NetLogo 6.2.2 User Manual: FAQ (Frequently Asked Questions)

# FAQ (Frequently Asked Questions)

NetLogo 6.2.2 User Manual

Feedback from users is very valuable to us in designing and improving NetLogo. We’d like to hear from you. (See Contacting Us.)

- General
  - Why is it called NetLogo?
  - How do I cite NetLogo or HubNet in a publication?
  - How do I cite a model from the Models Library in a publication?
  - Where and when was NetLogo created?
  - What programming language was NetLogo written in?
  - What’s the relationship between StarLogo and NetLogo?
  - Under what license is NetLogo released? Is the source code available?
  - Do you offer any workshops or other training opportunities for NetLogo?
  - Are there any NetLogo textbooks?
  - Is NetLogo available in other languages besides English?
  - Is NetLogo compiled or interpreted?
  - Has anyone built a model of <x>?
  - Are NetLogo models runs scientifically reproducible?
  - Will NetLogo and NetLogo 3D remain separate?
  - Can I run NetLogo on my phone or tablet?
- Downloading
  - Can I have multiple versions of NetLogo installed at the same time?
  - I’m on a UNIX system and I can’t untar the download. Why?
  - How do I install NetLogo unattended?
- Running
  - Can I run NetLogo from a CD, a network drive, or a USB drive?
  - Why is NetLogo so much slower when I unplug my Windows laptop?
  - Why does NetLogo bundle Java?
  - How come NetLogo won’t start up on my Linux machine?
  - When I try to install NetLogo on Windows, I see “Windows protected your PC”
  - When I try to start NetLogo on Windows I get an error “The JVM could not be started”. Help!
  - NetLogo won’t start on Mac OS Sierra (or later)
  - NetLogo won’t start on Windows or crashes suddenly on Mac OS Sierra
  - Can I run NetLogo from the command line, without the GUI?
  - Does NetLogo take advantage of multiple processors?
  - Can I distribute NetLogo model runs across a cluster or grid of computers?
  - Is there any way to recover lost work if NetLogo crashes or freezes?
  - Why is HubNet Discovery Not Working?
- Usage
  - When I move the speed slider all the way to the right, why does my model seem to stop?
  - Can I use the mouse to “paint” in the view?
  - How big can my model be? How many turtles, patches, procedures, buttons, and so on can my model contain?
  - Can I use GIS data in NetLogo?
  - My model runs slowly. How can I speed it up?
  - Can I have more than one model open at a time?
  - Can I change the choices in a chooser on the fly?
  - Can I divide the code for my model up into several files?
- Programming
  - How does the NetLogo language differ from other Logos?
  - How come my model from an earlier NetLogo doesn’t work right?
  - How do I take the negative of a number?
  - My turtle moved forward 1, but it’s still on the same patch. Why?
  - How do I keep my turtles on patch centers?
  - `patch-ahead 1` is reporting the same patch my turtle is already standing on. Why?
  - How do I give my turtles “vision”?
  - Can agents sense what’s in the drawing layer?
  - I’m getting numbers like 0.10000000004 and 0.799999999999 instead of 0.1 and 0.8. Why?
  - The documentation says that `random-float 1` might return 0 but will never return 1. What if I want 1 to be included?
  - Why is the number value in my monitor widget changing even though nothing is happening in my model?
  - How can I keep two turtles from occupying the same patch?
  - How can I find out if a turtle is dead?
  - Does NetLogo have arrays?
  - Does NetLogo have hash tables or associative arrays?
  - How can I use different patch “neighborhoods” (circular, Von Neumann, Moore, etc.)?
  - How can I convert an agentset to a list of agents, or vice versa?
  - How do I stop foreach?
  - I’m trying to make a list. Why do I keep getting the error “Expected a literal value”?
- BehaviorSpace
  - Why are the rows in my BehaviorSpace table results out of order?
  - How do I measure runs every n ticks?
  - I’m varying a global variable I declared in the Code tab, but it doesn’t work. Why?
- NetLogo 3D
  - Does NetLogo work with my stereoscopic device?
- Extensions
  - I’m writing an extension. Why does the compiler say it can’t find `org.nlogo.api`?

## General

### Why is it called NetLogo?

The “Logo” part is because NetLogo is a dialect of the Logo language.

“Net” is meant to evoke the decentralized, interconnected nature of the phenomena you can model with NetLogo, including network phenomena. It also refers to HubNet, the multiuser participatory simulation environment included in NetLogo.

### How do I cite NetLogo or HubNet in a publication?

If you use or refer to NetLogo, HubNet or a model from the NetLogo models library, we ask that you cite it as follows:

NetLogo itself: Wilensky, U. 1999. NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University. Evanston, IL.

