Many models created in earlier versions of NetLogo also work in NetLogo 5.0. However, some models will need changes. If an old model isn't working, this section of the User Manual may be able to help you.
What issues may arise depends on what version of NetLogo the model was created with.
This guide only covers changes most likely to cause issues for users. See the Release notes for more complete details on differences between versions.
In NetLogo 6.0, tasks have been replaced by anonymous procedures.
This means that
task is no longer a primitive, it's been replaced
by the new arrow syntax for creating
anonymous procedures. Similarly, question mark variables like
?2 are now just ordinary names in NetLogo and can
be used to name procedure variables, let variables, or anonymous procedure variables.
have been replaced by
To make this transition easier, we've added an automatic conversion step which
should allow most models saved in NetLogo 5 to be converted to use the new syntax
automatically. The autoconverter has been a substantial piece of effort and we've
tested it on all the models in the models library.
To use it, ensure the model compiles and run properly in NetLogo 5 or later,
then save it from NetLogo 5 or later.
Then, simply open the model in NetLogo 6. If all goes well, you'll see the converter
has changed code like
task [?1 + ?2] to
[ [?1 ?2] -> ?1 + ?2 ]. The question marks are meant to serve
as temporary placeholders for conversion.
They enable your model to run, but you can (and should) replace these
variables with meaningful names.
If you open a model with tasks and it has not been converted,
the autoconverter wasn't able to convert your model. Rather than attempt to
autoconvert your model and break something, the model will open, you will be
shown the appropriate errors and given a chance to edit your model.
While we have tested the autoconverter thoroughly, we expect there to be some cases it doesn't cover.
runresult to evaluate
strings containing tasks, the autoconverter will not change those strings to
be anonymous procedures. To make
as expected, look at strings in your model and change any which rely on
?-variables to instead rely on anonymous procedures
In NetLogo 6.0,
nobody is no longer a valid chooser value.
Just as you can't put
turtle 0 or
nobody refers to a non-literal value which
isn't supported in choosers. As part of this transition, choosers
nobody within a nested list)
will have all uses of
nobody changed to
opened in NetLogo 6.0.
In NetLogo 6.0, you must specify both plural and singular breed names.
In prior versions, declarations like
breed [mice] were legal,
but this support has been removed in 6.0. If you have models which use only
plural breed names, it is recommended that you convert them to specify both
names before opening in 6.0 since doing so will permit the NetLogo converter
to work most effectively on any other code in your model which needs conversion.
The NetLogo movie prims hadn't been updated in quite some time and
generated invalid quicktime movie files. They have been replaced by
prims in the new
vid extension. The full documentation
vid extension is available in the
Vid Extension section of the manual. As with all extensions,
users will need to include
vid in the
section of their NetLogo model.
Many of the movie primitives have direct parallels in the
extension which can be found in the following table:
vid:start-recorder optional-width optional-height
_recording-save-file-name to track the name of the active
recording, as well as adding the
to the model. You should verify that the conversion took place correctly.
There is no replacement for
The vid extension records frames at 25 per second, slightly more
than the default 15 frames-per-second of the movie prims.
If your recording is sensitive to framerate, consider recording
each existing frame twice (2/25 is fairly close to 1/15)
or consider using a postprocessing tool (like gstreamer or ffmpeg)
to adjust the video playback speed.
In NetLogo 6.0, expanded error-checking in the NetLogo compiler
causes models which define
undirected-link-breed [ undirected-links undirected-link ] and/or
directed-link-breed [ directed-links directed-link ] to error
for redefining a primitive reporter
If your model doesn't use
is-undirected-link? at all,
simply changing the breed names should resolve the error.
If you used either of the
there are several ways you might modify your model to account for this change.
If your model has no other breeded links, consider removing the link breed and using the
built-in link primitives.
If your model has other breeded links, but only of different directedness,
simply changing the breed name (and all related primitive names) should resolve the problem.
Note that in this case
continue to behave the same as before.
If your model has other breeded links of the same directedness,
the change will vary depending on your model. The breed name(s) must
be changed, but you must decide whether you used
to check link directedneess or to check that link breed membership.
If you used it to check link directedness leaving it as-is should keep the current behavior of the model.
Otherwise, it can simply be replaced by
hubnet-set-client-interface primitive was rendered obsolete
by the introduction of the HubNet client editor and end of support for calculator HubNet.
We have found it used in very few models and
have decided to remove it from the language.
