Install ROS-Melodic on Ubuntu 18 (Bionic): unmet dependencies / Unable to locate package

Question

To install ROS Melodic on Ubuntu 18.04 (Bionic), I am following the instructions given here.

$ sudo apt-get install ros-melodic-desktop-full
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 ros-melodic-desktop-full : Depends: ros-melodic-perception but it is not going to be installed
                            Depends: ros-melodic-simulators but it is not going to be installed
                            Depends: ros-melodic-urdf-sim-tutorial but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

Dependencies of ros-melodic-simulators & ros-melodic-urdf-sim-tutorial can be taken care of by installing gazebo from here.

Attempt to install ros-melodic-perception using apt results in,

ros-melodic-perception : Depends: ros-melodic-perception-pcl but it is not going to be installed

Consequently, attempt to install ros-melodic-perception-pcl results in,

ros-melodic-perception-pcl : Depends: ros-melodic-pcl-conversions but it is not going to be installed
                             Depends: ros-melodic-pcl-ros but it is not going to be installed

However, neither ros-melodic-pcl-conversions could be found,

$ sudo apt install ros-melodic-perception-pcl-conversions
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package ros-melodic-perception-pcl-conversions

nor ros-melodic-pcl-ros.

$ sudo apt install ros-melodic-perception-pcl-ros
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package ros-melodic-perception-pcl-ros

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As mentioned in the installation instructions, Ubuntu repositories have been configured to download from "main", "restricted," "universe," and "multiverse".

What is going wrong in there?

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Originally posted by Sumit Dey on ROS Answers with karma: 83 on 2018-10-25

Post score: 3

Answer 1

These are not the package names, which explains why apt can't find them:

E: Unable to locate package ros-melodic-perception-pcl-ros
E: Unable to locate package ros-melodic-perception-pcl-conversions

This should work without any additional fiddling:

apt-get install ros-melodic-desktop-full

So it's likely your system install is borked in some way. Try using aptitude instead of apt-get, which is much more clever about telling you what's wrong and how to fix it.

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Originally posted by paulbovbel with karma: 4518 on 2018-10-25

This answer was ACCEPTED on the original site

Post score: 3

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Original comments

Comment by Sumit Dey on 2018-10-25:
aptitude did the job. Thanks. Problem: vtk-7 was installed.. where as melodic needs vtk-6.

Comment by jurban1997 on 2018-11-10:
Failed for me. Does anyone QA these instructions?