Calculating totals

C# course · Beginner · Lesson 50/116

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 method. Each product contributes Price * 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 foreach, and add with sum += p.Price * p.Quantity. Finally return sum; returns the result.

Subtotal and 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.

Exercise Write the GetSubtotal method

Add a GetSubtotal method to the Cart class. Its signature must be public double GetSubtotal(). In the body start with double sum = 0;, walk each product with foreach (var p in items), add with sum += p.Price * p.Quantity, and return with return sum;. Main adds two products and prints GetSubtotal.

Show example solution

Inside Cart write public double GetSubtotal() { double sum = 0; foreach (var p in items) { sum += p.Price * p.Quantity; } return sum; }
Example solution:
using System;
using System.Collections.Generic;

class Product
{
    public string Name { get; set; }
    public double Price { get; set; }
    public int Quantity { get; set; }
}

class Cart
{
    private List<Product> items = new List<Product>();

    public void AddProduct(Product product)
    {
        items.Add(product);
    }

    public double GetSubtotal()
    {
        double sum = 0;
        foreach (var p in items)
        {
            sum += p.Price * p.Quantity;
        }
        return sum;
    }
}

class Program
{
    static void Main()
    {
        var cart = new Cart();
        cart.AddProduct(new Product { Name = "Notebook", Price = 25, Quantity = 2 });
        cart.AddProduct(new Product { Name = "Pen", Price = 10, Quantity = 3 });
        Console.WriteLine(cart.GetSubtotal());
    }
}

Test yourself

Write the foreach loop variable

double sum = 0;
foreach (____ p in items)
{
    sum += p.Price * p.Quantity;
}

Show answer

double sum = 0;
foreach (var p in items)
{
    sum += p.Price * p.Quantity;
}

What does subtotal mean?
```
// in a cart
```
- Tax-inclusive total
- Pre-tax intermediate total
- Number of items
- Discount amount
Show answer

Pre-tax intermediate total — Correct. Subtotal is the pre-tax intermediate total.
Match each concept to its meaning

Subtotal

Pre-tax total
Tax

Tax rate
Total

Tax-inclusive total
Quantity

Item count

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Calculating totals