Read from and write to a text file, including opening, reading line by line, writing and closing a file, so that data persists after the program ends.

What this dot point is asking

AQA wants you to read from and write to a text file in a program: opening a file, reading it line by line, writing data to it, and closing it, so that data is kept after the program ends.

Why use files

This is the same volatile-versus-non-volatile idea as RAM and secondary storage: a variable lives in RAM, which is cleared when the program stops, while a file lives on disk or SSD, which keeps its contents. Any program that must remember something for next time (a saved game, a contacts list, a log) writes that data to a file.

Reading from a file

fileRef <- OPEN('scores.txt')
WHILE NOT fileRef.endOfFile()
    line <- fileRef.readLine()
    OUTPUT line
ENDWHILE
fileRef.close()

The loop is controlled by an end-of-file test so it reads exactly as many lines as the file has, with no assumption about how many that is.

Writing to a file

Opening and closing

Processing a file line by line

The usual pattern for reading a text file is a loop controlled by an end-of-file test. You open the file, then while you are not at the end of the file you read the next line into a variable and process it (output it, search it, add a value to a running total), and when the loop ends you close the file. This pattern reads exactly as many lines as the file contains without needing to know the count in advance, which is why a WHILE loop with an end-of-file condition is used rather than a FOR loop. Processing each line as it is read also avoids loading a huge file into memory all at once.

Combining files with the rest of a program

File handling rarely stands alone; it combines with the other constructs. You might read each line of a scores file (iteration), convert the text to a number (type conversion), compare it with the current best (selection), and write the new high score back to the file (writing). Because input from a file arrives as strings, remember to convert any values you need to calculate with into numbers first. Closing the file at the end then guarantees the written data is saved and the file is released for other programs to use.

Try this

Q1. State one reason a program would write data to a file rather than a variable. [1 mark]

• Cue. So the data persists after the program ends, instead of being lost.

Q2. State why a file should be closed after writing to it. [1 mark]

• Cue. To save the changes and free the file for other use.