not sure which column you want to compare to columns B and C.  If the columns are named as A B C and E in stata, you can do, if compare by observation

gen long F=A*(E<=C & E>=B)+12*(!(E<=C & E>=B))

if you need to do loop, not sure how you want to output since each obs can have more than 1 output.

using column E to compare with B and C. You're a nice guy.

thanks, hope it works.  Happy new year!