On opening an existing model in 6.0, the autoconverter should remove all uses of
hubnet-set-client-interface from your code.
One of our goals in NetLogo 6.0 has been to make it easier to develop extensions and easy to develop more powerful extensions. To that end, we've bumped the extension API from 5.0 to 6.0. Existing extensions will need to recompile changing the "NetLogo-Extension-API-Version" in their jar's MANIFEST.MF from 5.0 to 6.0.
Some of the changes we've made to the extensions API include:
org.nlogo.api.Context now allows access to the current world and workspace objects
without requiring a cast to an
org.nlogo.api.Workspace has been introduced as a stable API for extensions to depend on.
In service of making it easier to build extensions, we've expanded and improved
the NetLogo Extension Plugin
sbt, the Scala Build Tool.
Sbt is a powerful tool for building JVM projects and can be used in projects that
use Scala, Java, or a combination of the two. We're now using the Extension Plugin to build
all of the bundled extensions and we strongly recommend extension authors take advantage
of the plugin as it makes configuring a NetLogo extension build extremely straightforward.
The plugin handles fetching the NetLogo jar which extensions compile against as
well as generation of a jar for the extension containing the appropriate metadata.
In 5.2, the hsb primitives have been changed to work with the standard scale values of 360 for hue, and 100 for saturation and brightness. This affects the primitives hsb, extract-hsb and approximate-hsb.
The old primitives, scaled to 255, are automatically transitioned to and have been renamed __hsb-old, __extract-hsb-old and __approximate-hsb-old.
The GoGo extension has been upgraded to use newer GoGo boards with the HID interface. Many of the older primitives no longer work, and will alert you to upgrading your GoGo board's firmware.
If you need to continue to use a serial interface, you can use the bundled gogo-serial extension. Change your model to use gogo-serial as opposed to gogo. More details [here](https://github.com/NetLogo/NetLogo/wiki/GoGo-Upgrade).
If you cannot upgrade to use the new HID extension nor the new serial extension, the original extension can be found at https://github.com/NetLogo/GoGo-RXTX-Extension
In 5.0, you don't have to put your plotting code in the Code tab anymore. Instead, you can put it inside the plots themselves, in the Interface tab.
Nonetheless, the old style and all of the existing plotting primitives are still supported. We recommend changing your model to use the new style, but if you don't, it should still work.
The following example shows how to change a model to use the new style. Suppose you have a typical NetLogo 4.1 model with one plot called "populations" and two pens called "robots" and "humans". The old code might look like:
plot count robots
plot count humans
Here are the steps to make the transition:
plot count robots command and paste it into
the Update Commands field for the robots pen in the plot edit dialog.
Remove it from the
plot count humans command can be moved in the
same way for the humans pen.
procedure, it doesn't actually do anything anymore! Remove it.
procedure calls in
do-plotting call should be changed
go, the do-plotting
call should be changed to
tick will both cause
plotting to happen automatically.
The resulting (much simpler) code looks like this:
For more details on how plotting works in NetLogo 5.0, see the Plotting Sections of the Programming Guide and the Interface Guide. For details on how plotting interacts with the tick counter, read on.
The way the tick counter works has changed in 5.0. Instead of being initially set to 0, the tick counter is initially blank.
You must use
reset-ticks to start the tick counter at 0
for the first time.
reset-ticks should go at the end of your setup procedure.
Putting it there will allow your model to work with 5.0's new
reset-ticks and plotting
In 5.0, you don't have to put your plotting code in the Code tab
anymore. Instead, you can put it inside the plots themselves, in the
Interface tab. Code inside plots is triggered by
tick. Resetting the tick counter runs plot setup code,
and then it also runs plot update code to plot the initial state of
the model. The initial state of the model won't be in place until
the end of
setup, so that's why
should go at the end.
In order for models from previous NetLogo versions to work in 5.0
without changes, when an old model is opened in 5.0, any occurrences
ca) are automatically changed to
__clear-all-and-reset-ticks, which combines the effects of
reset-ticks. The two underscores on
the name indicate that this is not a normal primitive, but exists
only for backwards compatibility.
You should remove
__clear-all-and-reset-ticks from your
code, replace it with
clear-all, and put
reset-ticks at the end of your setup procedure. (This
doesn't happen automatically because the structure of NetLogo
models is too free-form for an automatic converter to reliably make
the change for you.)
NetLogo 5.0 fully supports international characters cross-platform, using the Unicode character set. NetLogo 5.0 model files always represent Unicode characters using the UTF-8 encoding.
