Why is := allowed as an infix operator?

Question

I have come across the popular data.table package and one thing in particular intrigued me. It has an in-place assignment operator

:=

Answer 1

It's not just a colon operator but rather := is a single operator formed by the colon and equal sign (just as the combination of "<" and "-" forms the assignment operator in base R). The := operator is an infix function that is defined to be part of the evaluation of the "j" argument inside the [.data.table function. It creates or assigns a value to a column designated by its LHS argument using the result of evaluating its RHS.