Calculating totals

Learn to compute the subtotal and the tax-inclusive total.

Calculating totals

The subtotal of the cart is the amount before tax and extra fees. The getSubtotal method sums all prices in the prices list and returns a double.

To sum, we initialize a sum variable and walk each price with for-in: double sum = 0; for (var p in prices) { sum += p; } return sum;. The += operator adds the right side to the left variable. After the loop sum holds the total of all prices.

Tax and total

The subtotal is the amount before tax and extra fees. The tax-inclusive total uses subtotal * (1 + taxRate); for a 20% tax you apply a 1.20 multiplier.

In this project getSubtotal returns only the subtotal; the taxed total is 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 method

Add a getSubtotal method to the Cart class. Its signature is double getSubtotal(). In the body start with double sum = 0;, walk each price with for (var p in prices), add with sum += p; and return with return sum;. main already adds two products and prints getSubtotal. The output on a single line must be 35.0.

Show example solution

class Cart {
  List<String> names = [];
  List<double> prices = [];

  void addItem(String name, double price) {
    names.add(name);
    prices.add(price);
  }

  double getSubtotal() {
    double sum = 0;
    for (var p in prices) {
      sum += p;
    }
    return sum;
  }
}

void main() {
  var cart = Cart();
  cart.addItem("Notebook", 25.0);
  cart.addItem("Pen", 10.0);
  print(cart.getSubtotal());
}

Test yourself

Match each part to its job

double sum = 0

Initialize total
for (var p in prices)

Walk each price
sum += p

Add the price
return sum

Return the total
What does getSubtotal return?
```
double getSubtotal() { ... }
```
- Subtotal before tax
- Tax-inclusive total
- Number of products
- First product price
Show answer

Subtotal before tax — getSubtotal returns the subtotal before tax.

Write the return line

double getSubtotal() {
  double sum = 0;
  for (var p in prices) {
    sum += p;
  }
  ____
}

Show answer

double getSubtotal() {
  double sum = 0;
  for (var p in prices) {
    sum += p;
  }
  return sum;
}

Next lesson: Listing the cart

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