HubNet: Wilensky, U. & Stroup, W., 1999. HubNet. http://ccl.northwestern.edu/netlogo/hubnet.html. Center for Connected Learning and Computer-Based Modeling, Northwestern University. Evanston, IL.

### How do I cite a model from the Models Library in a publication?

The correct citation is included in the “Credits and References” section of each model’s Info tab.

### Where and when was NetLogo created?

NetLogo was first created in 1999 by Uri Wilensky at the Center for Connected Learning and Computer-Based Modeling, then at Tufts University in the Boston area. NetLogo grew out of StarLogoT, which was authored by Wilensky in 1997. In 2000, the CCL moved to Northwestern University, in the Chicago area. NetLogo 1.0 came out in 2002, 2.0 in 2003, 3.0 in 2005, 4.0 in 2007, 4.1 in 2009, and 5.0 in 2012.

### What programming language was NetLogo written in?

NetLogo is written mostly in Scala, with some parts in Java. (Scala code compiles to Java byte code and is fully interoperable with Java and other JVM languages.)

### What’s the relationship between StarLogo and NetLogo?

The original StarLogo began at the MIT Media Lab in 1989 and ran on the Connection Machine. Later versions were developed for Macintosh computers: MacStarLogo (1994, MIT) and StarLogoT (1997, Tufts).

Today there are two StarLogo descendants under active development: NetLogo (from the CCL at Northwestern University) and StarLogo TNG (from MIT). NetLogo is the most widely used agent-based modeling environment in both education and research. StarLogo TNG is distinguished by its programming interface based on colored blocks.

### Under what license is NetLogo released? Is the source code available?

NetLogo is free, open source software under the GPL (GNU General Public License), version 2, or (at your option) any later version.

Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at uri@northwestern.edu.

The source code is on GitHub, here. Development discussion is on the netlogo-devel group.

The User Manual is published under a Creative Commons Attribution-ShareAlike license (CC BY-SA 3.0).

Source code for all of the extensions bundled with NetLogo is on GitHub, here. Most of the extensions are in the public domain (CC0 notice). Other extensions are released under open source licenses. See each extension’s README for details.

The Code Examples in the Models Library are in the public domain (CC0 notice).

The rest of the models in the Models Library are provided under a variety of licenses. Some are public domain and some are open source, but most are under the Creative Commons Attribution-ShareAlike license (CC BY-NC-SA), which is not an open source license, though the models are free for noncommercial distribution and use.

See each model’s Info tab to check its particular license.

The models are in a public Git repository here.

### Do you offer any workshops or other training opportunities for NetLogo?

We offer workshops from time to time. If a workshop has been scheduled, we will announce it on the NetLogo Users Group.

### Are there any NetLogo textbooks?

The CCL has published a textbook (written by the author of NetLogo) that gives an introduction to agent-based modeling methods using NetLogo. It goes step by step with coding examples how to design, build, revise, and analyze models. And it presents some advanced techniques.

See www.intro-to-abm.com for more information on that textbook.

See the Textbooks section of our Resources page.

We at the CCL have hoped to write several more NetLogo textbooks. These could be aimed at different audiences, such as: middle school, high school, undergraduate course in modeling or complexity, practical guide for interested adults.

Unfortunately, we have not yet been able to find the time to make these happen. If anyone from the user community would like to collaborate on such a venture, please let us know. We would welcome it.

### Is NetLogo available in other languages besides English?

Volunteers have translated the user manual into Chinese and Czech. The translated versions are available from the NetLogo web site.

So far, the NetLogo user interface has been localized in English, Spanish, Chinese, Russian, and Japanese. All five languages are included in the standard download.

We are seeking volunteers to complete and improve these localizations and to translate the NetLogo software and manual into as many other languages as possible. If you’re able to help in this endeavor, please contact us.

### Is NetLogo compiled or interpreted?

Short answer: some of both.

Long answer: NetLogo does include a compiler that generates Java byte code. However, this compiler does not yet support the entire language, so some parts of user code remain interpreted. Note that our compiler generates Java byte code, and Java virtual machines have “just-in-time” compilers that in turn compile Java byte code all the way to native code, so much user code is ultimately translated to native code.

### Has anyone built a model of <x>?

Try looking at the NetLogo Models Library, the NetLogo Modeling Commons, our Community Models page, and our list of references to NetLogo in outside works.

You might also ask the question on the NetLogo Users Group and/or search past messages on the group.

