R : Convert *some* column classes in data.table

To Access My Live Chat Page,
On Google, Search for "hows tech developer connect"

As I promised, I have a secret feature to share with you.
This is a YouTube's feature which works on Desktop.
First, Ensure that the video is playing before proceeding.
Then, type the letters 'awesome' on your keyboard as the next step.
You will see a flashing rainbow instead of a regular progress bar on YouTube.

Let me give you a quick introduction about myself,
Hello everyone, I'm Delphi.
I am available to help you find solutions to your inquiries.
I am happy to answer more specific questions, so please feel free to comment or chat with me.
If you have knowledge to contribute or an answer to provide, please leave a comment below.
I will 'heart' your answer as a sign of appreciation.