Question

Assume the file data.txt exists and contains several lines of text. Write a short program using the while loop that displays each line in the file.

Short answer

Answer: To read and display each line from the file "data.txt," follow the steps below: 1. Open the file using the open() function in read mode: ```python file = open('data.txt', 'r') ``` 2. Initialize a variable 'line' to None, and set up a while loop that reads each line and prints it until the readline() method returns an empty string: ```python line = None while line != '': line = file.readline() print(line) ``` 3. Close the file to free up any associated resources using the close() method: ```python file.close() ``` Combining these steps, the full program would look like this: ```python file = open('data.txt', 'r') line = None while line != '': line = file.readline() print(line) file.close() ```

Step-by-step solution

Open the file data.txt

To read the content of the file, we first have to open it using the open() function in read mode (indicated by 'r') like this: ```python file = open('data.txt', 'r') ```

Set up the while loop

We'll now set up a while loop that reads each line and, for the time being, just prints it. We will start by initializing a variable 'line' to None, so our loop will continue until the readline() method returns an empty string (indicating the end of the file): ```python line = None while line != '': line = file.readline() print(line) ```

Close the file

After the while loop is finished, we need to close the file to free up any associated resources. We can do this using the close() method on our file object: ```python file.close() ```

Full program

Combining the three previous steps, the full program to display each line in 'data.txt' using a while loop would look like this: ```python file = open('data.txt', 'r') line = None while line != '': line = file.readline() print(line) file.close() ```

Key concepts

while loop

The while loop is a fundamental control flow statement in Python, often used to repeat a block of code as long as a condition is true. It continues to execute the code within it until the condition evaluates to False. In the context of reading files, a while loop can be handy because it allows you to continue reading from a file until there is nothing left to read.

Here’s how it generally works:

• First, you start with a condition, usually evaluated at the beginning of the loop.
• If the condition is True, the code block within the loop executes.
• After executing the code, the condition is re-evaluated.
• If the condition remains True, the loop continues. If not, it exits the loop.

In our file reading example, the condition checks if `line` is not equal to an empty string. This ensures that as long as there are lines to read, they will be printed. It’s crucial to always include a mechanism inside the loop to eventually meet the exit condition, preventing an infinite loop.

open() function

The open() function in Python is indispensable for file handling. It establishes communication between your Python program and a file, enabling data reading or writing.

Here’s how you use it:

• To open a file, you use `open(filename, mode)`, where `filename` is a string specifying the file path.
• `mode` determines how the file is to be opened:
  • `'r'` for reading, which is used in this example.
  • `'w'` for writing (and erasing any existing content).
  • `'a'` for appending data at the end of the file.
  • `'b'` for binary mode (combined with any of the above).

For our task, the open function is used to access 'data.txt' in read mode. This allows us to read all the lines of the file sequentially. Keep in mind, without opening a file, you can't read or write to it. Open returns a file object, which is essential for other file-related operations.

readline() method

The readline() method is a feature of Python's file object, and it is essential for reading files line by line. This allows you to process large files without loading the entire content into memory at once.

The method works as follows:

• When called, readline() reads the next line from the file.
• The method returns the line as a string, including the newline character at the end.
• If there are no more lines to read, it returns an empty string, which is useful to know when to stop reading.

In the example program, readline() is used within a while loop, allowing each line to be read and printed until the end of the file is reached. This technique is efficient because it reads one line at a time, making it suitable for processing large files in a manageable way.

file close() method

Closing a file is a crucial part of file handling in Python, and it is achieved using the close() method. Whenever a file is opened, whether for reading, writing, or appending, system resources are allocated to handle these operations.

Here's why the close() method is important:

• It releases the resources allocated during the open file operation.
• Ensures all buffered data is written to the file if you are in write or append mode.
• It’s a best practice that prevents data corruption and frees up memory.

In our example, close() is called once all file operations (in this case, reading) are complete. This makes the program more efficient and reduces the risk of running into issues like running out of available file handles in case more files are opened afterwards. Always remember: after finishing with a file, make sure to close it efficiently.