Understand the concept of subroutines. Students should know that a…

The idea

A subroutine is a named block of code written separately from the main program flow, described as ‘out of line’. It is executed by a call: writing its name in a program statement, usually with brackets and any arguments, such as calculateArea(5, 3). When the call runs, control jumps to the subroutine, the code inside executes, and control returns to the statement after the call. Naming the block once and calling it many times avoids duplicating code and makes programs easier to read and maintain. Learners should recognise a call in a program listing, explain what happens when it executes, and distinguish defining a subroutine from calling it.

What earns the marks

• States that a subroutine is a named block of code.
• Explains that the block is ‘out of line’, meaning it is written separately from the main sequence of statements.
• Describes that a subroutine is executed by a call, which is writing its name in a program statement.
• Explains that control transfers to the subroutine when called and returns afterwards.
• Identifies a subroutine call within a short program listing and distinguishes it from the definition.

Common mistakes and how to put them right

• Confusing the definition of a subroutine with a call to it.

  Put it right: The definition contains the code, the call executes it by name.

• Believing the subroutine code runs automatically without being called.

  Put it right: It runs only when its name appears in a call statement.

• Thinking ‘out of line’ means the code is deleted or unavailable.

  Put it right: It is stored separately but still part of the program.

Exam tips

• Look for the subroutine name followed by brackets to spot a call in a listing.
• Use the phrase ‘out of line’ when explaining why the block sits apart from the main code.
• Describe the sequence: call by name, execute the block, then return to the next statement.