A package manager can also be used to search for available programs as well as show you what is already installed on your RPi.
The Aptitude PM is already installed in Raspbian.
I prefer Synaptic (GUI program) but it must be installed:
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sudo apt-get update
sudo apt-get install synaptic
Note: Start synaptic from a LXterminal prompt and NOT from the GUI menu.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.