8.3 File handling

8.3.1 Purpose of storing data in a file

Data held in RAM is lost when the computer is switched off. If a program will need the data again, the data can instead be saved in a file, where it is stored permanently.

Once data has been saved in a file, it can be:

• accessed later by the same program
• accessed by another program
• sent to and used on another computer.

8.3.2 Using files

Every file is identified by its filename. For IGCSE Computer Science, you need to understand how a program can write a line of text or a single item of data to a file and then read it back.

The basic file-handling sequence

Pseudocode example from the textbook

The source example stores the filename in MyFile, writes a line of text, closes the file, reopens it for reading, reads the line and displays it.

DECLARE TextLine : STRING   // variables are declared as normal
DECLARE MyFile : STRING
MyFile ← "MyText.txt"

// writing the line of text to the file
OPEN MyFile FOR WRITE       // opens file for writing
  OUTPUT "Please enter a line of text"
  INPUT TextLine
  WRITEFILE, TextLine       // writes a line of text to the file
CLOSEFILE(MyFile)           // closes the file

// reading the line of text from the file
OUTPUT "The file contains this line of text:"
OPEN MyFile FOR READ        // opens file for reading
  READFILE, TextLine        // reads a line of text from the file
  OUTPUT TextLine
CLOSEFILE(MyFile)           // closes the file

What each pseudocode command is doing

Python example

# writing to and reading a line of text from a file
MyFile = open ("MyText.txt","w")
TextLine = input("Please enter a line of text ")
MyFile.write(TextLine)
Myfile.close()

print("The file contains this line of text")
MyFile = open ("MyText.txt","r")
TextLine = MyFile.read()
print(TextLine)
Myfile.close()

In the example, "w" is used when the file is opened for writing and "r" when it is opened for reading. The line entered by the user is written to the file, read back and then displayed.

Visual Basic example

'writing to and reading from a text file
Imports System.IO
Module Module1
  Sub Main()
    Dim textLn As String
    Dim objMyFileWrite As StreamWriter
    Dim objMyFileRead As StreamReader

    objMyFileWrite = New StreamWriter("textFile.txt")
    Console.Write("Please enter a line of text ")
    textLn = Console.ReadLine()
    objMyFileWrite.WriteLine(textLn)
    objMyFileWrite.Close()

    Console.WriteLine("The line of text is ")
    objMyFileRead = New StreamReader("textFile.txt")
    textLn = objMyFileRead.ReadLine
    Console.WriteLine(textLn)
    objMyFileRead.Close()
    Console.ReadLine()
  End Sub
End Module

The Visual Basic example imports System.IO, uses a StreamWriter to write the text, closes it, then uses a StreamReader to read the stored line back.

Java example

// writing to and reading from a text file
// try and catch implement exception handling; the textbook notes that
// exception handling is beyond IGCSE level
import java.util.Scanner;
import java.io.BufferedReader;
import java.io.PrintWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

class TextFile {
  public static void main(String[] args) {
    Scanner myObj = new Scanner(System.in);
    String textLn;
    try {
      FileWriter myFileWriter = new FileWriter("textFile.txt", false);
      PrintWriter myPrintWriter = new PrintWriter(myFileWriter);
      System.out.println("Please enter a line of text ");
      textLn = myObj.next();
      myPrintWriter.printf("%s" + "%n", textLn);
      myPrintWriter.close();
    } catch (IOException e) {
      e.printStackTrace();
    }
    try {
      FileReader myFileReader = new FileReader("textFile.txt");
      BufferedReader myBufferReader = new BufferedReader(myFileReader);
      textLn = myBufferReader.readLine();
      System.out.println(textLn);
      myFileReader.close();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
}

The Java example imports the file-handling classes it needs, writes the text using FileWriter and PrintWriter, then reads it back using FileReader and BufferedReader. The textbook explicitly states that the try/catch exception-handling concept is beyond IGCSE level.

Copying data from one text file to another

The chapter activity asks you to apply the same ideas to copy a line of text from one text file to another. The logic is:

OPEN source file FOR READ
READ the line into a variable
CLOSE the source file
OPEN destination file FOR WRITE
WRITE the stored line to the destination file
CLOSE the destination file

This is not a new kind of file operation: it combines the same read and write steps already used above.