C++
Calculating totals
Learn to compute the subtotal and the tax-inclusive total.
Calculating totals
To compute the subtotal of the products in the cart we will write a getSubtotal function. Each product contributes p.price * p.quantity; we walk all products and sum these to get the subtotal.
This is a classic accumulation pattern: start with double sum = 0;, walk each product with range-based for, and add with sum += p.price * p.quantity. Finally return sum; returns the result. The same approach will help you with stock counts, averages and many other calculations later.
Computing tax
The subtotal is the amount before tax and extra fees. To get the tax-inclusive total you use subtotal * (1 + taxRate); for an 18% tax you apply a 1.18 multiplier.
In this project getSubtotal returns only the subtotal; the taxed total can be computed in main. Separating the two makes it easy to update just the multiplier when the tax rate changes. This separation matters because tax rates vary from country to country.
Exercise Write the getSubtotal function
Add a getSubtotal function to the Cart struct. Its signature must be double getSubtotal(). In the body start with double sum = 0;, walk each product with for (const auto& p : items), add with sum += p.price * p.quantity and return with return sum;. main already adds two products and prints getSubtotal. The output must be 80.
Show example solution
#include <iostream>
#include <string>
#include <vector>
struct Product {
std::string name;
double price;
int quantity;
};
struct Cart {
std::vector<Product> items;
void addItem(Product p) {
items.push_back(p);
}
double getSubtotal() {
double sum = 0;
for (const auto& p : items) {
sum += p.price * p.quantity;
}
return sum;
}
};
int main() {
Cart cart;
Product p1;
p1.name = "Notebook";
p1.price = 25.0;
p1.quantity = 2;
cart.addItem(p1);
Product p2;
p2.name = "Pen";
p2.price = 10.0;
p2.quantity = 3;
cart.addItem(p2);
std::cout << cart.getSubtotal() << std::endl;
return 0;
}
Test yourself
-
Match each concept to its meaning
- Subtotal
- Pre-tax total
- Tax
- Tax rate
- Total
- Tax-inclusive total
- Quantity
- Item count
-
Write the getSubtotal body
double sum = 0;
for (const auto& p : items) {
____;
}
return sum;
Show answer
double sum = 0;
for (const auto& p : items) {
sum += p.price * p.quantity;
}
return sum;
-
What does subtotal mean?
// in a cart
- Number of items
- Tax-inclusive total
- Pre-tax intermediate total
- Discount amount
Show answer
Pre-tax intermediate total — Subtotal is the pre-tax intermediate total.
Next lesson: Listing the cart
