File Handling

🎯 Syllabus & Goals 3 min

Cambridge 8.3 · File handling Paper 2 · Algorithms, Programming and Logic

By the end of this lesson you can:

1. Explain why programs store data in files.
2. Open a file for READ or WRITE, read or write one line or one item of data, and close it — in pseudocode and Python.
3. Write a short algorithm that keeps data between runs of a program by using a file.

Textbook: Chapter 8, §8.3 (pp. 333–336)

Recap / Warm-Up 5 min

Last lesson: arrays hold many values under one name. But every variable and array lives in RAM while the program runs.

Quick starter

A game stores its high score in a variable. The player closes the game and restarts it. What is the high score now — and why? (Think back to Unit 3.)

🧠 Key Concept 14 min

1 · Why store data in a file?

• Data in RAM is lost when the computer is switched off; data saved to a file is stored permanently.
• The same program can use the data again later, e.g. the next time it runs.
• Another program can open and use the data.
• The file can be sent to other computers.

2 · Using a file — open, read or write, close

Every file is identified by its filename. For IGCSE you read and write one line of text or a single item of data.

Worked Example 12 min

Example 1 · Write a line of text, then read it back

DECLARE TextLine : STRING
DECLARE MyFile : STRING
MyFile ← "Message.txt"
// write the line of text to the file
OPENFILE MyFile FOR WRITE
OUTPUT "Please enter a line of text: "
INPUT TextLine
WRITEFILE MyFile, TextLine
CLOSEFILE MyFile
// read the line of text back from the file
OUTPUT "The file contains this line of text:"
OPENFILE MyFile FOR READ
READFILE MyFile, TextLine
OUTPUT TextLine
CLOSEFILE MyFile

# Write one line of text to a file, then read it back
my_file = open("Message.txt", "w")      # open for WRITE
text_line = input("Please enter a line of text: ")
my_file.write(text_line)                 # write the line
my_file.close()                          # close the file

print("The file contains this line of text:")
my_file = open("Message.txt", "r")      # open for READ
text_line = my_file.readline()           # read one line
print(text_line)
my_file.close()

Please enter a line of text: Meet at the library at 4pm
The file contains this line of text:
Meet at the library at 4pm

1. The filename is stored in a variable, MyFile.if the name changes, only one line is edited.
2. Open for WRITE → write → close.closing makes sure the text is actually saved to storage.
3. Open again, this time for READ, and read into TextLine.a file is opened in one mode at a time.

Example 2 · Keeping a high score between games

The file HighScore.txt holds one integer. Read it, compare it with this game's score, and save the new score if it is higher.

DECLARE HighScore, NewScore : INTEGER
OPENFILE "HighScore.txt" FOR READ
READFILE "HighScore.txt", HighScore
CLOSEFILE "HighScore.txt"
OUTPUT "Current high score: ", HighScore
OUTPUT "Your score this game: "
INPUT NewScore
IF NewScore > HighScore
  THEN
    OPENFILE "HighScore.txt" FOR WRITE
    WRITEFILE "HighScore.txt", NewScore
    CLOSEFILE "HighScore.txt"
    OUTPUT "New high score saved!"
  ELSE
    OUTPUT "No new high score"
ENDIF

# Keep the high score in a file so it survives after the program ends
score_file = open("HighScore.txt", "r")
high_score = int(score_file.readline())   # read one item, cast to INTEGER
score_file.close()
print("Current high score:", high_score)

new_score = int(input("Your score this game: "))
if new_score > high_score:
    score_file = open("HighScore.txt", "w")
    score_file.write(str(new_score))        # files store text, so cast back
    score_file.close()
    print("New high score saved!")
else:
    print("No new high score")

Output — two separate runs (the file started with 250)

Current high score: 250
Your score this game: 310
New high score saved!

Current high score: 310
Your score this game: 200
No new high score

The second run "remembers" 310 — even though every variable was wiped when the first run ended. That is the whole point of a file.

Try It Yourself 12 min

📝 Exam Practice 10 min

🗝️ Recap & Key Terms 3 min

Files keep data after a program ends. Always open (in READ or WRITE mode), read or write one line or item, then close. WRITE replaces the old contents. In Python, remember that files hold text: cast with int() after reading and str() before writing a number.

File

A collection of data stored by a computer program to be used again.

OPENFILE

Makes a file available to the program in a given mode (READ or WRITE).

READFILE

Reads a line of text / an item of data from an open file into a variable.

WRITEFILE

Writes a line of text / an item of data to a file opened for writing.

CLOSEFILE

Closes the file so the data is saved and the file is released.

Homework 1 min

Task (≤ 15 min): Write pseudocode that reads a number from "Visits.txt", adds 1 to it, outputs "You have visited n times", and writes the new number back to the file. [5]

DECLARE Visits : INTEGER
// read the old count
OPENFILE "Visits.txt" FOR READ
READFILE "Visits.txt", Visits
CLOSEFILE "Visits.txt"
// update and report it
Visits ← Visits + 1
OUTPUT "You have visited ", Visits, " times"
// save the new count
OPENFILE "Visits.txt" FOR WRITE
WRITEFILE "Visits.txt", Visits
CLOSEFILE "Visits.txt"