Kartik,

You can think of launch files like subroutines, and of course, a
subroutine needs to declare what arguments it takes and, optionally,
default values of those arguments:

---
Pseudocode:
main:
  call foo(arg1=false)

subroutine foo(arg1, arg2=true):
   if arg1: something
   if arg2: something_else

---

Launch file equivalent:

main.launch:
<launch>
  <include file="foo.launch">
    <arg name="arg1" value="false"/>
  </include>
</launch>

foo.launch:
<launch>
 <arg name="arg1"/>
 <arg name="arg2", default="true"/>

 <node if="$(arg arg1)" pkg="somepkg" type="something" name="something"/>
 <node if="$(arg arg2)" pkg="somepkg" type="something_else"
name="something_else"/>
</launch>
---

This is essentially what the documentation tells you but I found it
helped to think about the analogy between launch file arguments and
subroutine/function arguments.

Cheers,

Pat

On Fri, Jan 28, 2011 at 10:20 PM, Kartik Mohta <kartikmohta@gmail.com> wrote:
> Thanks, it works :)
>
> I think the documentation is still not very clear, maybe you can put
> the example there too.
>
> On Sat, Jan 29, 2011 at 1:08 AM, Ken Conley <kwc@willowgarage.com> wrote:
>> Hi Kartik,
>>
>> When using <args>, every launch file must be self-contained -- i.e. it
>> has to declare any args that it uses.   I updated the roslaunch/XML
>> documentation to make this more explicit.
>>
>> Thus, test2.launch:
>>
>> <launch>
>>   <arg name="name" />
>>   <param name="param" value="$(arg name)"/>
>> </launch>
>>
>> Hope this helps,
>> Ken
>>
>> On Fri, Jan 28, 2011 at 9:43 PM, Kartik Mohta <kartikmohta@gmail.com> wrote:
>>> Hi,
>>>
>>> Is there any help on how to properly use the $(arg) substitution in
>>> launch files? I followed the example given at
>>> http://www.ros.org/wiki/roslaunch/XML/arg but it gives me a "unused
>>> args" error.
>>>
>>> This is with ROS CTurtle on Ubuntu 10.10.
>>>
>>> $ cat test.launch
>>> <launch>
>>>    <include file="test2.launch">
>>>        <arg name="name" value="value"/>
>>>    </include>
>>> </launch>
>>>
>>> $ cat test2.launch
>>> <launch>
>>>    <param name="param" value="$(arg name)"/>
>>> </launch>
>>>
>>> $ roslaunch test.launch
>>> ...
>>> Checking log directory for disk usage. This may take awhile.
>>> Press Ctrl-C to interrupt
>>> Done checking log file disk usage. Usage is <1GB.
>>>
>>> unused args [name] for include of [test2.launch]
>>>
>>>
>>> --
>>> Kartik
>>> _______________________________________________
>>> ros-users mailing list
>>> ros-users@code.ros.org
>>> https://code.ros.org/mailman/listinfo/ros-users
>>>
>> _______________________________________________
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>
>
>
> --
> Kartik
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