### Are NetLogo models runs scientifically reproducible?

Yes. NetLogo’s pseudorandom number generator and agent scheduling algorithms are deterministic, and NetLogo always uses Java’s “strict math” library, which gives bit-for-bit identical results regardless of platform. But keep the following cautions in mind:

- If your model uses random numbers, then in order to get reproducible behavior, you must use the `random-seed` command to set the random seed in advance, so that your model will receive the exact same sequence of random numbers every time. Remember that agentsets are always in random order, so anything you do with agentsets uses random numbers.
- If your model uses the `every` or `wait` commands in such a way that affects the outcome of the model, then you may get different results on different computers, or even on the same computer, since the model may run at a different speed.
- In order to reproduce model runs exactly, you must be using the exact same version of NetLogo. The details of the agent scheduling mechanism and the random number generator may change between NetLogo versions, and other changes (bugfixes in the engine, language changes, and so forth) may also affect the behavior of your model. (Then again, they may not.)
- We have expended every effort to make NetLogo model runs fully reproducible, but of course this can never truly be an iron-clad guarantee, due not only to the possibility of random hardware failure, but also the possibility of human error in the design of: your model, NetLogo, your Java VM, your hardware, and so on.

### Will NetLogo and NetLogo 3D remain separate?

For now, yes. NetLogo 3D is included with NetLogo, but it is still a separate application.

Ideally a single unified application would support both 2D and 3D modeling. We would design the 3D world support so it doesn’t get in the way when you are building 2D models. Models built in NetLogo 3D might require changes in order to run in the hypothetical unified application.

### Can I run NetLogo on my phone or tablet?

No. Neither iOS, nor Android, nor Windows RT supports running Java applications such as NetLogo.

We are working on an alternate implementation of NetLogo on a JavaScript and HTML5 base, instead of Java. It will work on a variety of tablets and phones. We don’t know yet when it will be ready, and we expect that for a long time it will only support a subset of the features in desktop NetLogo. The many person-years of development effort that have gone into the Java version can’t cheaply or easily be replicated on another platform.

For technical details on this new project, go here.

## Downloading

### Can I have multiple versions of NetLogo installed at the same time?

Yes. When you install NetLogo, the folder that is created contains has the version number in its name, so multiple versions can coexist.

On Windows systems, whichever version you installed last will be the version that opens when you double click a model file in Windows Explorer. On Macs, you can control what version opens via “Get Info” in the Finder.

### I’m on a UNIX system and I can’t untar the download. Why?

Some of the files in the tarball have long pathnames, too long for the standard tar format. You must use the GNU version of tar instead (or another program which understands the GNU tar extensions). On some systems, the GNU version of tar is available under the name “gnutar”. You can find out if you are already using the GNU version by typing `tar --version` and seeing if the output says “tar (GNU tar)”.

### How do I install NetLogo unattended?

It depends on which platform you are using.

- Linux: Untar NetLogo into the appropriate place.
- Mac: Copy the NetLogo directory from the disk image into the Applications folder.
- Windows: The installer is an MSI which can be run using one of the following commands (these may require admin privileges):

      msiexec.exe /i C:\Path\to\NetLogo.msi ALLUSERS=1 ADDLOCAL=Shortcuts   :: To install with desktop shortcuts
      msiexec.exe /i C:\Path\to\NetLogo.msi ALLUSERS=1 ADDLOCAL=BaseInstall :: To install without desktop shortcuts

We also maintain an advanced installation wiki page which may have additional tips and tricks. We encourage users who have problems with unattended installation or ideas on how it could be improved to email feedback@ccl.northwestern.edu.

## Running

### Can I run NetLogo from a CD, a network drive, or a USB drive?

Yes. NetLogo runs fine from any file system, including read-only file systems.

### Why is NetLogo so much slower when I unplug my Windows laptop?

Your computer is switching to power saving mode when unplugged. It’s normal for this to reduce speed a little, but unfortunately there is a bug in Java that drastically slows down Swing applications, including NetLogo.

One workaround is to change the power settings on your computer so it doesn’t go into power saving mode when you unplug it. (If you do this, your battery won’t last as long.)

Another workaround is to run NetLogo with an option recommended by Oracle, by editing the NetLogo 6.2.2.vmoptions file, found in the NetLogo directory (under Program Files on your hard drive, unless you installed NetLogo in a different location). Add on a new line:

    -Dsun.java2d.ddoffscreen=false

You can see the details of the Java bug and vote for Oracle to fix it here.

### Why does NetLogo bundle Java?

Since Mac OS X Lion, Apple have encouraged Mac application developers to bundle Java. NetLogo bundles Java because it allows us to deliver a consistent, convenient experience to our users. Bundling Java allows us to test for compatibility once and avoid any bugs caused by version mismatches or Java configuration incompatibilities.

If users are interested in using Java on their system instead of the version of Java bundled with NetLogo, they can configure NetLogo to use a different Java runtime. Please note that this change is done at your own risk. We are unable to offer support for problems caused by running NetLogo with an alternate Java Runtime. To change the Java runtime used by NetLogo, open the NetLogo.cfg file and modify the `app.runtime` property to the path of your preferred Java Runtime.

