I would just like to point everyone to my c2d4u PPA, which has almost 4,000 R packages for LTS Ubuntu releases. The Trusty repository is starting to show its age, but the Xenial and Bionic are update to date. Usually updated every week.
R 3.5: https://launchpad.net/~marutter/+archive/ubuntu/c2d4u3.5
R 3.4: https://launchpad.net/~marutter/+archive/ubuntu/c2d4u
Also, binaries complied for R 3.4 are not compatible with R 3.5. Needed two repositories so that I didn't break systems with incompatible packages.