How do you return two values from a single method?

Question

When your in a situation where you need to return two things in a single method, what is the best approach?

I understand the philosophy that a method should do one t

Answer 1

This is not entirely language-agnostic: in Lisp, you can actually return any number of values from a function, including (but not limited to) none, one, two, ...

(defun returns-two-values ()
  (values 1 2))

The same thing holds for Scheme and Dylan. In Python, I would actually use a tuple containing 2 values like

def returns_two_values():
   return (1, 2)

As others have pointed out, you can return multiple values using the out parameters in C#. In C++, you would use references.

void 
returns_two_values(int& v1, int& v2)
{
    v1 = 1; v2 = 2;
}

In C, your method would take pointers to locations, where your function should store the result values.

void 
returns_two_values(int* v1, int* v2)
{
    *v1 = 1; *v2 = 2;
}

For Java, I usually use either a dedicated class, or a pretty generic little helper (currently, there are two in my private "commons" library: Pair<F,S> and Triple<F,S,T>, both nothing more than simple immutable containers for 2 resp. 3 values)

Answer 2

When your in a situation where you need to return two things in a single method, what is the best approach?

It depends on WHY you are returning two things. Basically, as everyone here seems to agree, #2 and #4 are the two best answers...

I understand the philosophy that a method should do one thing only, but say you have a method that runs a database select and you need to pull two columns. I'm assuming you only want to traverse through the database result set once, but you want to return two columns worth of data.

If the two pieces of data from the database are related, such as a customer's First Name and Last Name, I would indeed still consider this to be doing "one thing." On the other hand, suppose you have come up with a strange SELECT statement that returns your company's gross sales total for a given date, and also reads the name of the customer that placed the first sale for today's date. Here you're doing two unrelated things! If it's really true that performance of this strange SELECT statement is much better than doing two SELECT statements for the two different pieces of data, and both pieces of data really are needed on a frequent basis (so that the entire application would be slower if you didn't do it that way), then using this strange SELECT might be a good idea - but you better be prepared to demonstrate why your way really makes a difference in perceived response time.

The options I have come up with:

1 Use global variables to hold returns. I personally try and avoid globals where I can.

There are some situations where creating a global is the right thing to do. But "returning two things from a function" is not one of those situations. Doing it for this purpose is just a Bad Idea.

2 Pass in two empty variables as parameters then assign the variables inside the method, which now is a void.

Yes, that's usually the best idea. This is exactly why "by reference" (or "output", depending on which language you're using) parameters exist.

I don't like the idea of methods that have a side effects.

Good theory, but you can take it too far. What would be the point of calling SaveCustomer() if that method didn't have a side-effect of saving the customer's data? By Reference parameters are understood to be parameters that contain returned data.

3 Return a collection that contains two variables. This can lead to confusing code.

True. It wouldn't make sense, for instance, to return an array where element 0 was the first name and element 1 was the last name. This would be a Bad Idea.

4 Build a container class to hold the double return. This is more self-documenting then a collection containing other collections, but it seems like it might be confusing to create a class just for the purpose of a return.

Yes and no. As you say, I wouldn't want to create an object called FirstAndLastNames just to be used by one method. But if there was already an object which had basically this information, then it would make perfect sense to use it here.

Answer 3

Use var/out parameters or pass variables by reference, not by value. In Delphi:

function ReturnTwoValues(out Param1: Integer):Integer;
begin
  Param1 := 10;
  Result := 20;
end;

If you use var instead of out, you can pre-initialize the parameter.

With databases, you could have an out parameter per column and the result of the function would be a boolean indicating if the record is retrieved correctly or not. (Although I would use a single record class to hold the column values.)

Answer 4

You should also consider whether the design of your method is primarily returning a single value, and you are getting another value for reference along with it, or if you really have a single returnable thing like first name - last name.

For instance, you might have an inventory module that queries the number of widgets you have in inventory. The return value you want to give is the actual number of widgets.. However, you may also want to record how often someone is querying inventory and return the number of queries so far. In that case it can be tempting to return both values together. However, remember that you have class vars availabe for storing data, so you can store an internal query count, and not return it every time, then use a second method call to retrieve the related value. Only group the two values together if they are truly related. If they are not, use separate methods to retrieve them separately.