### How come NetLogo won’t start up on my Linux machine?

We bundle Oracle’s Java runtime when using NetLogo on Linux. If you would like to change the version of linux used, you can modify the .cfg files to point to a different version of java (see How big can my model be? for more information).

In theory, any Java 8 or later runtime will run NetLogo. In practice, some Java implementations aren’t high enough quality. Recent versions of OpenJDK should work; older ones may not. GNU libgcj does not work.

Ubuntu users should consult https://help.ubuntu.com/community/Java.

### When I try to install NetLogo on Windows, I see “Windows protected your PC”

Windows attempts to protect users from downloading malicious software by maintaining a list of “good” and “malicious” software. The first users to install any NetLogo release will see this warning. Later users may or may not see this warning. Before going any further, ensure you are protected. The CCL can only vouch for NetLogo downloads hosted on the CCL Website. NetLogo can be freely downloaded from the official NetLogo download page. If you obtain NetLogo from anywhere else, you install it at your own risk!

You can continue past the Windows prompt by taking the following steps:

- In the “Windows protected your PC” prompt, click “More Info”, the prompt will change
- In the changed prompt, click “Run Anyway” and continue with installation as normal

Note in order to see the “More Info” or “Run Anyway” options, you will need to run the installer as an administrator. If you do not see those options, right-click and choose “Run as administrator” and/or change your user account to an administrator account before installing.

We at the CCL are working on solving this problem and hope to free our users of the added installation difficulty soon!

### When I try to start NetLogo on Windows I get an error “The JVM could not be started”. Help!

A nearly certain fix is to use a text editor to edit the NetLogo.cfg file, changing 1024m to a smaller number, like 512m. This should permit NetLogo to start, although the lower heap size limit may affect your ability to run models with many agents. See How big can my model be? for information on model size and how to edit the cfg file.

If running with the lower heap size limit is unacceptable, read on.

Some Windows systems have trouble allocating large amounts of contiguous virtual memory. Upgrading to a newer version of Windows may help.

Running Windows in 64-bit mode instead of 32-bit mode may also help. Double check that Windows is actually running in 64-bit mode; see Microsoft’s FAQ page on 64-bit Windows.

### NetLogo won’t start on Mac OS Sierra (or later)

Some users have reported problems opening NetLogo 6 on Mac OS Sierra. We’ve been unable to determine a root cause for this problem, but we’re continuing to investigate. We’re looking for assistance from you if you run into this problem. It would be extremely helpful if you could run the following command in the “Terminal” application and send the output in an email to bugs@ccl.northwestern.edu.

    xattr -pl com.apple.quarantine /Applications/NetLogo\ 6.2.2/NetLogo\ 6.2.2.app

running it should give you a single line of output (something like `com.apple.quarantine: 01e1;58ac6af2;Firefox;F2E0B1E2-D203-4B05-8DF9-ABA58B52EFEA`, but yours will have different numbers, letters, and words). Please copy and paste this string into the email you send us.

There is also a partial workaround! Running the following command in the Terminal will enable users to run NetLogo without turning Gatekeeper completely off. The command is:

    sudo xattr -dr com.apple.quarantine /Applications/NetLogo\ 6.2.2/NetLogo\ 6.2.2.app

Note that if you plan to run NetLogo 3D, HubNet Client, or NetLogo Logging, you may also need to re-run that command once for each of those applications. If the workaround was not effective for you, please send us an email and let us know.

### NetLogo won’t start on Windows or crashes suddenly on Mac OS Sierra

Some users on Mac OS with discrete graphics cards experience sudden crashes of NetLogo when switching between integrated and discrete graphics. It’s possible to prevent these crashes by disabling automatic graphics switching in System Preferences.

Some Windows users may also be unable to open NetLogo due to graphics card settings or drivers. If you are unable to open NetLogo on Windows, you might be able to fix this by updating or reinstalling your graphics driver and/or disabling any graphics card utilities. Because NetLogo might be unable to start for a number of reasons, we encourage you to contact us if these steps aren’t effective in resolving the problem.

### Can I run NetLogo from the command line, without the GUI?

Yes. The easiest way is to set up your model run or runs as a BehaviorSpace experiment. No additional programming is required. See the BehaviorSpace Guide for details.

Another option is to use our Controlling API. Some light Java programming is required. See the Controlling API Guide for details and sample code.

### Does NetLogo take advantage of multiple processors?

Only when using BehaviorSpace. BehaviorSpace does parallel runs, one per processor.

For a single model run, only one processor is used.

We are seeking funding to make it possible to split a single model run across multiple processors or multiple computers.

