What does y -= m < 3 mean?

Question

While looking through some example C code, I came across this:

y -= m < 3; 

What does this do? It it some kind of condensed for loop or something? It's impossible to google for as far as I know.

Answer 1

m < 3 is either 1 or 0, depending on the truth value.

So y=y-1 when m<3 is true, or y=y-0 when m>=3