A-LevelComputer ScienceAlgorithm design and problem-solving9618/23 · May/June 2022 · Q78 Marks

The following pseudocode represents an algorithm intended to output the last three lines as they appear in a text file. Line numbers are provided for reference only. ```pseudocode 10 PROCEDURE LastLines(ThisFile : STRING) 11 DECLARE ThisLine : STRING 12 DECLARE Buffer : ARRAY[1:3] OF STRING 13 DECLARE LineNum : INTEGER 14 LineNum ← 1 15 OPENFILE ThisFile FOR READ 16 17 WHILE NOT EOF(ThisFile) 18 READFILE Thisfile, ThisLine // read a line 19 Buffer[LineNum] ← ThisLine 20 LineNum ← LineNum + 1 21 IF LineNum = 4 THEN 22 LineNum ← 1 23 ENDIF 24 ENDWHILE 25 26 CLOSEFILE ThisFile 27 FOR LineNum ← 1 TO 3 28 OUTPUT Buffer[LineNum] 29 NEXT LineNum 30 ENDPROCEDURE ```

(a)(i)There is an error in the algorithm. In certain cases, a text file will have at least three lines but the output will be incorrect. State how the output may be incorrect.[1]
(a)(ii)Describe the error in the algorithm and explain how it may be corrected. Description Explanation[4]
(b)The original algorithm is implemented and sometimes the last three lines of the text file are output correctly. State the condition that results in the correct output.[1]
(c)Lines 20 to 23 inclusive could be replaced with a single pseudocode statement. Write the pseudocode statement.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[1] (a)(i) The lines are output in an incorrect sequence / in the wrong order / not as they appear in the file
[4] (a)(ii) Description of error: If the final line of the file is not written into array element 3, then outputting the elements in the sequence 1 to 3 will give the error.
[4] (a)(ii) Explanation of error correction (Option 1): Attempt at description of 'shuffle' / Copy Buffer[2] to Buffer[1] AND copy Buffer[3] to Buffer[2] / Read a line from the file and write it to Buffer[3]
[4] (a)(ii) Explanation of error correction (Option 2): Store the index of the last element written to buffer (the last line of the file) / Replace the FOR loop with something that starts at index and then wraps around (MOD 3)
[4] (a)(ii) Explanation of error correction (Option 3 - two-loop solution, not using an array): Loop to read file to end to get number of lines / close and re-open file / read (and discard) lines to number of lines - 3, then loop to read and output last 3 lines
[1] (b) If the number of lines in the text file is a multiple of three
[2] (c) Correct answers include: LineNum ← (LineNum MOD 3) + 1 // ((LineNum + 3) MOD 3) + 1
[2] (c) One mark for assignment to LineNum making any use of MOD
[2] (c) One for completely correct statement

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