File-Processing

What Is File Processing?

File processing allows programs to store and retrieve data from external files.

Files can be opened in different modes: read, write, and append.

File Handling Functions in Python and Java

Operation	Python Function	Java Class/Method
Open a file	`open("file.txt", "r")`	`new FileReader("file.txt")`
Read from file	`file.read()`, `file.readline()`	`BufferedReader.readLine()`
Write to file	`file.write("text")`	`FileWriter.write("text")`
Append data	`open("file.txt", "a")`	`new FileWriter("file.txt", true)`
Close file	`file.close()` (or implicit with `with`)	`close()` on `BufferedReader` or `FileWriter`

Visualising File Modes

Imagine you have a text file, example.txt, containing three items. In a “line‐delimited” file each item sits on its own line; in a “CSV” file they share one line, separated by commas.

Line-Delimited File (1D array)

Initial Content

                Alpha

                Beta

                Gamma

After Write Mode (w)

                Delta
            

After Append Mode (a)

                Alpha

                Beta

                Gamma

                Delta

CSV-Delimited File (1D array)

Initial Content

                Alpha, Beta, Gamma
            

After Write Mode (w)

                Delta
            

After Append Mode (a)

                Alpha, Beta, Gamma, Delta
            

2D-Array File Content

Now imagine each line in example.txt is a “record” (an object), with comma-separated fields. Together they form a 2D array: rows of records, columns of fields.

Initial Content

                A1,B1,C1

                A2,B2,C2

After Write Mode (w)

                D1,D2,D3

                D4,D5,D6

After Append Mode (a)

Examples

For the following code samples, feel free to use either of the following files:

example.txt | example.csv

Example: Reading a 2D Array (CSV)

import csv

# Read CSV rows into a list of lists
with open("example.txt", "r") as f:
    reader = csv.reader(f)
    data = [row for row in reader]
    print("2D Data:", data)

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

public class Read2DExample {
    public static void main(String[] args) {
        List<String[]> data = new ArrayList<>();
        try (BufferedReader br = new BufferedReader(new FileReader("example.txt"))) {
            String line;
            while ((line = br.readLine()) != null) {
                data.add(line.split(","));
            }
        } catch (IOException e) {
            System.out.println("Read error: " + e.getMessage());
        }
        System.out.println("2D Data: " + data);
    }
}

Example: Writing a 2D Array (CSV)

import csv

# Sample 2D data
data = [["A1","B1","C1"], ["A2","B2","C2"]]

# Write rows to CSV (overwrite)
with open("example.txt", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerows(data)

import java.io.FileWriter;
import java.io.IOException;

public class Write2DExample {
    public static void main(String[] args) {
        String[][] data = { {"D1","D2","D3"}, {"D4","D5","D6"} };
        try (FileWriter fw = new FileWriter("example.txt")) {
            for (String[] row : data) {
                fw.write(String.join(",", row) + "\n");
            }
        } catch (IOException e) {
            System.out.println("Write error: " + e.getMessage());
        }
    }
}

Example: Appending a 2D Array (CSV)

import csv

# New rows to append
new_rows = [["E1","E2","E3"], ["E4","E5","E6"]]

# Append rows to CSV
with open("example.txt", "a", newline="") as f:
    writer = csv.writer(f)
    writer.writerows(new_rows)

import java.io.FileWriter;
import java.io.IOException;

public class Append2DExample {
    public static void main(String[] args) {
        String[][] newRows = { {"E1","E2","E3"}, {"E4","E5","E6"} };
        try (FileWriter fw = new FileWriter("example.txt", true)) {
            for (String[] row : newRows) {
                fw.write(String.join(",", row) + "\n");
            }
        } catch (IOException e) {
            System.out.println("Append error: " + e.getMessage());
        }
    }
}

Avoiding Common File Handling Errors

File Not Found:
Before reading, check existence with os.path.exists() in Python or File.exists() in Java. Handle missing files in except/catch blocks.
Permission Errors:
Ensure your program has read/write permissions. In Python, catch PermissionError; in Java, catch IOException and inspect e.getMessage().
Forgetting to Close Files:
Use Python’s with context manager or Java’s try-with-resources to auto-close. This prevents resource leaks and file locks.

Key Takeaways

File processing lets you read, write, and append data to external files.
Python uses open(), read(), write(), and context managers; Java uses FileReader, BufferedReader, and FileWriter.
Always handle missing files and permissions with proper exception handling.
Use context managers (Python) or try-with-resources (Java) to ensure files are closed and resources freed.