'Mauro Abrantes' ( https://stackoverflow.com/u/8126788/ ) and on the answer https://stackoverflow.com/a/64728242/ provided by the user 'JenB' ( https://stackoverflow.com/u/4346285/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Patch Variables in NetLogo

When working with NetLogo, one of the challenges developers encounter is how to effectively check the values of patch variables from the context of turtles. Specifically, many users often seek to check the state of patch variables like cheiro before making movement decisions. This post will delve into a common problem experienced while trying this and provide a straightforward solution to ensure your turtle interacts with the patches correctly.

The Problem

In a typical scenario, you may have a turtle that moves across patches that possess specific variables, such as cheiro and tempo. For example, your code might look something like this:

[[See Video to Reveal this Text or Code Snippet]]

However, if you attempt to run this code, you may encounter an error message saying: "You can't use PATCH-AHEAD in a patch-context, because PATCH-AHEAD is turtle-only." This error indicates that patches do not have movement references as turtles do.

Another Attempt

You might also try another approach:

[[See Video to Reveal this Text or Code Snippet]]

Unfortunately, this results in an error stating: "WITH expected this input to be an agentset, but got a number instead." This further complicates the situation as it shows that patch-ahead cannot be evaluated in this manner when used incorrectly.

The Solution

To successfully check for the value of a patch variable, you need to access the variable from the perspective of the turtle. Instead of asking the patch about its properties, you should reference the patch directly from the turtle context. Here’s how you can do it right:

Correct Code Structure

Modify your approach to look like this:

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the Solution

Understanding the Syntax:

The brackets [cheiro] around cheiro are telling NetLogo to look for the variable within the context of the specified patch.

Functionality:

patch-ahead 1 gives you the patch that is one step in front of the turtle.

The expression [cheiro] of patch-ahead 1 is then able to retrieve the value of the variable cheiro of that specific patch correctly.

Example Implementation:

Here is how it may look in practice within your NetLogo program:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By restructuring your approach to check variable values appropriately, you can avoid errors and allow your turtles to make informed decisions based on patch characteristics. The key takeaway is recognizing the context in which you're operating, whether it’s a turtle or a patch, and accessing variables accordingly. Happy coding in NetLogo!