How to iterate over rows with vectorization

Question

x = [\'A\',\'AA\',\'AABA\',\'AAL\',\'AAP\',\'AAPL\',\'ABBV\',\'ABC\',\'ABMD\',\'ABT\']
list12 = file.loc[file[\'Firm Name\'].isin(x)]
#file3
def findindex(list12,A,B