Structured programming and subroutines (procedures and functions)

What you'll learn

What a subroutine is, and how it is called from another part of a program.
The difference between procedures and functions.
How parameters, return values and local variables pass and protect data.
Why a structured approach makes programs easier to write, test and maintain.

Starting point: programs are built from smaller actions

Before subroutines, you need to be comfortable with the three basic programming constructs:

Sequence means instructions run in order.
Selection means the program chooses between paths, usually with IF.
Iteration means instructions repeat, usually with FOR, WHILE or REPEAT.

As programs get bigger, writing everything in one long sequence becomes hard to read. Structured programming solves this by breaking a program into smaller named blocks.

Subroutines

A subroutine is a named block of code that sits “out of line” from the main program. It only runs when it is called, which means another part of the program executes it by writing its name in a statement.

Definition

Subroutine

A subroutine is a named block of code that can be run by calling its name from another part of the program.

For example, a game might have subroutines called DisplayMenu, GetPlayerMove and CalculateScore. The main program can call these whenever it needs them, instead of repeating the same code.

Diagram showing a main program calling a procedure and a function, with parameters, a return value, and a local variable

Why use subroutines?

Subroutines are useful because they:

make code easier to read by hiding detail inside named blocks
reduce repetition, because the same subroutine can be reused
make testing easier, because each part can be checked separately
make debugging easier, because faults can be narrowed down to one block
allow different programmers to work on different parts of a program
make the program easier to maintain and update later

Key Idea

The main benefit

Subroutines let you split a large problem into smaller named tasks, so the program becomes easier to understand, reuse, test and change.

Example

Splitting a quiz program into subroutines

A quiz program needs to show instructions, ask five questions, calculate the score and display the result. A structured version could be planned like this:

Choose the main tasks that happen in the program: showing instructions, asking questions, calculating marks and displaying feedback.
Turn each task into a clear subroutine name, such as DisplayInstructions, AskQuestion, CalculateScore and ShowResult.
Decide which parts need data. AskQuestion might need the question text and the correct answer, so those can be passed in as parameters.
Decide which parts produce data. CalculateScore may need to return the updated score to the main program.

Procedures and functions

There are two common types of subroutine in GCSE programming.

A procedure is a subroutine that performs an action but does not return a value. For example, DisplayMenu might print a menu on the screen.

A function is a subroutine that returns a value to the part of the program that called it. For example, CalculateArea might return the calculated area.

Definition

Procedure and function

A procedure carries out a task and does not return a value. A function carries out a task and returns a value to the calling routine.

The calling routine is the part of the program that calls the subroutine. This could be the main program or another subroutine.

Procedure example

SUBROUTINE DisplayMenu()
    OUTPUT "1. Start game"
    OUTPUT "2. View scores"
    OUTPUT "3. Quit"
ENDSUBROUTINE

DisplayMenu()

Here, DisplayMenu() is called by writing its name. It outputs text, but it does not send back a result.

Function example

SUBROUTINE CalculateTotal(price, quantity)
    total ← price * quantity
    RETURN total
ENDSUBROUTINE

amountToPay ← CalculateTotal(3, 4)

Here, CalculateTotal returns a value. The returned value is stored in amountToPay.

Example

Tracing a function call

Consider this subroutine call:

amountToPay ← CalculateTotal(3, 4)

where:

SUBROUTINE CalculateTotal(price, quantity)
    total ← price * quantity
    RETURN total
ENDSUBROUTINE

Match the parameter values to the parameter names: price becomes 3 and quantity becomes 4.
Substitute the values into the calculation: $\times quantity$ , so $\times 4$ .
The subroutine returns 12, so the calling routine stores 12 in amountToPay.

Common Mistake

Forgetting to store the return value

If a function returns a value, the calling routine usually needs to use or store it, for example answer ← CalculateTotal(3, 4). Just writing CalculateTotal(3, 4) may run the function but lose the returned result.

Parameters: passing data into a subroutine

A parameter is a value passed into a subroutine so the subroutine has the data it needs to work.

Definition

Parameter

A parameter is data passed into a subroutine when it is called. In AQA GCSE exam material, the term parameter is used for both the variable in the subroutine heading and the value supplied in the call.