### Can I distribute NetLogo model runs across a cluster or grid of computers?

Many of the same comments in the previous answer apply. It is not possible to split a single model run across multiple computers, but you can have each machine in a cluster doing one or more separate, independent model runs, using either BehaviorSpace or our Controlling API. We don’t have automated support for splitting the runs across clusters, so you’ll need to arrange that yourself.

Various users are already using NetLogo on clusters, with a variety of hardware and software. You can seek them out on the NetLogo Users Group.

### Is there any way to recover lost work if NetLogo crashes or freezes?

Yes. NetLogo auto-saves files as you are working on them. The path to the auto-save file depends on whether or not the NetLogo model has been saved.

For NetLogo models which have been saved, a hidden file with the name “.filename.tmp.nlogo” will be created in the same directory as the NetLogo model.

For unsaved NetLogo models, autosave files can be found in your OS-specific java temporary directory. The files are named according to the following format: `autosave_yyyy-MM-dd.HH_mm_ss.nlogo` where the time and date are the time and date the model was opened. The exact path will depend on your operating system:

- On Mac OS, the temporary directory is `/var/folders/68/<30-character-alphanumeric-sequence>/T/`. The 30-character-alphanumeric-sequence is unique to each machine.
- On Windows the logs can be found in `C:\Users\<user>\AppData\Local\Temp`, where `<user>` is the logged in user.
- On most non-Mac Unix-like systems the temp directory is `/tmp`.

### Why is HubNet Discovery Not Working?

HubNet discovery provides the ability for HubNet clients running on the same local network as the HubNet server to automatically display the IP address and current activity of the server. This uses IPv4 multicast over a local area network - it won’t identify HubNet servers on the internet or on different networks. Some networks also do not support multicast traffic and it will simply not work on those networks. For HubNet Discovery to work, the server must be broadcasting to the same network that the client is polling for messages. Often, both the client and server are only connected to a single network and there is no need for further configuration.

Since NetLogo 6.0.3, we have tools in place to allow clients and/or servers connected to multiple networks to select the network on which they will broadcast/listen. When running a HubNet server, this is done by selecting the appropriate “broadcast network” via selection dropdown when starting a HubNet server. This preference will be remembered until NetLogo is restarted, so if you end up needing to broadcast on a different network, restart NetLogo. HubNet clients also see a similar dropdown when selecting which activity to join. Unlike HubNet server, clients may change the network on which they listen at any time. HubNet discovery will only work when both client and server are listening/broadcasting on the same network, so some trial and error may be required to figure out a working configuration. Generally speaking, the shared network is the one on which the client and the server have the most similar IP address prefixes. For instance, if the server selects to broadcast on a network where it has IP address 192.168.0.101, a client should prefer listening on a network where it has an IP like 192.167.0.203 over a network where it has an IP like 10.5.0.101. Even when the same network is selected, it is possible that that network may not support IPv4 multicast, in which case discovery will not work and clients must enter IP addresses manually.

Note that the change made in 6.0.3 simply enable selection of the network on which clients and servers broadcast/listen on. The HubNet server still listens on all network connections, so all listed IP addresses given in the server broadcast network selection dropdown are potentially valid for a HubNet client (even a client on a different network) to connect to. Similarly HubNet clients may connect to any HubNet server reachable from their computer.

## Usage

### When I move the speed slider all the way to the right, why does my model seem to stop?

The only way NetLogo can make your model run faster is by updating the view less frequently. As you move the speed slider to the right, view updates become less and less frequent. Since view updates take time, that means more speed.

However, fewer updates also means that the updates come farther apart. When several seconds pass between updates, it may seem like your model has stopped. It hasn’t. It’s running at full speed. Watch the tick counter! (If your model uses it. If it doesn’t, watch something else, like a plot.)

To get a feel for what’s going on, try moving the slider to the right gradually rather than suddenly. If you find the updates too infrequent at the rightmost position, just don’t push the slider that far.

### Can I use the mouse to “paint” in the view?

NetLogo does not have a built-in set of painting tools for painting in the view. But with only a few lines of code, you can add painting capability to your model. To see how it’s done, look at Mouse Example, in the Code Examples section of the Models Library. The same techniques can be used to let the user interact with your model using the mouse in other ways, too.

Another possibility is to create an image in another program and import it using the import items on the File menu or the `import-*` primitives.

### How big can my model be? How many turtles, patches, procedures, buttons, and so on can my model contain?

We have tested NetLogo with models that use hundreds of megabytes of RAM and they work fine. We haven’t tested models that use gigabytes of RAM, though. Theoretically it should work, but you might hit some limits that are inherent in the underlying Java VM and/or operating system (either designed-in limits, or bugs).

The NetLogo engine has no fixed limits on size. By default, though, NetLogo ships with a one-gigabyte ceiling on how much total RAM it can use. If your model exceeds that limit, you’ll get an OutOfMemoryError dialog.

