Question

Write a void function definition for a function called add_tax. The function add_tax has two formal parameters: tax_rate, which is the amount of sales tax expressed as a percentage, and cost, which is the cost of an item before tax. The function changes the value of cost so that it includes sales tax.

Short answer

Question: Define a void function called "add_tax" that takes two parameters: tax_rate and cost. Calculate the cost of an item after including sales tax and update the cost with the new value. Answer: The "add_tax" function is defined as follows: ```cpp void add_tax(double tax_rate, double& cost) { double tax_amount = (cost * tax_rate) / 100; cost = cost + tax_amount; } ``` This function calculates the tax amount by multiplying the cost by the tax_rate and dividing by 100. The cost is then updated by adding the tax_amount to the original cost.

Step-by-step solution

Understand the void function and parameters

A void function does not return a value and its result is directly reflected on the input parameters when working with reference types or pointer types, like in this case with the cost variable. The formal parameters are tax_rate and cost. Tax_rate is the sales tax percentage, and cost is the price of the item before tax.

Calculate the tax amount

To calculate the tax amount, we need to multiply the cost by the tax_rate and then divide by 100 to convert percentage to a decimal form. The formula is: tax_amount = (cost * tax_rate) / 100

Update the cost to include tax

Once the tax amount is calculated, we need to update the cost variable to include the tax. We can do this by simply adding the tax_amount to the current cost. The formula is: cost = cost + tax_amount Now, let's put it all together in the function definition: ```cpp void add_tax(double tax_rate, double& cost) { double tax_amount = (cost * tax_rate) / 100; cost = cost + tax_amount; } ``` Note that we use double& for cost parameter so that it's updated within the function and reflects the changes when the function finishes execution.

Key concepts

Void Functions

In C++ programming, a 'void function' is a type of function that does not return any value. Essentially, when you create a void function, you are instructing the code to perform operations internally without returning a result to the calling code.
The primary characteristic of a void function is its 'void' keyword, which denotes that no value will be returned. For example, in the function `add_tax`, the keyword 'void' indicates that this function will only perform the task of modifying the `cost` variable, but it will not return any data back to the caller.
Void functions are particularly useful for tasks that involve modifying variables or when a return value is not necessary. Another advantage is the clarity and straightforwardness they offer when writing and reading your code.

Parameters

Parameters in C++ are variables that provide a way to pass information to a function. They act as placeholders for the actual values that will be used in the function's operations. When you define a function, you declare what variables it expects, called 'formal parameters'.
For our `add_tax` function, the parameters are `tax_rate` and `double& cost`. The `tax_rate` represents the sales tax percentage, and its role is to hold the tax rate specific to that function call. The `cost` parameter, denoted by `double&`, signifies its reference nature, allowing changes made within the function to affect the original variable.
Parameters enhance the flexibility of your code by enabling the same function to perform similar tasks on different sets of data without needing multiple, almost identical function definitions.

Sales Tax Calculation

Calculating sales tax in programming involves determining a portion of a cost, based on a given tax rate. The basic formula to calculate sales tax is:
\[ \text{tax\_amount} = \left(\frac{\text{cost} \times \text{tax\_rate}}{100}\right) \]
This formula multiplies the `cost` of an item by the `tax_rate`, which is often represented as a percentage. To convert a percentage to a decimal, divide by 100. This calculation yields the additional cost incurred by the sales tax, which you then add to the original cost to obtain the final price.
Understanding this process is essential for writing code that dynamically calculates the final price of goods or services, ensuring the correct application of tax.

Pass-by-Reference

The concept of 'pass-by-reference' in C++ allows you to pass variables to functions such that any modification to these variables within the function affects the original data outside the function. This is achieved by using the '&' symbol when declaring a parameter in a function's definition, indicating that this parameter should accept a reference to the original variable.
In the context of the `add_tax` function, `double& cost` is a reference parameter, which means that the function works directly with the original `cost` variable. Changes to `cost` inside the function persist even after the function concludes, making it ideal for scenarios where you want the function to modify the input data permanently.
Pass-by-reference minimizes memory usage by avoiding copies of large data structures and provides a straightforward way to allow functions to modify input parameters directly.