Computer Science 4CP0 | Paper 2 Mock 01 | Written Paper 2

A sports centre records lap times from a swimming lane sensor. The program reads a number of times in seconds and must reject any time below 15 or above 180. Valid times are added to totalTime. Table 1 shows four inputs used while testing the validation code. The variable validCount is also increased only for valid data. The programmer needs to choose the correct Boolean operator so that invalid input is identified reliably.

(a) Identify the two input numbers that should not be added to totalTime. [1]
(b) Complete the condition: IF lapTime < 15 ______ lapTime > 180 THEN [2]
(c) Write pseudocode that displays "Invalid time" when the condition is true. [3]

Messages and keys work for a mountain rescue team that sends short encrypted words between two radio stations. A new volunteer suggests using the same Caesar cipher key of 1 for every message because it is easy to remember. The coordinator tests the plaintext word HELP and receives ciphertext IFMQ. The coordinator also wants a brief check of whether this encryption method is suitable for confidential rescue data.

(a) Identify the ciphertext letter produced from plaintext H. [1]
(b) State the Caesar shift used by the volunteer. [1]
(c) Write the plaintext word obtained by decrypting IFMQ. [4]
(d) Give one reason why a fixed Caesar key of 1 is weak encryption. [1]

Engineers use a small logic circuit in a solar-powered rescue beacon on a sailing boat. Fig. 1 shows the circuit. Input N is 1 when the navigation computer reports a distress position. Input R is 1 when the crew presses the radio switch. Input B is 1 when the backup battery has enough charge. The transmitter output X must be 1 only when the battery is available and either N or R is 1. Each test is saved as binary data in a log file so that engineers can identify a fault after a voyage.

Table 1 contains output data from six checks of the circuit.

(a) Identify the two gates used in Fig. 1. [2]
(b) State the checks in Table 1 for which the recorded output is correct. [3]
(c) Give the correct output X for check 5 and state why. [4]
(d) Complete the two missing input combinations needed for a full truth table, and give X for each combination. [4]
(e) Write a Boolean expression for X. [4]

Fig. 1 shows a warehouse robot program. Each robot reads a shelf code and a number of items to collect. The main algorithm calls one subprogram to validate the shelf code and another to calculate the travel distance. Table 1 gives data from four robot test runs. Shelf codes from A01 to A20 are on the left aisle, and B01 to B20 are on the right aisle.

If validShelf returns FALSE, the program must display "Code error" and must not call travelMetres. The programmer is checking that data is passed correctly between subprograms.

(a) Identify the return data type of each subprogram in Fig. 1. [2]
(b) State the two invalid shelf codes in Table 1 and give the displayed message for either one. [3]
(c) Complete the main algorithm using the correct subprogram calls. [4]

INPUT shelfCode
IF __________ THEN
  distance <- __________
  DISPLAY distance
ELSE
  DISPLAY "Code error"
ENDIF

(d) Use Table 1 to write two test cases for travelMetres, including an input and its expected returned number. Give one reason why it should not be called for C03. [5]

A greenhouse controller uses a program to open roof vents only when conditions need cooling. Fig. 1 shows the Boolean decision in the algorithm. The controller reads a temperature input and a humidity input every 100 seconds. Table 1 gives four test cases prepared by a technician. The program should display the word OPEN if the temperature is above 28 degrees C, or if humidity is above 85% and it is not raining.

Table 1

(a) Identify the two logical operators shown in Fig. 1. [3]
(b) Use Table 1 to state the test letters for which the display is OPEN. [4]
(c) Complete the condition in this pseudocode.
IF temperature > 28 ___ (humidity > 85 ___ raining) THEN
  DISPLAY "OPEN" [4]
(d) Write a condition that tests whether the vent must remain closed, using the same three input variables. [6]

Fig. 1 shows a grid used by a youth orchestra to conceal seat codes on printed tickets. The grid maps plaintext letters to ciphertext letters with a shift of 1. Table 1 shows ticket data for three performances, including the number of seats and the encrypted code length. A volunteer has written an algorithm to check whether every output word has the same number of letters as its input word.

Fig. 1: ticket cipher grid

Table 1: ticket program data

(a) Identify the ciphertext letter for plaintext Z. [1]
(b) State the Caesar shift used by the grid. [1]
(c) Identify the performance with 100 seats sold. [1]
(d) Give one reason why the number of letters does not change after this encryption. [1]
(e) Complete: plaintext WAX becomes ciphertext ______. [2]

The table below shows results from testing a program that checks visitor pass codes at a community workshop. A pass code must contain six characters and begin with the word WK. The programmer has opened the test file in an editor because some outputs are erroneous. The program accepts a code when both rules are true.

The condition currently used in the program is:
IF LEFT(code, 2) = "WK" OR LENGTH(code) = 6 THEN

(a) Identify the test number that shows an error in the program output. [1]
(b) Give two further pass-code inputs that should be used to test the program, including one boundary example. [2]
(c) Complete the corrected Boolean operator needed between the two tests in the condition. [1]
(d) State the correct display for test number 2 after the code has been corrected. [1]
(e) Write a suitable name for the file that could store the test data, including a file extension. [1]

Fig. 1 is a flowchart from a cinema booking program. It categorises a screen as large when it has more than 100 seats. Table 1 records data from a test run. The developer used this example because 100 is a boundary number. One output in the table is not correct for the stated rule.

(a) Identify the erroneous comparison operator in Fig. 1. [1]
(b) Complete the expected displays for input values 100 and 101. [2]
(c) Write the corrected condition that should be used in the decision. [2]
(d) Give two values that are suitable boundary test data for this program. [2]