Reading from and
Writing to Files

Understanding File Input and Output

File I/O (Input/Output) is the process of reading data from a file and writing data to a file. Most programs interact with files to save data persistently—for example, saving user data, logs, configurations, or reports.

Why File I/O is Important

Files allow programs to:

• Store data that needs to persist beyond the program's execution
• Read structured or unstructured information from external sources
• Log operations, errors, and system activity

Opening a File

Before performing any operation on a file, it must be opened using a file handler or reference.

// Open a file for reading
file = open("data.txt", mode="read")

// Open a file for writing (creates if not exists)
file = open("output.txt", mode="write")

// Open a file for appending (writes at end)
file = open("log.txt", mode="append")

Question: What happens if the file doesn't exist in reading mode?

Answer: It usually throws an error because reading expects the file to be present. Writing or appending mode can create the file if it doesn't exist.

Reading from a File

When reading, data is extracted from the file and loaded into memory. You can read:

• The entire file at once
• Line by line
• A specific number of characters

// Read entire file content
content = file.readAll()

// Read one line
line = file.readLine()

// Read fixed number of characters
chunk = file.readChars(100)

Output:

Hello, this is a sample text file.
It has multiple lines of content.

Question: Why would you read line by line instead of the whole file?

Answer: Reading line-by-line is memory efficient for large files and allows processing content incrementally.

Writing to a File

Writing stores content into a file. Writing mode overwrites existing content, while append mode adds to the end.

// Write to file
file.write("This is a new file.")

// Append to existing file
file.append("New log entry recorded.")

Output:

This is a new file.
New log entry recorded.

Question: What if you open a file in write mode and it already has data?

Answer: It will overwrite the existing content unless opened in append mode.

Closing a File

Always close a file after completing operations to release system resources and ensure data is saved properly.

file.close()

Tip: Some languages provide automatic handling (like using blocks or context managers) to ensure files are closed, even if an error occurs.

Putting It All Together

// Complete flow to read and write
inputFile = open("input.txt", mode="read")
data = inputFile.readAll()
inputFile.close()

outputFile = open("result.txt", mode="write")
outputFile.write("Processed Data:\n" + data)
outputFile.close()

Output:

Processed Data:

Points to Remember

• Always open files in the correct mode (read/write/append).
• Reading loads content into memory; use efficiently for large files.
• Writing replaces content unless using append mode.
• Always close your files to avoid data loss or resource leaks.