Launching multiple nodes in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to launch a single node named 'talker' in ROS.

ROS

<launch>
  <node pkg="roscpp_tutorials" type="talker" name="[1]"/>
</launch>

Drag options to blanks, or click blank then click option'

Alistener

Bpublisher

Crosnode

Dtalker

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different name than the node type without reason.

Leaving the name attribute empty.

The name attribute sets the node's name. Here, it should be 'talker' to match the node type.

2fill in blank

medium

Complete the code to launch two nodes: 'talker' and 'listener'.

ROS

<launch>
  <node pkg="roscpp_tutorials" type="talker" name="talker"/>
  <node pkg="roscpp_tutorials" type="[1]" name="listener"/>
</launch>

Drag options to blanks, or click blank then click option'

Aroscore

Btalker

Clistener

Dpublisher

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same type for both nodes.

Using an unrelated node type.

The second node's type should be 'listener' to match its name and function.

3fill in blank

hard

Fix the error in the launch file to correctly launch two nodes.

ROS

<launch>
  <node pkg="roscpp_tutorials" type="talker" name="talker"/>
  <node pkg="roscpp_tutorials" type="listener" name="[1]"/>
</launch>

Drag options to blanks, or click blank then click option'

Aroscore

Blistener

Ctalker

Dpublisher

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same name for both nodes.

Using an unrelated name.

The second node's name should be 'listener' to match its type and avoid conflicts.

4fill in blank

hard

Fill both blanks to launch two nodes with remapped topics.

ROS

<launch>
  <node pkg="roscpp_tutorials" type="talker" name="talker">
    <remap from="chatter" to="[1]"/>
  </node>
  <node pkg="roscpp_tutorials" type="listener" name="listener">
    <remap from="chatter" to="[2]"/>
  </node>
</launch>

Drag options to blanks, or click blank then click option'

Achatter_out

Bchatter_in

Cchatter

Dtopic

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same remap name for both nodes causing conflicts.

Not remapping topics when needed.

The talker remaps 'chatter' to 'chatter_out' and the listener remaps 'chatter' to 'chatter_in' to avoid topic conflicts.

5fill in blank

hard

Fill all three blanks to launch two nodes with namespaces and remapped topics.

ROS

<launch>
  <group ns="[1]">
    <node pkg="roscpp_tutorials" type="talker" name="talker">
      <remap from="chatter" to="[2]"/>
    </node>
  </group>
  <group ns="[3]">
    <node pkg="roscpp_tutorials" type="listener" name="listener"/>
  </group>
</launch>

Drag options to blanks, or click blank then click option'

Arobot1

Brobot1/chatter_out

Crobot2

Drobot2/chatter_in

Attempts:

3 left

💡 Hint

Common Mistakes

Not using namespaces causing topic collisions.

Incorrect remap paths.

The first group uses namespace 'robot1' and remaps 'chatter' to 'robot1/chatter_out'. The second group uses namespace 'robot2'.

Practice

1. What is the main purpose of a ROS launch file when working with multiple nodes?

easy

A. To debug a single ROS node

B. To write the source code of ROS nodes

C. To start several ROS nodes together with one command

D. To stop all running ROS nodes automatically

Solution

1. Step 1: Understand the role of launch files and compare options

   Launch files are used to start multiple ROS nodes at once, simplifying the process. Only "To start several ROS nodes together with one command" correctly describes starting multiple nodes together; others describe unrelated tasks.

2. Final Answer:

   To start several ROS nodes together with one command -> Option C

3. Quick Check:

   Launch file = start multiple nodes [OK]

Hint: Launch files group nodes to run with one command [OK]

Common Mistakes:

• Thinking launch files write node code
• Confusing launch files with debugging tools
• Assuming launch files stop nodes

2. Which of the following is the correct XML tag to define a node in a ROS launch file?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall ROS launch file syntax and check options

   The correct tag to start a node is <node> with attributes pkg, type, and name. Only <node pkg="package_name" type="node_type" name="node_name" /> uses the correct <node> tag; others use invalid tags like <launch>, <start>, <run>.