Previous versions of NetLogo allowed Unicode characters to be used in some contexts. However, model files were saved in the platform's default encoding, which on most systems was something other than UTF-8. Characters were handled correctly on the same platform (e.g. two Windows machines), but could be altered if the model was moved between platforms (e.g. from Windows to Mac or vice versa).
When opening an existing model in NetLogo 5.0, if the model contains international or other non-ASCII characters, the characters may be interpreted incorrectly, because they were originally written in a platform-specific encoding, but then read back in in UTF-8.
If only a few characters are affected, you might find it easiest just to fix them manually.
But if you expect a large number of characters to be affected, and you want them translated automatically, you can use a third party utility to re-encode your .nlogo file from its original encoding into UTF-8. After conversion, open the model in NetLogo 5.0 and all characters should be correct.
NetLogo 5.0 uses the Markdown markup language to allow you to format your Info tab, including headers, bold and italics, images, and so forth.
Earlier versions of NetLogo used a custom markup language with much more limited capabilities.
When opening a model from an older version, NetLogo 5.0 translates your old markup into Markdown. Most of the time this produces good results, but you may want to check the results yourself and make sure that your Info tab still looks good.
In NetLogo 5.0 every model has a "target frame rate" which affects the default speed at which the model runs, when the speed slider is in the middle, on the "normal speed" setting.
The default target frame rate for new models, and for models that were created in earlier versions of NetLogo, is 30 frames per second. If you are using tick-based updates, as we recommend for most models, then that translates to 30 ticks per second.
If your model runs slower in 5.0 than it ran in 4.1, it's probably just because its speed is being limited by this rate. If you want, you can press the Settings button in the Interface tab and change the frame rate to a higher number.
Some old models used the
every command to set a default
speed. In most case this can be now removed from the code, and the
target frame rate setting used instead.
The underlying data structure for NetLogo lists has changed.
In NetLogo 4.1, a NetLogo list was represented internally as a singly
linked list. Some operations on singly linked lists are fast (such as
butfirst) but others are slow because
they could require traversing the whole list (such as
In NetLogo 5.0, lists are now actually trees internally. As a result, some operations are a little slower, but other operations are drastically faster on long lists. See the Lists section of the Programming Guide for details.
Some models may run a little slower with the new data structure, especially if you make heavy use of short lists. But other models will run faster -- perhaps dramatically faster.
Some special ways of writing list-processing code that were useful in
NetLogo 4.1 are no longer needed in 5.0. For example, since in 4.1
fput was fast and
lput was slow, modelers sometimes
built up lists in reverse order using
fput, perhaps calling
reverse later to restore the intended order. In NetLogo 5.0,
you don't need to code that way anymore.
lput are the same speed.
If you are the author of an extension, you will need to recompile it against the 5.0 NetLogo.jar and lib directory for it to work with 5.0.
You may also need to be aware of the following changes:
The code for specifying the syntax of a primitive has changed
slightly, for example
Syntax.TYPE_STRING is now
Syntax.StringType(). (From Java, the pair of parentheses at
the end is required. In Scala, you can omit them.)
One significant change is that
longer has a public constructor. Instead, there are two new ways to
If you have a
java.lang.Iterable, you can copy the contents
into a fresh
LogoList by passing it to the static method
LogoList.fromJava(). See the array extension source code for
a sample usage.
Or, to build up a new list one item a time, use
org.nlogo.api.LogoListBuilder. The Extensions Guide has sample code showing the
In prior NetLogo versions, the extensions API required that each extension primitive have its own separate top-level class with a no-argument constructor. These limitations have now been lifted. Also, api.Primitive objects are now made only once, when the extension is loaded, instead of every time the Code tab was recompiled.
The following syntax is no longer supported:
set [<variable>] of <agent> <value>
Commands of this form must be rewritten using
ask <agent> [ set <variable> <value> ]
Or, if the new value must be computed by the asking agent and not by the agent whose variable is being set:
;; OPTION #1 (using let): let new-value <value> ask <agent> [ set <variable> new-value ] ;; OPTION #2 (using myself): ask <agent> [ set <variable> [value] of myself ]
So for example, this:
set [color] of turtle 0 red
Can be rewritten as:
ask turtle 0 [ set color red ]
It is not necessary to use
myself since red
is red from the point of view of both agents.
set [color] of turtle 0 color
Must be rewritten as:
let new-color color ask turtle 0 [ set color new-color ]or
ask turtle 0 [ set color [color] of myself ]
in order not to change the meaning, since the two agents may have
different starting values for
color. The form using
myself is briefer, but the former using
let may be
considered clearer, depending on context and individual preference.
