Break Function without return

Question

I have a function

function foo(a)
    if a > 5 
        a = 5
    end
    some_more_code
end

If the if-statement is true I would like to end the function but I don't want to return anything - to change the value of a is all I need.

How do I do that?

Answer 1

You can write (note that I have also changed the syntax of function definition to make it more standard for Julia style):

function foo(a)
    if a > 5 
        a = 5
        return
    end
    # some_more_code
end

Just use the return keyword without any expression following it. To be precise in such cases Julia returns nothing value of type Nothing from a function (which is not printed in REPL and serves to signal that you did not want to return anything from a function).

Note though that the value of a will be only changed locally (within the scope of the function), so that outside of the function it will be unchanged:

julia> function foo(a)
           if a > 5 
               a = 5
               return
           end
           # some_more_code
       end
foo (generic function with 1 method)

julia> x = 10

julia> foo(x)

julia> x
10

In order to make the change visible outside of the function you have to make a to be some kind of container. A typical container for such cases is Ref:

julia> function foo2(a)
           if a[] > 5 
               a[] = 5
               return
           end
           # some_more_code
       end
foo2 (generic function with 1 method)

julia> x = Ref(10)
Base.RefValue{Int64}(10)

julia> foo2(x)

julia> x[]
5