Files (Cambridge International A-Level Computer Science): Exam Questions

An algorithm will output the last three lines from a text file Result.txt

The lines need to be output in the same order as they appear in the file.

Assume:

• Three variables LineX, LineY and LineZ will store the three lines. These are of type string and all three variables have been initialised to an empty string.
• The file exists and contains at least three lines.

The algorithm to output the lines is expressed in eight steps.

Complete the steps.

1. Open the file ...........................................................................
2. Loop until ...........................................................................
3. ........................................................................... and store in ThisLine
4. Assign LineY to LineX
5. Assign LineZ to LineY
6. Assign ThisLine to LineZ
7. After the loop, ...........................................................................
8. Output LineX, LineY, LineZ

The requirement changes, and the algorithm will now output three lines from the file, starting from a given line number.

The modified algorithm will be implemented as a function which will:

• be called with an integer parameter representing the given line number
• output three lines, starting at the given line
• return TRUE if the 3 lines are output, or FALSE if it was not possible to output the 3 lines.

Describe the changes that need to be made to steps 2 to 8 of the algorithm given in part (a).

A weather station records the total rainfall for each day.

At the end of each day the program adds one new line to the text file Rainfall.txt. This file already contains the totals for all the previous days. The totals for the previous days must not be lost.

Explain why the program stores the daily totals in a text file rather than in an array, and why the file is opened in APPEND mode rather than WRITE mode.

A program writes a message to a log file each time an error occurs.

The text file ErrorLog.txt already exists and contains the messages from earlier runs of the program. These messages must not be lost.

The procedure AddError() takes one parameter Message of type STRING. It adds Message to ErrorLog.txt as a new line, then closes the file.

The first and last lines of the procedure are given.

Complete the procedure by writing the three pseudocode statements that are missing.

A weather station stores its temperature readings in the text file Readings.txt.

Each line of the file holds one reading as a numeric string, for example 18.4. The number of readings in the file is not known, and the file may be empty.

Write pseudocode for a function CountAbove() that:

• takes one parameter Limit of type REAL
• opens Readings.txt for reading
• reads every line in the file
• counts the readings that are greater than Limit
• closes the file
• returns the count as an INTEGER, returning 0 if the file is empty.

You may use the built-in functions EOF() and STRTONUM().