Students should be able to describe how data is passed out of a subroutine…

The idea

A subroutine that produces a result hands it back to the calling code through a return value. Inside the subroutine, a return statement ends execution and supplies the value; the call site captures it, for example total = addScores(4, 7), where addScores returns 11 and the variable total receives it. The value travels out only through that return, not through the subroutine's local variables, which disappear when it finishes. A function is expected to return a value, whereas a procedure typically performs an action without returning one. The returned value can be stored, printed, tested in a condition or passed into another subroutine. Describe the route: the subroutine computes a result, executes return with that value, control passes back to the caller, and the caller assigns or uses the returned data.

What earns the marks

• A return value is the data a subroutine sends back to the code that called it.
• A return statement ends the subroutine and supplies the value to the caller.
• The calling code captures the returned value, for example by assigning it to a variable.
• Functions normally return a value; procedures normally do not.
• The returned value can then be stored, displayed, tested or passed on.
• Local variables inside the subroutine are not the mechanism for passing data out.

Common mistakes and how to put them right

• Thinking a subroutine passes data out by leaving its local variables visible to the caller.

  Put it right: The value leaves only through the return statement.

• Confusing parameters (data passed in) with return values (data passed out).

  Put it right: Parameters go in, return values come out.

• Assuming every subroutine returns a value.

  Put it right: Procedures typically return nothing while functions return a value.

• Believing the return value appears automatically without the caller using it.

  Put it right: The caller must capture or use the returned value.

Exam tips

• Name the three stages: subroutine computes, return supplies the value, caller captures it.
• Use a short code example such as result = double(5) to show the value leaving and being stored.
• Contrast parameters going in with return values coming out to show you understand direction.