Defensive Design & Testing (OCR GCSE Computer Science): Exam Questions

Customers at a hotel can stay between 1 and 5 (inclusive) nights and can choose between a basic room or a premium room.

Complete the following test plan to check whether the number of nights is validated correctly.

The area of a circle is calculated using the formula π × r2 where π is equal to 3.142 and r is the radius.

A program is written to allow a user to enter the radius of a circle as a whole number between 1 and 30, then calculate and output the area of the circle.

01 radius = 0

02 area = 0.0

03 radius = input("Enter radius")

04 if radius < 1 OR radius > 30 then

05 print("Sorry, that radius is invalid")

06 else

07 area = 3.142 * (radius ^ 2)

08 print (area)

09 endif

Explain, using examples from the program, two ways to improve the maintainability of the program.

The program should only allow values from 0 to 300 inclusive as valid inputs. If the data entered breaks this validation rule, an error message is displayed.

Complete the following program to output "Invalid input" if the data does not meet the validation rule.

You must use either:

• OCR Exam Reference Language, or

• a high-level programming language that you have studied.

mins = input("Enter minutes played: ")

if mins < 0 …………………… mins …………………… then

…………………………… ("Invalid input")

endif

A teacher writes an algorithm to store the name of the game a student plays each night (for example "OCR Zoo Simulator").

variable.length returns the number of characters in variable. variable.upper returns the characters in variable in upper case.

valid = false

while(valid == false)

gameName = input("Enter the game name")

if (gameName.length > 0) AND (gameName.length < 20)

gamesPlayed = gameName.upper

valid = true

print("Valid game name")

else

print("Game name is not valid")

endif

endwhile

The algorithm needs testing to make sure the IF-ELSE statement works correctly.

Identify two different pieces of test data that can be used to test different outputs of the algorithm.

Give the output from the program for each piece of test data.

OCRBlocks is a game played on a 5 × 5 grid. Players take it in turns to place blocks on the board.

The board is stored as a two-dimensional (2D) array with the identifier gamegrid

Fig. 6.1 shows that players A and B have placed three blocks each so far.

The function checkblock() checks whether a square on the board has been filled. When checkblock(4,2) is called, the value "A" is returned.

function checkblock(r,c)

if gamegrid[r,c] == "A" or gamegrid[r,c] == "B" then

outcome = gamegrid[r,c]

else

outcome = "FREE"

endif

return outcome

endfunction

When checkblock(-1,6) is called, an error is produced

State why this function call will produce an error.

Describe how validation could be added in to the checkblock() function to stop this error from occurring.

A vending machine has the following options available.

Users insert coins into the vending machine and then enter the two character item code of their selection. If the user has inserted enough money, the vending machine will release the chosen item and output any change required. If the user enters an invalid item code then a suitable error message is displayed.

The vending machine is tested before it is released.

Explain the purpose of testing the vending machine.

Describe the difference between iterative testing and final testing.

Complete the following test plan for the vending machine

Louise writes a program to work out if a number entered by the user is odd or even. Her first attempt at this program is shown.

01 num = input(“enter a number”) 

02 if num MOD 2 >= 0 then 

03     print(“even”) 

04 else 

05     pritn(“odd”) 

06 endif

The program contains a syntax error on line 05.

State what is meant by a syntax error.

Give a corrected version of line 05 that fixes the syntax error.

Elliott plays football for OCR FC. He wants to create a program to store the results of each football match they play and the names of the goal scorers. Elliott wants individual players from the team to be able to submit this information.

Describe two examples of defensive design that should be considered when developing this program.

An algorithm stores the position of a character on a straight line as an integer.

A user can move the character left or right.

The following algorithm:

• Generates one random number between 1 and 512 (inclusive) and stores it as position.

• Prompts the user to input a direction to move ("left" or "right").

• Continues to take input until a valid direction is entered.

p ← RANDOM_INTEGER(1, 512)
OUTPUT "The position is ", p
a ← ""
WHILE a ≠ "left" AND a ≠ "right"
    a ← USERINPUT("Enter direction, left or right:")
ENDWHILE

Describe two ways to improve the maintainability of the algorithm.

A hotel has nine rooms that are numbered from room 0 to room 8.

The number of people currently staying in each room is stored in an array with the identifier room.

The index of room represents the room number.

Array room

The following program counts how many people are currently staying in the hotel.

for count = 1 to 8

total = 0

total = total + room[count]

next count

print(total)

When tested, the program is found to contain two logic errors.

Describe how the program can be refined to remove these logic errors.

Louise writes a program to work out if a number entered by the user is odd or even. Her first attempt at this program is shown.

01 num = input(“enter a number”)

02 if num MOD 2 >= 0 then

03 print(“even”)

04 else

05 pritn(“odd”)

06 endif

The program contains a logic error on line 02.

State what is meant by a logic error.

Give a corrected version of line 02 that fixes the logic error.

Describe the purpose of input validation in defensive programming.

Write a Python function that validates if an input string is a valid email address (contains "@" and ".").

Describe the differences between authentication and input validation.

The height a student jumps in the high jump needs to be input and validated.

The height is entered in centimetres (cm) and must be between 40.0 and 180.0 inclusive.

Write an algorithm to:

• take the height jumped as input

• output "VALID" or "NOT VALID" depending on the height input

You must use either:
• OCR Exam Reference Language, or
• a high-level programming language that you have studied.

The algorithm is tested using a range of tests.

Complete the table to identify an example of test data for each type of test.