If you are using BehaviorSpace, note that doing runs in parallel will multiply your RAM usage accordingly.

Each platform contains “.cfg” files containing JVM settings. There is one cfg file for each sub-application (NetLogo, NetLogo 3D, HubNet Client, etc.) Although the file location varies by platform, the process for changing it is the same. Locate the section of the file that looks like the following:

    [JVMOptions]
    # there may be one or more lines, leave them unchanged
    -Xmx1024m
    # there may be one or more lines, leave them unchanged

Modify the value immediately following `-Xmx`, changing it to the amount of space you need, save the file, and restart NetLogo. Platform specific notes follow:

- Windows: The file will typically be in `C:\Program Files\NetLogo 6.2.2\app`, unless you are running 32 bit NetLogo on a 64 bit Windows, then it will be in `C:\Program Files (x86)\NetLogo 6.2.2\app`.
- Mac OS X: The file for NetLogo will be located at: `/Applications/NetLogo 6.2.2/NetLogo 6.2.2.app/Contents/Java/NetLogo.cfg` For NetLogo 3D and the other applications, you will find the file in the corresponding location for each application package. You can reach the cfg file by control-clicking the application in the Finder and choosing “Show Package Contents” from the popup menu.

  Please note that depending on your version of Mac OS X, changing the cfg file may break application signing. If this happens, follow Apple’s directions on this page to temporarily allow apps from “Anywhere”, run NetLogo once, then restore the setting to “Mac App Store and Identified Developers”.

- Other: The cfg files will be located in the `netlogo-6.2.2/app` folder after untarring.

By default, Mac builds of NetLogo bundle a 64-bit JVM, which should be able to make use of as much memory as the lesser of available system memory and the value following `-Xmx`. Windows and Linux will bundle a 32-bit or 64-bit JVM, depending on which version you have downloaded. It is recommended that you install 64-bit NetLogo on all 64-bit operating systems for best performance.

### Can I use GIS data in NetLogo?

Yes, many users are using GIS data with NetLogo. The most complete way to do that is with the GIS extension. See the GIS Extension Guide.

A simpler way is to use `import-pcolors`, but that only works for importing maps that are images, not maps in other formats.

It is also possible to write NetLogo code that reads GIS data using our file I/O primitives such as `file-open`. For example, see the Grand Canyon model in the Earth Science section of Sample Models.

### My model runs slowly. How can I speed it up?

Here’s some ways to make it run faster without changing the structure of the code:

- Use tick-based view updates, not continuous updates.
- Decrease the frequency of view updates by pushing the speed slider to the right, or turn updates off using the checkbox.
- If your model is using all available RAM on your computer, then installing more RAM should help. If your hard drive makes a lot of noise while your model is running, you probably need more RAM.
- Use turtle size 1, 1.5, or 2, as the 2D renderings for these sizes are cached by NetLogo. (This only affects graphics speed in the 2D view, not computation speed.)

In many cases, though, if you want your model to run faster, you may need to make some changes to the code. Usually the most obvious opportunity for speedup is that you’re doing too many computations that involve all the turtles or all the patches. Often this can be reduced by reworking the model so that it does less computation per time step. The members of the NetLogo Users Group may be able to help with this.

The profiler extension is useful for identifying which parts of your code are taking the most time.

Unless you are running the exact same strings over and over, using `run` and `runresult` are much slower than running code directly; you should avoid using these primitives on fresh strings in performance-critical code.

### Can I have more than one model open at a time?

One instance of NetLogo can only have one model open at a time. (Unfortunately, it is unlikely that this will change in a future version, due to the engineering difficulties involved.)

You can have multiple models open by opening multiple instances of NetLogo, though. On Windows and Linux, simply start the application again. On a Mac, you’ll need to duplicate the application (not the whole folder, just the application itself) in the Finder, then open the copy. (The copy takes up only a very small amount of additional disk space.)

### Can I change the choices in a chooser on the fly?

No.

### Can I divide the code for my model up into several files?

On an experimental basis, this is available using the `__includes` keyword.

## Programming

### How does the NetLogo language differ from other Logos?

This is answered in detail at the end of the Programming Guide.

### How come my model from an earlier NetLogo doesn’t work right?

See the Transition Guide for help.

### How do I take the negative of a number?

Any of these ways:

    (- x)
    -1 * x
    0 - x

With the first way, the parentheses are required.

### My turtle moved forward 1, but it’s still on the same patch. Why?

If you have disabled wrapping at the world edges in your model, then the turtle might be at a world edge and unable to move any further. You can test for this using `can-move?`.

Assuming the turtle isn’t hitting a world edge, moving forward 1 is only guaranteed to take a turtle to a new patch if the turtle’s heading is a multiple of 90 (that is, exactly north, south, east, or west).