Parameters make subroutines flexible. Instead of writing separate subroutines for every possible input, one subroutine can work with different values.

For example:

SUBROUTINE DisplayGreeting(name)
    OUTPUT "Hello " + name
ENDSUBROUTINE

DisplayGreeting("Amina")
DisplayGreeting("Ben")

The same subroutine is reused, but the parameter value changes.

More than one parameter

Subroutines can require more than one parameter. The order matters because the values are matched to the names in the subroutine heading.

SUBROUTINE ShowLogin(username, attemptsLeft)
    OUTPUT username
    OUTPUT attemptsLeft
ENDSUBROUTINE

ShowLogin("student1", 2)

Here, username gets "student1" and attemptsLeft gets 2.

Example

Matching multiple parameters

A subroutine is called like this:

ShowRectangleArea(5, 8)

and its heading is:

SUBROUTINE ShowRectangleArea(width, height)

Match the first value to the first parameter: width becomes 5.
Match the second value to the second parameter: height becomes 8.
If the subroutine calculates $\times height$ , it calculates $\times 8$ , giving 40.

Tip

Check the order

When a subroutine has several parameters, read the heading and the call together. The first value goes into the first parameter, the second value into the second parameter, and so on.

Return values: passing data out

A return value is data sent back from a function to the calling routine.

Definition

Return value

A return value is the result passed back from a function to the routine that called it.

Return values are useful when a subroutine calculates or finds something that the rest of the program needs.

For example:

SUBROUTINE IsAdult(age)
    IF age >= 18 THEN
        RETURN true
    ELSE
        RETURN false
    ENDIF
ENDSUBROUTINE

canVote ← IsAdult(19)

The function returns true, so canVote stores true.

A procedure does not return a value, but it can still perform useful actions, such as outputting text, changing the screen, or saving data.

Local variables

A variable is a named storage location for data. A local variable is a variable declared or created inside a subroutine.

Definition

Local variable

A local variable is a variable that belongs to a subroutine. It usually only exists while that subroutine is running and is only accessible inside that subroutine.

This “only accessible inside” idea is called scope. The scope of a variable describes where in the program it can be used.

Local variables are good practice because they:

prevent one subroutine accidentally changing another part of the program
allow the same variable name to be used safely in different subroutines
make subroutines easier to test because they depend mainly on parameters and return values
reduce unexpected side effects

For example:

SUBROUTINE CalculateAverage(mark1, mark2)
    total ← mark1 + mark2
    average ← total / 2
    RETURN average
ENDSUBROUTINE

total and average are local variables. The main program should not rely on them existing after the subroutine has finished.

Common Mistake

Trying to use a local variable outside its subroutine

If average is created inside CalculateAverage, the main program should use the returned value instead. It should not try to access average directly after the subroutine ends.

Example

Using a local variable safely

A program calls:

result ← CalculateAverage(10, 14)

where:

SUBROUTINE CalculateAverage(mark1, mark2)
    total ← mark1 + mark2
    average ← total / 2
    RETURN average
ENDSUBROUTINE

Pass the values into the parameters: mark1 becomes 10 and mark2 becomes 14.
Calculate the local variable total: $t o t a l = 10 + 14$ , so total becomes 24.
Calculate the local variable average: $\div 2$ , so average becomes 12.
Return 12 to the calling routine, so result becomes 12. The local variables total and average are no longer needed by the main program.

The structured approach to programming

The structured approach means designing programs as a set of clear, well-organised parts rather than one long block of code.

In this topic, structured programming includes:

modularised programming: splitting the program into modules or subroutines
clear interfaces: making it obvious what data goes into and out of each subroutine
parameters: passing data into subroutines
return values: passing results back out
local variables: keeping temporary data inside the subroutine that needs it
documentation: using sensible names and comments to explain purpose where helpful

Definition

Interface

An interface is the agreed way one part of a program communicates with another, such as the subroutine name, its parameters and its return value.

A well-structured subroutine has a clear purpose. If you cannot describe what it does in one short phrase, it may be trying to do too much.

Key Idea

Structured programming

Structured programming is about controlling complexity: break the program into modules, give each module a clear job, and pass data through parameters and return values rather than relying on hidden shared data.