Prior to NetLogo 4.0, a dead turtle's who number (stored in the
who turtle variable) could
be reassigned to a later newborn turtle. In NetLogo 4.0, who numbers
are never reused until who numbering is reset to 0 by the
This change in behavior may break a few old models.
crt gives the new turtles random colors and random integer
cro assigns colors sequentially and gives the
turtles sequential equally spaced headings, with the first turtle
facing north (heading of 0).
Prior to NetLogo 4.0, the
crt command behaved the way
cro does now. If your old model depends on the
"ordered" behavior, you will need to change your code to
cro instead of
It is common for old models that used
crt to contain extra
commands to randomize the new turtles' headings, for example
rt random 360 or
set heading random 360. These
commands are no longer necessary when used inside
Prior to NetLogo 4.0, the
+ (addition) operator could be
used to concatenate strings and join lists. In current NetLogo,
+ only works on numbers. To concatenate strings, use the
word primitive; to join
lists together, use the
sentence primitive. This
language change was made to increase the speed of code that uses
print "There are " + count turtles + " turtles."
print (word "There are " count turtles " turtles.")
Likewise, if you need to concatenate lists, use SENTENCE.
This change is not handled automatically when converting old models; users will need to change their code by hand.
We know this change will be awkward for users who are used to the old syntax. We have made this change for efficiency and consistency. We can implement an addition operator that only adds numbers much more efficiently than one that handles several different data types. Because addition is such a common operation, NetLogo's overall speed is affected.
The observer may no longer use
turtles-on patch, and
instead. Note that
patch now rounds its inputs (before it
only accepted integer inputs).
NetLogo 3.1 had supports for using links to connect turtles to make networks, graphs, and geometric figures. The links were themselves turtles.
In NetLogo 4.0, instead of links being turtles, links are now an independent fourth agent type, right alongside observer, turtles, patches. The primitives involving links are no longer considered experimental; they are now fully part of the language.
Models that use the old, experimental turtle-based link primitives will need to be updated to use link agents. The differences are not huge, but hand updating is required.
Links are documented in the Links section of the Programming Guide, and in the NetLogo Dictionary entries for the link primitives. See the Networks section of the Models Library for example models that use links. There are also some link-based Code Examples.
First you will need to remove any breeds called "links" if
you are only using one type of links then you will not have to use
breeds at all. If you are using multiple types of links see
Commands and reporters that contain the word "links" (like
__create-links-with, etc.) will automatically be converted
to the new form without underscores (
However, primitives that use a different breed name (such as
"edges") will not be converted. You will need to remove the
underscores by hand and unless you are declaring a link breed with
that name you will need to change the breed designation to
remove-link(s)-with/from/to no longer exist.
Instead you should ask the links in question to
ask turtle 0 [ __remove-links-with link-neighbors ]
ask turtle 0 [ ask my-links [ die ] ]
Several of the layout commands have slightly different inputs, the
first two inputs are generally a turtle agentset and a link agentset
to perform the layout on. See the dictionary entries for details.
You may also need to rearrange the declaration of turtles-own
variables, since links were once actually turtles. Any variables that
apply to links should be moved into a
Since links are no longer turtles they no longer have the built-in
turtle variables (though some of the link variables are the same such
label. If you formerly used the
location of link turtles you will now need to calculate the midpoint
of the link. This is fairly simple in a non-wrapping world.
to-report link-xcor report mean [xcor] of both-ends end to-report link-ycor report mean [ycor] of both-ends endit is a little bit trickier in a wrapping world but still fairly straightforward.
to-report link-xcor let other-guy end2 let x 0 ask end1 [ hatch 1 [ face other-guy fd [distance other-guy] of myself / 2 set x xcor die ] ] report x end
and similarly for
We have replaced three different language constructs,
values-from with a
of construct (no
color-of turtle 0
[color] of turtle 0
value-from turtle 0 [size * size]
[size * size] of turtle 0
mean values-from turtles [size]
mean [size] of turtles
of is used with a single agent, it reports a
single value. When used with an agentset, it reports a list of
values (in random order, since agentsets are always in random
Note that when opening old models in the new version,
values-from will automatically be
converted to use "of" instead, but some nested uses of
these constructs are too complex for the converter and must be
converted by hand.
ask command is now
serial rather than concurrent. In other words, the asked agents will
run one at a time. Not until one agent completely finishes the entire
body of the
ask does the next agent start.