2. Final Answer:

   <node pkg="package_name" type="node_type" name="node_name" /> -> Option A

3. Quick Check:

   Node tag = <node> [OK]

Hint: Use <node> tag with pkg, type, and name attributes [OK]

Common Mistakes:

• Using <launch> or <start> instead of <node>
• Forgetting to close the tag properly
• Mixing up attribute names

3. Given this launch file snippet:

<launch>
  <node pkg="turtlesim" type="turtlesim_node" name="sim1" />
  <node pkg="turtlesim" type="turtlesim_node" name="sim2" />
</launch>

What happens when you run roslaunch with this file?

medium

A. The launch file causes a syntax error and no nodes start

B. Only one turtlesim node starts named sim1

C. Two nodes start but both have the same name sim1 causing a conflict

D. Two turtlesim nodes named sim1 and sim2 start simultaneously

Solution

1. Step 1: Analyze node definitions and ROS naming rules

   Two nodes are defined with different names: sim1 and sim2, both from turtlesim package. Nodes with unique names can run simultaneously without conflict.

2. Final Answer:

   Two turtlesim nodes named sim1 and sim2 start simultaneously -> Option D

3. Quick Check:

   Unique node names = both run [OK]

Hint: Different node names allow multiple nodes to run [OK]

Common Mistakes:

• Assuming same package means one node only
• Thinking duplicate names cause silent errors
• Believing launch files always cause syntax errors

4. You wrote this launch file snippet:

<launch>
  <node pkg="demo_pkg" type="demo_node" name="node1">
</launch>

What is the error and how to fix it?

medium

A. Missing closing tag for <node>, fix by adding </node>

B. Wrong attribute name 'type', fix by changing to 'exec'

C. Missing package name, fix by adding pkg attribute

D. Extra closing tag needed for <launch>, fix by adding </launch>

Solution

1. Step 1: Check XML tag completeness and verify attributes

   The <node> tag is opened but not closed properly; it needs a closing </node> tag. Attributes pkg and type are correct; <launch> tag is closed properly.

2. Final Answer:

   Missing closing tag for <node>, fix by adding </node> -> Option A

3. Quick Check:

   Unclosed node tag = add </node> [OK]

Hint: Every opened tag must be closed properly [OK]

Common Mistakes:

• Confusing 'type' with 'exec' attribute
• Ignoring missing closing tags
• Assuming launch tag is not closed

5. You want to launch three nodes from different packages in one launch file:

<launch>
  <node pkg="pkg1" type="node1" name="node1" />
  <node pkg="pkg2" type="node2" name="node2" />
  <node pkg="pkg3" type="node3" name="node3" />
</launch>

How can you modify this launch file to add a parameter use_sim_time set to true for all nodes?

hard

A. Add <param name="use_sim_time" value="true" /> inside each <node> tag

B. Add <param name="use_sim_time" value="true" /> directly inside <launch> tag

C. Add <arg name="use_sim_time" default="true" /> inside each <node> tag

D. Add <env name="use_sim_time" value="true" /> inside each <node> tag

Solution

1. Step 1: Understand parameter scope in launch files and compare options

   Parameters placed directly inside <launch> apply globally to all nodes. Adding <param name="use_sim_time" value="true" /> directly inside <launch> sets it once for all; inside each <node> repeats unnecessarily; options B and C use wrong tags (<env>, <arg>).

2. Final Answer:

   Add <param name="use_sim_time" value="true" /> directly inside <launch> tag -> Option B

3. Quick Check:

   Global param inside <launch> = all nodes get it [OK]

Hint: Put common params inside <launch> for all nodes [OK]

Common Mistakes:

• Adding param inside each node causing repetition
• Using <arg> or <env> instead of <param>
• Placing param outside launch tag