1. Higher Computing Science
Procedures, Functions and
Parameters
Welcome to the 1st Higher Computing homework webinar.
We’re going to look at Procedures and Functions, and how they are used with parameters
Procedures and functions are really just ways of making your code more efficient and more easily understood. If you use them your code will be more readable and of course more maintainable.

2. Decomposition
When you break your initial programming problem down into smaller more easily solvable sub problems, you will want to code these sub problems as separate sub procedures or functions.
Sub-procedures and functions can be tested individually before a program is complete. This makes code easier to debug.
As well as making problems more manageable, these sub procedures and functions can be re-used, making your job of writing code easier.

3. Local Variables
A Local variable only has scope within the sub- program where it has been defined.
PROCEDURE looper()
FOR counter FROM 1 TO 10 # Local variable
SEND counter TO DISPLAY
END FOR
END PROCEDURE
Lets start with a little revision on variables
Some programming languages require you to define variables explicitly with a name and a data type before they are used even if this is not the case it is often a good idea to declare your variables making it easier to keep track of them.

4. Why use local variables?
Local variables mean that they only have a value inside the sub-program they are declared in.
Using local variables means that sub-programs are “self contained” and can be reused without affecting the value of variables elsewhere in the program
The first point is important as it makes it much more difficult to make a mistake by changing the value of a variable accidentally
Ideally re-use means not just re-used within the program they have been written for, but re-used again in other programs – they become part of your own bank of pre-written modules, saving development time as a result.
Self contained sub-programs make code easy to test and debug.

5. Why use local variables?
You can use the same variable name again in a different sub-program
The memory used by a local variable is freed up when its sub-program has finished running
Some additional advantages of using local variables

6. Global Variables
Global variables are variables which have scope throughout a program.
Any change to the value of a global variable within a sub-program will change its value anywhere else it occurs in the code.
Obviously we want to change the values of variables inside a sub program so how do we avoid using global variables?
We can control which sub programs are allowed to change the value of a variable by passing them as parameters to that sub program. This is what we are going to look at in this lesson

7. Global Variables
Global variables should be avoided as they make your procedures and functions less portable and your code less maintainable
The memory required by a global variable is locked up for the entire time your program is running
Using Global variables can cause unexpected changes if variables with the same name are used elsewhere in the program

8. A simple procedure This procedure can only perform one task
PROCEDURE eightStars()
FOR counter FROM 1 TO 8 # Local variable
SEND " * " TO DISPLAY
END FOR
END PROCEDURE
A very simple procedure which prints 8 stars. Not much use if you want to print any other number of stars,

9. Parameters
A parameter is a variable which is used to modify how a sub program behaves
A parameter is declared in the definition of a sub program (called a formal parameter)
Parameters make sub programs more flexible and more portable
We are going to look at how a subprogram can use a variable passed to it from outside (called a parameter)
Sometimes the sub program will only use the value of that variable, sometimes that sub program will be able to change its value. We are going to look at these two types of parameter.

10. A procedure with a value parameter
This procedure is more flexible as it can be called with any value
PROCEDURE stars (INTEGER numberOfStars)
FOR counter FROM 1 TO numberOfStars DO
SEND " * " TO DISPLAY
END FOR
END PROCEDURE
Number_of Stars is a formal parameter – ie it is being declared in the procedure definition.
Some languages require a type definition as well. In this case it is an integer

11. Calling the stars procedure with an actual parameter
PROCEDURE stars (INTEGER numberOfStars)
FOR counter FROM 1 TO numberOfStars DO
SEND " * " TO DISPLAY
END FOR
END PROCEDURE
DECLARE totalStars AS INTEGER INITIALLY 0
SEND " How many stars?" TO DISPLAY
RECEIVE totalStars FROM KEYBOARD
stars (totalStars)
If the input to the question “How many stars” was 10 then the result would be:
* * * * * * * * * *
We could call the stars procedure with any value we like.

