j - Arithmetic mean forwards vs backwards? -

i'm familiar way of doing arithmetic mean in j:

+/ % # 

but it's shown here as

# %~ +/ 

are these 2 versions interchangeable, , if not when should use 1 versus other?

dyadic ~ reverses arguments of verb. x f~ y equivalent y f x. use ~ when you, um, want reverse arguments of verb.

one of common uses forks , hooks composition. example, because y f (g y) (f g) y can use ((f~) g) y when need (g y) f y.