S├ębastien Portebois (2) [Avatar] Offline
#1
In the introduction paragraphs (before 19.1), there's a duplicate "are":

Maps are especially useful for unstructured data where the keys are are determined while a program is running.
(note the duplicate are)
Nathan Youngman (43) [Avatar] Offline
#2
Thanks for pointing that out. I'll get it corrected.