AS · Practice questions · Text files

Open, read, close.

Six original Cambridge-style questions on text file handling in 9618 pseudocode, with trace tables and mark points.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

One mode at a time.

Why: variables are lost when the program ends; a file stores data permanently for later use.
Modes: READ, WRITE (new file, old data lost), APPEND (add to the end).
Read loop: WHILE NOT EOF(file), READFILE file, StringVar, process, ENDWHILE.
Finish: CLOSEFILE every file you opened, after the loop.

Explain

[2 marks]

Explain why a program that keeps a list of high scores needs to store them in a file.

Variables and arrays are held in RAM and are lost when the program ends. ✓
A file is held on secondary storage, so the scores are still available the next time the program runs. ✓

Recall

[3 marks]

Describe the effect of opening a text file in each of these modes: READ, WRITE, APPEND.

READ: data can be read from the file, starting at the first line. ✓
WRITE: a new file is created for writing; any existing data in the file is lost. ✓
APPEND: data is written after the existing data in the file. ✓

Trace

[4 marks]

Stock.txt contains five lines: Pen, Ink, (blank line), Pad, Pin. State the output of this pseudocode.

Count ← 0
OPENFILE "Stock.txt" FOR READ
WHILE NOT EOF("Stock.txt")
    READFILE "Stock.txt", Item
    IF Item <> "" THEN
        IF MID(Item, 1, 1) = "P" THEN
            OUTPUT Item
        ENDIF
    ELSE
        Count ← Count + 1
    ENDIF
ENDWHILE
CLOSEFILE "Stock.txt"
OUTPUT Count

Pen is output (starts with P). ✓
Ink is not output; the blank line adds 1 to Count. ✓
Pad and Pin are output. ✓
Final output 1 (the number of blank lines). ✓

Write

[5 marks]

Names : ARRAY[1:30] OF STRING is a global array; unused elements hold the empty string. Write pseudocode for a procedure SaveNames() that writes every non-empty name to a new file Names.txt, one per line.

PROCEDURE SaveNames() ... ENDPROCEDURE. ✓
OPENFILE "Names.txt" FOR WRITE before the loop. ✓
FOR Index ← 1 TO 30 ... NEXT Index. ✓
IF Names[Index] <> "" THEN WRITEFILE "Names.txt", Names[Index]. ✓
CLOSEFILE "Names.txt" after the loop. ✓

Write

[6 marks]

Words.txt contains an unknown number of lines. Write pseudocode to read the lines into Words : ARRAY[1:100] OF STRING, stopping at the end of the file or when the array is full, and then output how many lines were stored.

Declare a counter Count ← 0 and a STRING variable for the line. ✓
OPENFILE "Words.txt" FOR READ. ✓
WHILE NOT EOF("Words.txt") AND Count < 100. ✓
READFILE "Words.txt", LineIn, then Count ← Count + 1. ✓
Words[Count] ← LineIn inside the loop, after the increment. ✓
CLOSEFILE "Words.txt" and OUTPUT Count after the loop. ✓

Analysis

[3 marks]

A program adds one line to Visitors.txt each time a visitor signs in. It uses OPENFILE "Visitors.txt" FOR WRITE. At the end of the day the file holds only one name. Explain why and describe how to correct the program.

Opening in WRITE mode creates a new file each time. ✓
So every earlier name is lost and only the last visitor remains. ✓
Open the file FOR APPEND, so each name is added after the existing lines (and close it after writing). ✓

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