Fastest way to check if a value exists in a list

Question

What is the fastest way to know if a value exists in a list (a list with millions of values in it) and what its index is?

I know that all values in the list are uniqu

Answer 1

Code to check whether two elements exist in array whose product equals k:

n = len(arr1)
for i in arr1:
    if k%i==0:
        print(i)