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Procedures, Functions, Arrays and File Handling: Question 10

Syllabus 8.2, 8.3

Structured 6 marks

A small sports club stores its members' names in a text file, register.txt, one name per line, so the membership list survives between seasons. The pseudocode below reads every name from the file into a one-dimensional array, Names, declared with room for up to 10 names, and counts how many names were actually read.

DECLARE Names : ARRAY[1:10] OF STRING
DECLARE NameCount : INTEGER
DECLARE NextName : STRING

NameCount ← 0
OPENFILE "register.txt" FOR READ
WHILE NOT EOF("register.txt")
    READFILE "register.txt", NextName
    NameCount ← NameCount + 1
    Names[NameCount] ← NextName
ENDWHILE
CLOSEFILE "register.txt"
OUTPUT NameCount

register.txt currently contains exactly four lines, in this order: Amara, Beth, Chidi, Dev.

(a) State the value output for NameCount after this code runs. [1]

(b) State the value of Names[3] after this code runs. [1]

(c) State what Names[5] to Names[10] hold after this code runs, and explain why. [2]

(d) A club administrator wants to add a fifth member, Esi, to the end of register.txt without erasing the four names already stored there. State the file mode that OPENFILE should use for this, and explain why OPENFILE "register.txt" FOR WRITE would not achieve this. [2]

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Worked solution

Part (a): Tracing NameCount

The loop runs once for every line in register.txt, incrementing NameCount and storing that name in Names[NameCount] each time, until EOF("register.txt") becomes TRUE:

ReadNextNameNameCount after this step
1stAmara1
2ndBeth2
3rdChidi3
4thDev4

After the 4th read, EOF("register.txt") becomes TRUE, so the loop stops. OUTPUT NameCount displays 4. [1 mark]

Part (b): Reading a single array element

From the trace above, Names[3] is assigned during the 3rd read, when NextName = "Chidi". So Names[3] = "Chidi". [1 mark]

Part (c): What the unused array elements hold

Names was declared with room for 10 names, ARRAY[1:10] OF STRING, but the loop only ever assigns to Names[1] through Names[4], because NameCount stops increasing once all 4 lines of register.txt have been read and EOF("register.txt") becomes TRUE. The statement Names[NameCount] ← NextName is therefore never executed with NameCount equal to 5, 6, 7, 8, 9 or 10.

As a result, Names[5] to Names[10] are never given a value by this code at all, they hold no defined data, they are simply the unused remainder of the array’s declared capacity. [2 marks]: [1] for recognising these elements are never assigned, [1] for the correct reason (NameCount only reaches 4, matching the 4 lines in the file).

Part (d): Adding a new name without erasing the existing register

To add Esi to the end of register.txt while keeping Amara, Beth, Chidi and Dev intact, the file must be opened with:

OPENFILE "register.txt" FOR APPEND

OPENFILE "register.txt" FOR WRITE would not work here, because opening a file in write mode creates a fresh, empty file (erasing any data already stored under that filename) rather than adding new data to what is already there. If FOR WRITE were used, the four existing names would be lost the moment the file was opened, before Esi was even written. FOR APPEND, by contrast, opens the existing file and positions any newly written data after its current content, so the four existing names are preserved and Esi is simply added after them. [2 marks]: [1] for stating FOR APPEND, [1] for correctly explaining why FOR WRITE would erase the existing data.

Final answers

  • (a) NameCount = 4
  • (b) Names[3] = “Chidi”
  • (c) Names[5] to Names[10] hold no defined value, they are never assigned by this code
  • (d) Use OPENFILE "register.txt" FOR APPEND; FOR WRITE would erase the four existing names