12. Formal and Actual Parameters
Formal Parameters are variables which are declared when a sub-procedure or function is defined
Actual parameters are used when a sub-procedure or function is called
A quick re-cap on parameters
Formal and actual

13. Formal and Actual parameters
Formal parameter
PROCEDURE stars (INTEGER numberOfStars) FOR counter FROM 1 TO numberOfStars DO SEND " * " TO DISPLAY END FOR END PROCEDURE DECLARE totalStars AS INTEGER INITIALLY 0 SEND " How many stars?" TO DISPLAY RECEIVE totalStars FROM KEYBOARD stars (totalStars)
Formal parameters are variables which are part of the definition of the sub procedure
Actual parameters are variables or values used when the sub procedure is being used (called)
Actual parameter

14. Functions

15. How a function differs from a procedure
A function returns a value
A procedure performs a sequence of actions
Function names are usually nouns
Functions return a value so need to be declared as returning a particular data type
Procedure names are usually verbs

16. Built in mathematical functions
int()
sqr()
abs()

17. Using mathematical functions
SET value TO -56
SET newValue TO abs(value)
SEND newValue TO DISPLAY
Result would be 56

18. Using mathematical functions
SET value TO 4
SET newValue TO sqr(value)
SEND newValue TO DISPLAY
Result would be 2

19. Using mathematical functions
SET value TO 3.4
SET newValue TO int(value)
SEND newValue TO DISPLAY
Result would be 3

20. String functions Left$() Right$() Len() Mid$()
Some languages have built in string functions like these

21. Using String Functions
SET myString TO "banana"
SET newString TO left$(myString, 2)
SEND newString TO DISPLAY
newString would have the value "ba"

22. Using String Functions
SET myString TO "banana"
SET newString TO right$(myString,2)
SEND newString TO DISPLAY
newString would have the value "na"

23. Using String Functions
SET myString TO "banana"
SET newString TO mid$(myString, 2, 3)
SEND newString TO DISPLAY
newString would have the value "nan"

24. The validInteger function
FUNCTION validInteger()RETURNS INTEGER
DECLARE userInput AS INTEGER INITIALLY 0
RECEIVE userInput FROM KEYBOARD
WHILE userInput < 1 OR userInput > 100 DO
SEND "Input must be between 1 and 100" TO DISPLAY
END WHILE
RETURN userInput
END FUNCTION
As well as built in functions, you can create your own user-defined functions. This one will only accept values between qand including 1 and 100

25. validInteger function
We would call this function within a program:
DECLARE number AS INTEGER INITIALLY 0
SET number TO validInteger
To give the variable number a value between 1 and 100
This could have been written as a procedure with a parameter but using a function makes the code more readable when it is used in a program

26. The validInteger function with value parameters
FUNCTION validInteger(lowerLimit, upperLimit)RETURNS INTEGER
DECLARE userInput AS INTEGER INITIALLY 0
RECEIVE userInput FROM (INTEGER) KEYBOARD
WHILE (userInput < lowerLimit) OR (userInput > upperLimit) DO
SEND "Input must be between" & lowerLimit "and" & upperLimit TO DISPLAY
RECEIVE userInput FROM KEYBOARD
END WHILE
RETURN userInput
END FUNCTION

27. Calling the validInteger function
We could call this function with actual parameters, 1 and 50 to return a number between 1 and 50:
SET numberToUse TO validInteger(1,50)
or we could call it with the actual parameters 1 and inputRange which is a variable which has a value assigned elsewhere in the program:
SEND "What is the maximum input value" TO SCREEN
RECEIVE maxInput FROM (INTEGER) KEYBOARD
SET numberToUse TO validInteger(1,maxInput)
This function call would return a value between 1 and maxInput.
So if we wanted several different inputs from the user with different upper and lower limits, we can use the same input validation function each time, but with different parameters.

28. Why user defined functions?
Functions return a value
Functions make code more readable
Functions, like procedures can be reused so make programming more efficient

29. Parameters passed by Value
A parameter passed by value is one which is passed into a procedure or function and whose value is used by that procedure
When a sub procedure is called with a variable passed by value, a copy of that variable is made using its formal parameter while the procedure is running.
The copy of the variable is destroyed when the procedure has completed, so the memory can be reused
A quick re-cap on parameters.

