Answer 1

MITRESH POONIA
Could you please describe in detail what you intend to do?

Your question is ambiguously to me.

Frequency can be derived from the number of cars passing a street crossing in a specified time interval, or frequency can refer to extracting frequencies out of a digitized sound signal by means of fourier analysis.

Answer 2

It could also refer to how often a letter appears in a sentence or statement.
It could be as simple as implementing a count function

Answer 3

There are many ways to achieve this, I have attached an example below for you.
Copy it for yourself as I normally delete after a couple of days

https://code.sololearn.com/cqR8yt4u8Mmm/?ref=app