It’s because the turtle might not be standing in the center of a patch. It might be near the corner of a patch. For example, suppose your turtle is close to the southwest corner of a patch and is facing northeast. The length of the patch diagonal is 1.414… (the square root of two), so `fd 1` will leave the turtle near the northeast corner of the same patch.

If you don’t want to have to think about these issues, one possibility is to write your model in such a way that your turtles always come to rest on patch centers. See next question.

### How do I keep my turtles on patch centers?

A turtle is on a patch center when its `xcor` and `ycor` are integers.

You can move a turtle to the center of its current patch with either of these two equivalent commands:

    move-to patch-here
    setxy pxcor pycor

But you’ll never need to do that if you never allow turtles off of patch centers in the first place.

The `sprout` command creates turtles on patch centers. For example:

    ask n-of 50 patches [ sprout 1 [ face one-of neighbors4 ] ]

Another way for a turtle to start on a patch center is with a command such as this line of turtle code, which moves it to the center of a random patch:

    move-to one-of patches

Once a turtle is on a patch center, as long as its heading always stays an exact multiple of 90 (that is to say, due north, east, south, or west), and as it long as it moves forward or back by integer amounts, it will always land on patch centers.

See Random Grid Walk Example, in the Code Examples section of the Models Library, to see these code snippets in use.

### `patch-ahead 1` is reporting the same patch my turtle is already standing on. Why?

See the answer two answers up. It’s the same issue.

This might not be the meaning of “ahead” you were expecting. With `patch-ahead`, you must specify the distance ahead that you want to look. If you want to know the next patch a turtle would cross into if it moved forward continuously, it is possible to find that out. See Next Patch Example, in the Code Examples section of the Models Library.

### How do I give my turtles “vision”?

You can use `in-radius` to let a turtle see a circular area around it.

Several primitives let the turtle “look” at specific points. The `patch-ahead` primitive is useful for letting a turtle see what is directly in front of it. If you want the turtle to look in another direction besides straight ahead, try `patch-left-and-ahead` and `patch-right-and-ahead`.

If you want the turtle to have a full “cone” of vision, use the `in-cone` primitive.

You can also find out the next patch a turtle would cross into if it moved forward continuously. See Next Patch Example, in the Code Examples section of the Models Library.

### Can agents sense what’s in the drawing layer?

No. If you want to make marks that agents can sense, use patch colors.

### I’m getting numbers like 0.10000000004 and 0.799999999999 instead of 0.1 and 0.8. Why?

See the “Math” section of the Programming Guide for a discussion of this issue.

### The documentation says that `random-float 1` might return 0 but will never return 1. What if I want 1 to be included?

It really doesn’t matter. Even if 1 were a possible result, it would only come up approximately 1 in 2^64 tries, which means you’d be waiting hundreds of years before it ever came up exactly 1.

Nonetheless, if you are convinced that it really must be possible to get 1, you can use `precision` to round your answer to a certain number of decimal places. For example:

    print precision (random-float 1) 10
    0.2745173723

(If you use this method, note that 0 and 1 are only half as likely to come up as other answers. To see why this is so, consider the case where you only keep one digit after the decimal point. Results between 0 and 0.5 get rounded to 0, but results between 0.5 and 1.5 get rounded to 1; the latter range is twice as large. If you want 0, 0.1, 0.2, …, 0.9, and 1 to all be equally likely, an alternative is to write `random 11 / 10`; this gives all 11 answers with equal probability.)

### Why is the number value in my monitor widget changing even though nothing is happening in my model?

This is caused by a combination of a few things:

1. NetLogo’s use of floating point numbers, which can produce small accuracy issues. See Floating point accuracy in the NetLogo programming guide.
2. Agentsets such as `turtles` are always returned in a random order.
3. Monitors re-run their reporter calculation constantly, even when you are not running any model code with a forever button or through the command center.

So the monitor constantly re-runs its `mean [xcor] of turtles` reporter, but the `turtles` agentset gives the turtles in a random order, and so the floating-point inaccuracies for `mean` will accumulate in a slightly different way each time due to the order differences. The end result is you see very slightly different numbers flashing through the monitor widget while nothing is happening.

You see the same problem doing `sum [xcor] of turtles`, `variance [xcor] of turtles`, or `standard-deviation [xcor] of turtles` - anytime you’re reducing a bunch of floating point numbers from an agentset into a single value. You can also see the problem running your reporter code directly in the command center repeatedly, without a monitor widget at all.