Note that even the old
was never truly concurrent; we simulated concurrent execution by
interleaving execution among the agents using a turn-taking mechanism
described in the NetLogo FAQ.
We have made this change because in our experience, users often wrote
models that behaved in unexpected ways due to the simulated
concurrency, but rarely wrote models that benefited from the
simulated concurrency. Models exhibiting unexpected behavior could
usually be fixed by adding the
command in the right places, but it was difficult for users to know
whether that command was needed and if so, where.
In NetLogo 4.0,
is no longer necessary unless your model uses
ask-concurrent (or a
turtle or patch forever button containing code that depends on
simulated concurrency). In most models, all uses of
without-interruption can be removed.
The simulated concurrency formerly employed by "ask" is still accessible in three ways:
primitive instead of
to get the old simulated concurrency. (We don't recommend this,
ask-concurrent as well.
ask itself is always serial regardless of the
context in which it is used, however.
In our own Models Library, models that make use of this concurrency are rare. A prominent example, though, is Termites, which uses a concurrent turtle forever button.
NetLogo now has a built-in tick counter for representing the passage of simulated time.
In most models the tick counter will be integer-valued, but if you
want to use smaller increments of time, you can use the
tick-advance command to
advance the tick counter by any positive amount, including fractional
amounts. Some Models Library models that use
are Vector Fields and the GasLab models.
The value of the tick counter is displayed in the toolbar at the top of the Interface tab. (You can use the Settings... button in the toolbar to hide the tick counter, or change the word "ticks" to something else.)
In the past, NetLogo always tried to update the view about 20 times a
second. We're now calling that "continuous" view
updates. The biggest problem with it was that you usually want
updates to happen between model ticks, not in the middle of a tick,
so we had a checkbox on buttons that (by default) forced a display
update after every button iteration. That made sure updates happened
between ticks, but it didn't get rid of the intermediate updates.
You had to use
display to lock them out.
We still support continuous updates. They are the default when you
start up NetLogo. But most Models Library models now use tick-based
updates. With tick-based updates, updates happen only when the tick
counter advances. (The
display command can be used to
force additional updates; see below.)
The advantages of tick-based updates as we see them are as follows:
buttons are no longer affected by the speed slider. This was a real
annoyance with the old speed slider. (The annoyance persists for
models that use continuous updates, though.)
As mentioned above, most models in our Models Library now use tick-based updates.
Even for models that would normally be set to tick-based updates, it may be useful to switch to continuous updates temporarily for debugging purposes. Seeing what's going on within a tick, instead of only seeing the end result of a tick, could help with troubleshooting.
If you switch your model to use tick-based updates, you'll also
need to add the
command to your code, otherwise the view won't update. (Note that
the view still always updates when a button pops up or a command
entered in the command center finishes, though. So it's not like
the view will just stay frozen indefinitely.)
Here are the steps to follow to convert your model to use ticks and tick-based updates in NetLogo 4.0:
to your go procedure, at or near the end. In Models Library models we
tick after the agents move but before any
plotting commands. That's because the plotting commands might
contain something like
plotxy ticks ... and we want the new
value of the tick counter used, not the old one. Most models
don't refer to the tick counter in their plotting commands, but
nonetheless, for consistency and to avoid mistakes we suggest always
tick before the plotting commands.
Some models will require some additional changes:
tick command and
ticks reporter instead.
(If your model uses fractional increments of time, use
tick-advance instead of
tick.) If you had a monitor for that variable, you can get
rid of it; there's now a tick counter in the toolbar.
the tick counter to zero. If you don't use
in your setup procedure, then you may need to add
reset-ticks to reset the
counter to zero.
display to prevent view
updates from happening in the middle of go, you can get rid of them.
display command to
force additional view updates so the user can see what is going on.
Previous versions of NetLogo had a speed slider that could be used to make models run slower, so you can see what's going on.
In NetLogo 4.0, the slider can be used to speed up models as well. It does this by updating the view less frequently. Updating the view takes time, so the fewer updates, the faster the model runs.
The default position of the slider is in the center. When you're at the center, the slider says "normal speed".
As you move the slider away from the center position, the model will gradually run faster or slower.
