Margaret uses a program to help her determine the selling price for each batch of ice cream based on the production cost, profit and tax - Edexcel - GCSE Computer Science - Question 6 - 2019 - Paper 1

Defining values as constants is considered good programming practice for several reasons:

1. Readability

Using meaningful constant names (like TAX and PROFIT) makes the code easier to read and understand. Anyone reviewing the code can immediately grasp the purpose of these values, unlike when raw numbers are used.

2. Maintainability

If a fixed value that is not a constant needs to be changed, it often requires searching through the entire code to find all instances. However, if constants are used, only the definition of the constant needs to be updated, which significantly reduces the chance of errors. This makes the codebase easier to maintain.

3. Prevention of Magic Numbers

Using fixed values within the code can lead to what is known as 'magic numbers'. These are values that appear without context, making it hard to determine their significance. Constants provide context and meaning to these values, leading to better structured and more professional code.

4. Error Minimization

When values are defined as constants, the risk of accidentally changing them during code modifications is minimized. This is particularly crucial in large applications where such changes can lead to bugs that are hard to trace.

5. Ease of Updating

If the value of a constant needs to be altered (for example, changing the tax rate), updating the constant in one location propagates this change throughout the entire code, ensuring consistency and reducing the possibility of errors due to oversight.

In summary, using constants instead of fixed values enhances the clarity, maintainability, and reliability of the code.