Why structured programming helps

The structured approach has similar benefits to subroutines, but at the whole-program level:

Readability: the overall program flow is easier to follow.
Maintainability: changes can often be made in one module.
Testing: each module can be tested with known inputs and expected outputs.
Reusability: useful subroutines can be used in other programs.
Teamwork: different people can work on different modules if the interfaces are clear.
Reliability: local variables reduce accidental interference between parts of the program.

Example

Choosing a clean interface

Suppose a program needs to calculate a discounted price. A poor design might make the subroutine use hidden variables from the main program. A better design is:

SUBROUTINE CalculateDiscountedPrice(price, discountPercent)
    discount ← price * discountPercent / 100
    newPrice ← price - discount
    RETURN newPrice
ENDSUBROUTINE

Identify the data the subroutine needs from outside: it needs price and discountPercent.
Pass those values in as parameters, so the subroutine does not depend on hidden variables elsewhere.
Identify the one result the caller needs: the discounted price.
Return that result using RETURN newPrice, so the calling routine can store or output it.

Exam technique

In the exam

If asked to define a subroutine, mention that it is a named block of code that can be called by writing its name.
If asked about parameters and return values, say parameters pass data into a subroutine and return values pass data back out to the calling routine.
If asked why local variables are good practice, focus on scope: they usually only exist while the subroutine runs and are only accessible inside it.

Self review

Check yourself

What is the difference between a procedure and a function?
Why is using a parameter better than relying on a variable from the main program?
What usually happens to a local variable when its subroutine has finished running?

Structured programming and subroutines (procedures and functions)

What you'll learn

What a subroutine is, and how it is called from another part of a program.
The difference between procedures and functions.
How parameters, return values and local variables pass and protect data.
Why a structured approach makes programs easier to write, test and maintain.

Starting point: programs are built from smaller actions

Before subroutines, you need to be comfortable with the three basic programming constructs:

Sequence means instructions run in order.
Selection means the program chooses between paths, usually with IF.
Iteration means instructions repeat, usually with FOR, WHILE or REPEAT.

As programs get bigger, writing everything in one long sequence becomes hard to read. Structured programming solves this by breaking a program into smaller named blocks.

Subroutines

Definition

Subroutine

A subroutine is a named block of code that can be run by calling its name from another part of the program.

For example, a game might have subroutines called DisplayMenu, GetPlayerMove and CalculateScore. The main program can call these whenever it needs them, instead of repeating the same code.

Diagram showing a main program calling a procedure and a function, with parameters, a return value, and a local variable

Why use subroutines?

Subroutines are useful because they:

make code easier to read by hiding detail inside named blocks
reduce repetition, because the same subroutine can be reused
make testing easier, because each part can be checked separately
make debugging easier, because faults can be narrowed down to one block
allow different programmers to work on different parts of a program
make the program easier to maintain and update later

Key Idea

The main benefit

Subroutines let you split a large problem into smaller named tasks, so the program becomes easier to understand, reuse, test and change.

Example

Splitting a quiz program into subroutines

A quiz program needs to show instructions, ask five questions, calculate the score and display the result. A structured version could be planned like this:

Choose the main tasks that happen in the program: showing instructions, asking questions, calculating marks and displaying feedback.
Turn each task into a clear subroutine name, such as DisplayInstructions, AskQuestion, CalculateScore and ShowResult.
Decide which parts need data. AskQuestion might need the question text and the correct answer, so those can be passed in as parameters.
Decide which parts produce data. CalculateScore may need to return the updated score to the main program.

Procedures and functions

There are two common types of subroutine in GCSE programming.

A procedure is a subroutine that performs an action but does not return a value. For example, DisplayMenu might print a menu on the screen.

A function is a subroutine that returns a value to the part of the program that called it. For example, CalculateArea might return the calculated area.

Definition

Procedure and function

A procedure carries out a task and does not return a value. A function carries out a task and returns a value to the calling routine.

The calling routine is the part of the program that calls the subroutine. This could be the main program or another subroutine.

