If you want to use binary packages I highly recommend to use the PPA
here in order to get the newest version:
https://launchpad.net/~ginggs/+archive/ubuntu/deal.ii-backports
After that make sure that you actually install the developer
version. All in one:
sudo add-apt-repository ppa:ginggs/d
I downloaded the source code for deal.II in Ubuntu, which contains the
example files. (I'm a beginner.) I noted, however, that a binary is
available for Ubuntu (18.04) and installed that, instead of compiling the
library. Then I attempted to run step-1.cc from the source code package,
after det