At very high speeds, view updates become very infrequent and may be separated by several seconds. It may feel like the model is actually running slower, since the updates are so infrequent. But watch the tick counter, or other indicators such as plots, and you'll see that yes, the model really is running faster. If the infrequent updates are disconcerting, don't push the slider so far over.
When using tick-based updates, slowing the model down does not cause additional view updates. Rather, NetLogo simply pauses after each tick.
When using continuous updates, slowing the model down means view
updates become more closely spaced. If you push the speed slider more
than halfway to the left, the model will be running so slowly that
you can watch turtles moving one at a time! This is new in NetLogo
4.0; in previous NetLogo versions, no matter how slowly you ran a
model, you would never see the agents in an
ask moving one
at a time; all the agents in an
ask always appeared to move
NetLogo no longer maintains an internal distinction between integers and floating point numbers. So for example:
observer> print 3 3 observer> print 3.0 3.0 observer> print 1 + 2 3 observer> print 1.5 + 1.5 3.0 observer> print 3 = 3.0 true
(The last line shows that although the distinction between integer 3 and floating point 3.0 was maintained, the two numbers were still considered equal.)
observer> print 3 3 observer> print 3.0 3 observer> print 1 + 2 3 observer> print 1.5 + 1.5 3 observer> print 3 = 3.0 true
We expect that only rare models will be negatively impacted by this change.
A benefit of this change is that NetLogo now supports a much larger range of integers. The old range was -2,147,483,648 to 2,147,483,647 (around +/- 2 billion); the new range is +/-9,007,199,254,740,992 (around +/- 9 quadrillion).
NetLogo 3.1 (and some earlier versions) included primitives called
patches-from that were
occasionally useful for building agentsets. In NetLogo 4.0, these
primitives have been replaced with new primitives called
patch-set that are much more
flexible and powerful. (
link-set exists as well.) See
the entries for these primitives in the NetLogo Dictionary. Models
that use the old
need to be altered by hand to use the new primitives.
In NetLogo 3.1 RGB and HSB colors could be approximated as NetLogo
colors using the
hsb primitives. These have
been renamed to
now expect inputs in the range 0-255, not 0-1.
The full RGB spectrum is now available in NetLogo so it may no longer be necessary to use these primitives at all. You can set any color variable to a three-item RGB list, with values in the 0-255 range, and get that exact color. See the Color section of the Programming Guide for details.
In previous versions
__tie was provided as an experimental
feature. As of NetLogo 4.0 links have a
tie-mode variable which can
be set to
"fixed". In 4.0
tie is now a link-only primitive.
This means that to tie turtle 1 to turtle 0 you write:
ask turtle 0 [ create-link-to turtle 1 [ tie ] ]
See the Tie section of the programming guide for details.
If your model is behaving strangely or incorrectly, it may be because
since NetLogo 3.1, agentsets are now always in random order. In prior
versions of NetLogo, agentsets were always in a fixed order. If your
code depended on that fixed order, then it won't work anymore.
How to fix your model to work with randomized agentsets depends on
the details of what your code is doing. In some situations, it is
helpful to use the
to convert an agentset (random order) into a list of agents (fixed
order). See "Lists of agents" in the Lists section of the Programming Guide.
If you are seeing pieces of turtle shapes wrapping around the view edges, it's because NetLogo 3.0 allowed you to turn off such wrapping in the view without affecting the behavior of the model. Since NetLogo 3.1, if you don't want the view to wrap you must make it so the world doesn't wrap, using the new topology feature. Making this change may require other changes to your model, though. See the Topology section of the Programming Guide for a thorough discussion of how to convert your model to take advantage of this new feature.
Many models made in NetLogo 3.0 or earlier use
world-width random world-height to scatter turtles randomly,
random-float. It only works
if world wrapping is on.
(Why? Because when wrapping is on, you can set coordinates of turtles to numbers beyond the edge of the world and NetLogo will wrap the turtle to the other side. But in worlds that don't wrap setting the x or y coordinates of a turtle to a point outside the bounds of the world causes a runtime error. The world wrap settings were added in NetLogo 3.1. See the Topology section of the Programming Guide for more information.)
To fix your model so that it works regardless of the wrapping settings, use one of these two commands instead:
setxy random-xcor random-ycor setxy random-pxcor random-pycor
The two commands are a bit different. The first command puts the turtle on a random point in the world. The second command puts the turtle on the center of a random patch. An even more concise way to put a turtle on the center of a random patch is:
move-to one-of patches