30. Parameters passed by Reference
When a sub procedure is declared with a formal parameter passed by reference then when it is called with an actual parameter its value is changed by that sub procedure.
When a variable is passed by reference the reference is to the memory location of the variable itself. No copy is being made.
NB Not all programming languages make the distinction between value and reference parameters. Some languages need you to declare a parameter as a reference parameter otherwise it assumes that you are intending to use it as a value parameter

31. Parameters passed by Reference
The value of the variable changes as a result of calling the sub procedure with it as a parameter
PROCEDURE swap (a, b)
DECLARE temp AS INTEGER INITIALLY 0
SET temp TO a
SET a TO b
SET b TO temp
END PROCEDURE
Note that temp is a local variable. It only has a value inside the swap procedure

32. Parameters passed by Reference
Write a program which asks for two values (stored in the variables a and b) prints their values, then swaps the contents of the variables around and prints the new values

33. Parameters passed by Reference
PROCEDURE get_values()
RECEIVE a (INTEGER) FROM KEYBOARD
RECEIVE b (INTEGER) FROM KEYBOARD
END PROCEDURE
PROCEDURE DisplayAndSwapValues()
SEND "The value of a is " & a TO DISPLAY
SEND "The value of b is " & b TO DISPLAY
swap (a, b)
PROCEDURE swap (a, b)
DECLARE temp AS INTEGER INITIALLY 0
SET temp TO a
SET a TO b
SET b TO temp

34. Parameters passed by Reference
Example: Write a program allows the user to swap the positions of any two values in an array

35. The printArray procedure
PROCEDURE printArray (numbers)
FOR counter = 0 TO 5 DO
SEND numbers[counter] TO DISPLAY
END FOR
END PROCEDURE

36. The swap procedure PROCEDURE swap (a, b)
DECLARE temp AS INTEGER INITIALLY 0
SET temp TO a
SET a TO b
SET b TO temp
END PROCEDURE
Note that temp is a local variable. It only has a value inside the swap procedure

37. Parameters passed by Reference
DECLARE numbers AS ARRAY OF INTEGER INITIALLY [1,3,5,7,9 11]
PROCEDURE mainProgram (numbers)
printArray (numbers)
SEND "Please enter index position 1 to swap" TO DISPLAY
RECEIVE index1 (INTEGER) FROM KEYBOARD
SEND "Please enter index position 2 to swap" TO DISPLAY
RECEIVE index2 (INTEGER) FROM KEYBOARD
swap (numbers[index1], numbers[index2] )
END PROCEDURE
Index1 and index2 are local variables inside the program
The numbers array is a reference parameter in the mainProgram procedure definition
The numbers array is passed to the printArray sub program

38. Parameter Passing
Parameter passing by value creates a copy of the variable when the sub procedure is called - this is inefficient if the variable is an array.
Arrays are normally passed as reference parameters because of the memory space they occupy.

39. Summary
A local variable only has scope within the-sub procedure where it has been defined. A global variable has scope throughout a program. Using global variables makes code less readable, less modular and thus less maintainable
The advantages of using parameter passing rather than global variables is to increase the modularity, portability readability and maintainability of the code.
A formal parameter is one declared in the definition of a procedure or function
An actual parameter is one used when calling a procedure or function
A value parameter is one which is passed into a sub-procedure and whose value is copied, used by that procedure then discarded.
A reference parameter is a variable which is passed into a sub-procedure and then out again and whose value is changed by that sub- procedure

40. Summary
A function returns a value, a procedure performs a sequence of actions
A built in function is supplied as part of the development environment, a user defined function is one created by the programmer.
Using procedures and functions improves the readability and maintainability of code
Procedures and functions which use local variables make code more portable
Procedures and functions can be tested independently making code easier to debug

41. Question Examples
2016 Q10b, c, d 2015 Q14c Specimen Paper Q9c Example Paper Q9b(ii) Example Paper Q11a, b Old Higher 2012 Q22 Old Higher 2011 Q10c

42. 2016 Q10b
(b) Mrs McColl implements the program using global variables. Another teacher suggests that she makes use of parameter passing instead. State two benefits of using parameter passing rather than global variables.

43. 2016 Q10b
Using Global variables can cause unexpected changes if variables with the same name are used in the program The code is less readable because it is not obvious how data is flowing between procedures Memory used by local variables is reused once the procedure has completed its task but global variables will always be using memory resources Procedures declared with parameters can be re-used making the code more modular and the code more portable

44. 2016 Q10c
(c) Parameters are used to pass data between subprograms. Parameters can be passed by reference or passed by value. Explain why passing by value is more demanding on system resources when the data being passed is held in an array.