Procedure example

SUBROUTINE DisplayMenu()
    OUTPUT "1. Start game"
    OUTPUT "2. View scores"
    OUTPUT "3. Quit"
ENDSUBROUTINE

DisplayMenu()

Here, DisplayMenu() is called by writing its name. It outputs text, but it does not send back a result.

Function example

SUBROUTINE CalculateTotal(price, quantity)
    total ← price * quantity
    RETURN total
ENDSUBROUTINE

amountToPay ← CalculateTotal(3, 4)

Here, CalculateTotal returns a value. The returned value is stored in amountToPay.

Example

Tracing a function call

Consider this subroutine call:

amountToPay ← CalculateTotal(3, 4)

where:

SUBROUTINE CalculateTotal(price, quantity)
    total ← price * quantity
    RETURN total
ENDSUBROUTINE

Match the parameter values to the parameter names: price becomes 3 and quantity becomes 4.
Substitute the values into the calculation: $\times quantity$ , so $\times 4$ .
The subroutine returns 12, so the calling routine stores 12 in amountToPay.

Common Mistake

Forgetting to store the return value

Parameters: passing data into a subroutine

A parameter is a value passed into a subroutine so the subroutine has the data it needs to work.

Definition

Parameter

Parameters make subroutines flexible. Instead of writing separate subroutines for every possible input, one subroutine can work with different values.

For example:

SUBROUTINE DisplayGreeting(name)
    OUTPUT "Hello " + name
ENDSUBROUTINE

DisplayGreeting("Amina")
DisplayGreeting("Ben")

The same subroutine is reused, but the parameter value changes.

More than one parameter

Subroutines can require more than one parameter. The order matters because the values are matched to the names in the subroutine heading.

SUBROUTINE ShowLogin(username, attemptsLeft)
    OUTPUT username
    OUTPUT attemptsLeft
ENDSUBROUTINE

ShowLogin("student1", 2)

Here, username gets "student1" and attemptsLeft gets 2.

Example

Matching multiple parameters

A subroutine is called like this:

ShowRectangleArea(5, 8)

and its heading is:

SUBROUTINE ShowRectangleArea(width, height)

Match the first value to the first parameter: width becomes 5.
Match the second value to the second parameter: height becomes 8.
If the subroutine calculates $\times height$ , it calculates $\times 8$ , giving 40.

Tip

Check the order

When a subroutine has several parameters, read the heading and the call together. The first value goes into the first parameter, the second value into the second parameter, and so on.

Return values: passing data out

A return value is data sent back from a function to the calling routine.

Definition

Return value

A return value is the result passed back from a function to the routine that called it.

Return values are useful when a subroutine calculates or finds something that the rest of the program needs.

For example:

SUBROUTINE IsAdult(age)
    IF age >= 18 THEN
        RETURN true
    ELSE
        RETURN false
    ENDIF
ENDSUBROUTINE

canVote ← IsAdult(19)

The function returns true, so canVote stores true.

A procedure does not return a value, but it can still perform useful actions, such as outputting text, changing the screen, or saving data.

Local variables

A variable is a named storage location for data. A local variable is a variable declared or created inside a subroutine.

Definition

Local variable

A local variable is a variable that belongs to a subroutine. It usually only exists while that subroutine is running and is only accessible inside that subroutine.

This “only accessible inside” idea is called scope. The scope of a variable describes where in the program it can be used.

Local variables are good practice because they:

prevent one subroutine accidentally changing another part of the program
allow the same variable name to be used safely in different subroutines
make subroutines easier to test because they depend mainly on parameters and return values
reduce unexpected side effects

For example:

SUBROUTINE CalculateAverage(mark1, mark2)
    total ← mark1 + mark2
    average ← total / 2
    RETURN average
ENDSUBROUTINE

total and average are local variables. The main program should not rely on them existing after the subroutine has finished.

Common Mistake

Trying to use a local variable outside its subroutine

If average is created inside CalculateAverage, the main program should use the returned value instead. It should not try to access average directly after the subroutine ends.

