To give you an idea of the “why”: A yes/no problem is an NP if you can prove the answer is “yes” by making an incredibly lucky guess, plus some moderate amount of work.
For example: Can I make a tour connecting the capitals of the 48 main states of the USA in at most 10,000 miles? If the answer is “yes” then I make a lucky guess in which order to visit the ...
In a way, they are opposites.
A stream is something you read data from or write data to, indefinitely. Yes, a stream may be closed, but you don't know whether or when that will happen.
A stream is not an entity on your system that holds, or produces, or stores data. Rather, it is a gateway, a pipe, that can be attached to such an entity.
There is a strong ...
The data structure itself is unambiguously called a contiguous array. Depending on the context and language I would call it "initialized" or "having only non-null elements" or similar to describe the contents.