45. 2016 Q10c
Passing a parameter by value means that a copy of that parameter is made while the procedure is running. This means that more memory resources are being used and more processor instructions are needed to manage it.

46. 2016 Q10d
d) Mrs McColl’s program is modular and makes use of functions. Explain what is meant by a function.

47. 2016 Q10d
A function returns a single value The value of a function can be assigned to a variable eg. Uservalue = validInteger (1, 10)

48. 2015 Q 14c
An algorithm is implemented to validate the applicant’s data from the application form shown. There are two subprograms at lines two and three. The parameters for these subprograms are not shown. Name a parameter that should be passed at line 2, state the type of parameter passing used and justify your answer.
Line 1 REPEAT
Line 2 Enter_applicant_data(…)
Line 3 Validate_form_data(…)
Line 4 UNTIL <form data is valid>

49. 2015 Q 14c
The parameter could be any item from the form such as title, first name, surname etc.
It will be passed by reference because the value will be updated and then passed back out of the sub-program
PROCEDURE validate_form_data(gender)
RECEIVE gender FROM (CHARACTER) KEYBOARD
WHILE gender ≠ "M" AND gender ≠ "F"
< error message>
END WHILE
END PROCEDURE

50. Specimen Paper Q 9c
Jeanette works for a bank and has downloaded a taxCode function, from their online library. Bank employees receive an annual salary and bonus pay and Jeanette’s program stores these values in variables salary and bonus. It also stores the employee’s tax code in a variable called code.
Using pseudocode or a language with which you are familiar, write an algorithm for a subroutine that will:
Ask the user for the values for variables salary and bonus
Use the function to assign the variable code
Display code on screen

51. Specimen Paper Q 9c
PROCEDURE calculateCode RECEIVE salary FROM KEYBOARD RECEIVE bonus FROM KEYBOARD SET code TO taxCode (salary, bonus) SEND code TO DISPLAY END PROCEDURE

52. Example Paper Q9b(ii)
When implementing code, a programmer can limit the scope of a variable. Explain what is meant by scope and how it can be limited. Scope is the section of code in which a variable can be used and has a value You can create a local variable by declaring it inside sub-program or by defining a formal parameter which is used inside that sub-program.

53. Example Paper Q11a
A function used to perform an arithmetical operation is shown below:
FUNCTION combinations (INTEGER number) RETURNS INTEGER
SET factor TO 1
FOR counter FROM 1 TO number DO
SET factor TO counter * factor
END FOR
RETURN factor
END FUNCTION
State the formal parameter in the function.
The formal parameter is number

54. Example Paper Q11b
A program has two variables called items and permutations which contain the values 3 and 0 respectively, as shown below: Explain what is meant by an actual parameter, by referring to the following line of code used to call the function above: SET permutations TO combinations(items) State the values contained in items and permutations after execution of the line of code. Explain your answer
The actual parameter is the one used when calling the combinations procedure

55. Example Paper Q11b SET permutations TO combinations(items)
FUNCTION combinations (INTEGER number) RETURNS INTEGER
SET factor TO 1
FOR counter FROM 1 TO number DO
SET factor TO counter * factor
END FOR
RETURN factor
END FUNCTION
SET permutations TO combinations(items)
State the values contained in items and permutations after execution of the line of code. Explain your answer
We call the function with the actual parameter items (value is 3)
Factor is set initially to 1
From counter 1 to 3 we set factor to counter * 3
Factor = 1 X 1 =1 2 X 1 =2 then 3 X 2 = 6

56. Example Paper Q11b items = 3 permutations = 6
items is unchanged by the function (it is still 3)
We call the function with the actual parameter items (value 3)
The loop executes three times
factor is set initially to 1
For counter from 1 to 3 we set factor to counter * 3
factor = 1 X 1 =1 then 2 X 1 =2 then 3 X 2 = 6

57. Old Higher 2012 Q22
A travel agent uses a suite of software to help advertise holidays and make bookings. Part of the pseudocode that was written for the software is:
IF cost_per_person < 500 THEN
SET band TO "cheap"
END IF
IF cost_per_person >= 500 AND cost_per_person < 2000 THEN
SET band TO "medium"
IF cost_per_person >= 2000 THEN
SET band TO "expensive"

58. Old Higher 2012 Q22
(c) When the above is implemented as a subroutine, state whether the variable “cost_per_person” would be passed by reference or value. Justify your answer.
Each holiday booking is assigned a unique reference code. The software which creates this code uses concatenation within a user-defined function.
(d) Explain the term concatenation.
(e) Explain the term function.