Example

Using a local variable safely

A program calls:

result ← CalculateAverage(10, 14)

where:

SUBROUTINE CalculateAverage(mark1, mark2)
    total ← mark1 + mark2
    average ← total / 2
    RETURN average
ENDSUBROUTINE

Pass the values into the parameters: mark1 becomes 10 and mark2 becomes 14.
Calculate the local variable total: $t o t a l = 10 + 14$ , so total becomes 24.
Calculate the local variable average: $\div 2$ , so average becomes 12.
Return 12 to the calling routine, so result becomes 12. The local variables total and average are no longer needed by the main program.

The structured approach to programming

The structured approach means designing programs as a set of clear, well-organised parts rather than one long block of code.

In this topic, structured programming includes:

modularised programming: splitting the program into modules or subroutines
clear interfaces: making it obvious what data goes into and out of each subroutine
parameters: passing data into subroutines
return values: passing results back out
local variables: keeping temporary data inside the subroutine that needs it
documentation: using sensible names and comments to explain purpose where helpful

Definition

Interface

An interface is the agreed way one part of a program communicates with another, such as the subroutine name, its parameters and its return value.

A well-structured subroutine has a clear purpose. If you cannot describe what it does in one short phrase, it may be trying to do too much.

Key Idea

Structured programming

Why structured programming helps

The structured approach has similar benefits to subroutines, but at the whole-program level:

Readability: the overall program flow is easier to follow.
Maintainability: changes can often be made in one module.
Testing: each module can be tested with known inputs and expected outputs.
Reusability: useful subroutines can be used in other programs.
Teamwork: different people can work on different modules if the interfaces are clear.
Reliability: local variables reduce accidental interference between parts of the program.

Example

Choosing a clean interface

Suppose a program needs to calculate a discounted price. A poor design might make the subroutine use hidden variables from the main program. A better design is:

SUBROUTINE CalculateDiscountedPrice(price, discountPercent)
    discount ← price * discountPercent / 100
    newPrice ← price - discount
    RETURN newPrice
ENDSUBROUTINE

Identify the data the subroutine needs from outside: it needs price and discountPercent.
Pass those values in as parameters, so the subroutine does not depend on hidden variables elsewhere.
Identify the one result the caller needs: the discounted price.
Return that result using RETURN newPrice, so the calling routine can store or output it.

Exam technique

In the exam

If asked to define a subroutine, mention that it is a named block of code that can be called by writing its name.
If asked about parameters and return values, say parameters pass data into a subroutine and return values pass data back out to the calling routine.
If asked why local variables are good practice, focus on scope: they usually only exist while the subroutine runs and are only accessible inside it.

Self review

Check yourself

What is the difference between a procedure and a function?
Why is using a parameter better than relying on a variable from the main program?
What usually happens to a local variable when its subroutine has finished running?

Structured programming and subroutines (procedures and functions)

What you'll learn

Starting point: programs are built from smaller actions

Subroutines

Why use subroutines?

Procedures and functions

Procedure example

Function example

Parameters: passing data into a subroutine

More than one parameter

Return values: passing data out

Local variables

The structured approach to programming

Why structured programming helps

Recap questions

You've reached the end

Structured programming and subroutines (procedures and functions)

What you'll learn

Starting point: programs are built from smaller actions

Subroutines

Why use subroutines?

Procedures and functions

Procedure example

Function example

Parameters: passing data into a subroutine

More than one parameter

Return values: passing data out

Local variables

The structured approach to programming

Why structured programming helps

Recap questions

You've reached the end

Structured programming and subroutines (procedures and functions)

What you'll learn

Starting point: programs are built from smaller actions

Subroutines

Why use subroutines?

Procedures and functions

Procedure example

Function example

Parameters: passing data into a subroutine

More than one parameter

Return values: passing data out

Local variables

The structured approach to programming

Why structured programming helps

Recap questions

You've reached the end

Structured programming and subroutines (procedures and functions) Revision Guide

Structured programming and subroutines (procedures and functions)

What you'll learn

Starting point: programs are built from smaller actions

Subroutines

Why use subroutines?

Procedures and functions

Procedure example

Function example

Parameters: passing data into a subroutine

More than one parameter

Return values: passing data out

Local variables

The structured approach to programming

Why structured programming helps

Recap questions

You've reached the end

Structured programming and subroutines (procedures and functions) Revision Guide