I have a list of ("A", "B", "C", "D") and I want to get the index of "C" in the list (it's position in the list), how?

Given a list,

x:("A","B","C","D")

There are several ways we could find the index of a specific element.

Building a dictionary:

(x dict keys x)["C"]

Querying:

first extract index where value="C" from x

Arithmetic:

sum (keys x) * x="C"

Variants of these methods can also be used to search for several items in parallel:

(x dict keys x) @ "C","A"
# (2,0)
extract index where value in "C","A" from x
# (0,2)
extract index where value="C" from "ABCBABBCC"
# (2,7,8)

If you're ever specifically searching within strings, consider rtext.find[] (which can optionally ignore case, too).

first @ rtext.find["ALPHABETICAL APPLES" "A"]
# (0,4,10,13)