walk in vs walk-in

walk in is not a noun while walk-in is a noun.

Word	Noun	Adjective	Verb	Adverb
walk in	No	No	Yes	No
walk-in	Yes	Yes	No	No

walk in (noun)	walk-in (noun)
	a small room large enough to admit entrance
	an assured victory (especially in an election)
	an operative who initiates his own defection (usually to a hostile country) for political asylum
	person who walks in without having an appointment

walk in (adjective)	walk-in (adjective)
	(of e.g. closets or refrigerators) extending very far enough back to allow a person to enter

walk in (verb)	walk-in (verb)
enter by walking