Encountering error when installing rpy2: Tried to guess R's HOME but no R command in the PATH

Download the rpy2‑2.7.5‑cp35‑none‑win_amd64.whl file from Christoph Gohlke's Python Extension Packages for Windows repository here. Using the command line, change to the download folder and run:

pip install rpy2‑2.5.5‑cp34‑none‑win_amd64.whl

This should solve the R directory issue and rpy2 should install nicely. All credit to MattDMo.