The fixes are straightforward:

- Sort your numbers before you calculate: `mean sort [xcor] of turtles`, `sum sort [xcor] of turtles`, `variance sort [xcor] of turtles`. If the numbers are in the same order you’ll still have small floating-point inaccuracies, but they’ll be the the same every time so you won’t see the values change. This is probably the best solution, but it can be slow if you have a really large agentset.
- Change the `Decimal places` setting of your monitors to a number where you don’t notice the changing values. Since the differences in results should be small, this is usually possible.

### How can I keep two turtles from occupying the same patch?

See One Turtle Per Patch Example, in the Code Examples section of the Models Library.

### How can I find out if a turtle is dead?

When a turtle dies, it turns into `nobody`. `nobody` is a special value used in NetLogo used to indicate the absence of a turtle or patch. So for example:

    if turtle 0 != nobody [ ... ]

You could also use `is-turtle?`:

    if is-turtle? turtle 0 [ ... ]

### Does NetLogo have arrays?

Nearly all models should just use lists for this.

The usual motivation for using arrays in other programming languages is that they provide fast random access (`item`) and mutation (`replace-item`). But NetLogo’s lists, even though they are immutable, now provide near constant time performance on these operations.

Lists in earlier versions of NetLogo (4.1 and 4.0) were simple singly-linked lists and therefore these operations took linear time. The data structure underlying NetLogo’s lists now is the immutable Vector class from the Scala collections library. It uses 32-wide hash array mapped tries, as implemented by Tiark Rompf, based in part on work by Phil Bagwell and Rich Hickey.

If you are certain you want to use raw, mutable JVM arrays in your model, they are provided by the array extension. See the Arrays & Tables section of the User Manual.

### Does NetLogo have hash tables or associative arrays?

Yes, using the table extension. See the Arrays & Tables section of the User Manual.

### How can I use different patch “neighborhoods” (circular, Von Neumann, Moore, etc.)?

The `in-radius` primitives lets you access circular neighborhoods of any radius.

The `neighbors` primitive gives you a Moore neighborhood of radius 1, and the `neighbors4` primitive gives you a Von Neumann neighborhood of radius 1.

For Moore or Von Neumann neighborhoods of a larger radius, see Moore & Von Neumann Example in the Code Examples section of the Models Library.

### How can I convert an agentset to a list of agents, or vice versa?

If you want the list in a particular order, use the `sort` or `sort-by` primitives. The Lists section of the Programming Guide explains how to do this. See also Ask Ordering Example, in the Code Examples section of the Models Library.

If you want the list in a random order, here’s how:

    [self] of <agentset>

Because all operations on agentsets are in random order, the resulting list is in random order.

To convert a list of agents to an agentset, use the `turtle-set`, `patch-set`, or `link-set` primitives.

### How do I stop foreach?

To stop a foreach from executing you need to define a separate procedure that contains only the foreach, for example:

    to test
      foreach [1 2 3] [ i ->
        if i = 2 [ stop ]
        print i
      ]
    end

This code will only print the number 1. The `stop` returns from the current procedure so nothing after the foreach will be executed either. (If the procedure is a reporter procedure, use `report` instead of `stop`.)

### I’m trying to make a list. Why do I keep getting the error “Expected a literal value”?

If a list contains only literal values, you can write it down just by putting square brackets around it, like `[1 2 3]`.

If you want your list to contain items that may vary at runtime, the list cannot be written down directly. Instead, you build it using the list primitive.

## BehaviorSpace

### Why are the rows in my BehaviorSpace table results out of order?

This is normal when doing multiple runs in parallel. For a discussion of the issue, see the section on parallel runs in the BehaviorSpace Guide section of the User Manual.

### How do I measure runs every n ticks?

Use `repeat` in your experiment’s go commands, e.g.:

    repeat 10 [ go ]

to measure the run after every 10 ticks. Essentially you are making one experiment step equal 10 ticks.

### I’m varying a global variable I declared in the Code tab, but it doesn’t work. Why?

It’s probably because your setup commands or setup procedure are using `clear-all`, causing the values set by BehaviorSpace to be cleared.

One possible workaround is to change your experiment’s setup commands to preserve the value of the variable, e.g.:

    let old-var1 var1
    setup
    set var1 old-var1

This works because even `clear-all` doesn’t clear the values of local variables made with `let`

Another possible workaround is to change your model’s setup procedure to use more specific clearing commands to clear only what you want cleared.

## NetLogo 3D

### Does NetLogo work with my stereoscopic device?

NetLogo supports fullscreen exclusive mode. If that is all your device needs then, possibly yes. However, it can be tricky to get it working. We don’t have any such devices so it is difficult for us to make the process easier. If your device needs something else, for example, quadbuffers enabled, the answer is probably no.

## Extensions

### I’m writing an extension. Why does the compiler say it can’t find `org.nlogo.api`?

You need to add NetLogo.jar to your classpath when compiling. `NetLogo.jar` is included with NetLogo.