Question

Write a program that reads cach line in a file, reverses its lines, and writes them to another file. For example, if the file input. txt contains the lines Nary had a liftle lanb Its fleece was white as snow And everyuhere that Mary went The lanb was sure to go. and you run reverse input, txt output, txt then output, txt contains The lanb was sure to go. And everyuhere that Mary went Its fleece was white as snow Mary had a little lanb.

Short answer

Read input lines into a list, reverse the list, then write the reversed lines to the output file.

Step-by-step solution

Open Files

First, open the 'input.txt' file in read mode. This file contains the text that you want to reverse line by line. Also, prepare to open 'output.txt' in write mode, as this is where the reversed lines will be stored.

Read Input File

Read all the lines from the 'input.txt' file. You can use the readlines() method to read each line into a list where each element is a line from the file.

Reverse Lines

Once you have all the lines stored in a list, use Python's slicing feature to reverse this list. This means you will write the last line of the 'input.txt' first in 'output.txt', and continue in reverse order until the first line.

Write to Output File

Open 'output.txt' in write mode and iterate through the reversed list. Write each line into this file. After writing each line, make sure to close the file after the write process is complete to ensure all data is properly saved.

Confirm Output

After executing the program, check 'output.txt' to ensure the lines from 'input.txt' have been correctly reversed and saved. Each line in the output file should be in the reverse order of the input file.

Key concepts

Reading Files

Reading files in Python is a fundamental skill that allows you to access text or data stored in files. To begin reading a file, you first need to open it using the `open()` function. You specify the filename and the mode, such as `'r'` for reading. This prepares the file to be read:

• Use open('filename.txt', 'r') to open a file in read mode.
• The file contents can be read using methods like read(), readline(), or readlines().
• read() fetches the whole content as a single string.
• readline() allows you to read a file line by line.
• readlines() returns a list of all lines in the file, which is convenient for iterating line by line.

After reading the content, it is good practice to close the file using the close() method. This frees up system resources that were being used by the file.

Writing Files

Writing to files allows you to store data and create files dynamically within your programs. The `open()` function is used to create or open a file in write (`'w'`), append (`'a'`), or both read and write modes. When in write mode, if the specified file does not exist, it will be created:

• To open a file for writing, use open('filename.txt', 'w').
• Writing to the file involves using the write() method, where you pass in the string data you want to store.
• It's essential to manage the file stream properly by closing the file with close() after writing.
• Remember that opening a file in write mode will overwrite the content if the file already exists. Use append mode ('a') to add to the existing data without deleting the existing content.

Reversing List Elements

Reversing a list is a handy technique, especially when manipulating ordered data. In Python, this can be done efficiently with slicing or by using specific methods:

• Lists can be reversed by using the slicing technique: list[::-1].
• This method produces a copy of the list containing elements in reverse order.
• Alternatively, use the reverse() method, which reverses elements in place, modifying the original list.
• Remember that some methods, like slicing, return a new list, while others, like reverse(), do not create a new list but change the existing one.

Reversing in-place can be more memory-efficient for large lists since it doesn't require additional space for a new list.

File I/O Operations

File I/O (Input/Output) operations are essential for interacting with files, allowing you to read, write, and manipulate file data. Understanding these operations is crucial for managing file-based resources efficiently. Here's a simple guide:

• Input operations involve reading data from files, which requires opening the file in read mode.
• Output operations involve writing data to files, for which you'll use write or append modes.
• The open() function is fundamental, serving as the gateway to performing both reading and writing tasks.
• Always ensure you handle files with context managers like with statements to avoid resource leaks. This automatically closes the file after operations are complete.

By mastering File I/O, you can create programs capable of persistent data storage and retrieval, enhancing the functionality of your applications.