59. Old Higher 2012 Q22
The variable cost_per_person would be passed by value because it is not going to be changed by the program
The term concatenation means to join two or more strings together to make one string
A function is a section of code which returns a single value

60. Old Higher 2009
A cinema ticket system allows customers to select and pay for their own
tickets.
The top level algorithm is:
1. Get ticket details
2. Calculate cost
3. Display cost and accept payment
The module calculateCost uses the number of tickets and the category of
ticket to calculate the total payment due. It uses the parameters described below.
(a) State the most suitable data type for the parameter called cost.
Parameter
Description
amount
Number of tickets
category
adult, child, student, OAP
cost
Total cost of required tickets

61. Old Higher 2009
The module calculateCost uses the number of tickets and the category of ticket to calculate the total payment due. It uses the parameters described below.
The most suitable data type for the parameter cost is REAL since it will be storing a decimal value
Parameter
Description
amount
Number of tickets
category
adult, child, student, OAP
cost
Total cost of required tickets

62. Old Higher 2009
Parameter
Description
amount
Number of tickets
category
adult, child, student, OAP
cost
Total cost of required tickets
1. Get ticket details
2. Calculate cost
3. Display cost and accept payment
Parameters can either be passed by value or by reference.
(i) Identify one parameter that is passed by value to the module calculateCost. Justify your answer.
(ii) Identify one parameter that is passed by reference to the module calculateCost. Justify your answer.

63. Old Higher 2009
1. Get ticket details
2. Calculate cost
3. Display cost and accept payment
Amount or Category can be passed by value into calculateCost since they are not being changed
Cost will be passed by reference since it is being passed out of calculateCost and into the Display cost and accept payment procedure

64. Old Higher 2009 The program will make use of a 1-D array.
When creating, or declaring, a 1-D array for use in a program, a name must be given to the array. State two other items that should be specified when the array is created.
(ii) Explain why it is a more efficient use of system resources to pass an array by reference rather than by value.

65. Old Higher 2009
When creating, or declaring, a 1-D array you need to state the number of items in the array (its index) and the data type (integer, string boolean etc)
(ii) Passing an array by reference is more efficient than passing by value because arrays consume large amounts of memory and since when a parameter is passed by value, a copy is made, memory is consumed unnecessarily.

66. Another example The design for a program is shown below.
1. Initialise variables
2. Enter pupil marks
3. Count the number of pupils with less than 45 marks
4. Display results.
(b) Step 3 of the program uses the parameters pupilMark and noNeedingSupport.
(i) State the variable types for these parameters.
(ii) For each parameter, state if it should passed by value or by reference in step 3?

67. Another example The design for a program is shown below.
1. Initialise variables
2. Enter pupil marks
3. Count the number of pupils with less than 45 marks
4. Display results.
(i) and noNeedingSupport would be INTEGER
(ii) In step 3, pupilMark would be passed by value and noNeedingSupport would be passed by reference (because it is passed out and then into step 4)

68. Summary
A local variable only has scope within the-sub procedure where it has been defined. A global variable has scope throughout a program. Using global variables makes code less readable, less modular and thus less maintainable
The advantages of using parameter passing rather than global variables is to increase the modularity, portability readability and maintainability of the code.
A formal parameter is one declared in the definition of a procedure or function
An actual parameter is one used when calling a procedure or function
A value parameter is one which is passed into a sub-procedure and whose value is copied, used by that procedure then discarded.
A reference parameter is a variable which is passed into a sub-procedure and then out again and whose value is changed by that sub- procedure

69. Summary
A function returns a value, a procedure performs a sequence of actions
A built in function is supplied as part of the development environment, a user defined function is one created by the programmer.
Using procedures and functions improves the readability and maintainability of code
Procedures and functions which use local variables make code more portable
Procedures and functions can be tested independently making code easier to debug