Updating quantity

C# course · Beginner · Lesson 51/116

Learn to manage the cart with UpdateQuantity and RemoveProduct.

Updating quantity

To change a product's quantity in the cart we will write an UpdateQuantity method. Signature: public void UpdateQuantity(string name, int quantity). In the body, walk the products with foreach and update the Quantity property of the matching one.

We will also add a RemoveProduct method to drop a product from the cart. This uses a for loop that walks indices backward and calls items.RemoveAt(i); going backward keeps removal safe from index shifts.

Backward removal

Deleting elements while walking a list with foreach is dangerous; the compiler raises an error. Instead, a for loop should walk backward from the last index: for (int i = items.Count - 1; i >= 0; i--).

Going backward handles the shift of subsequent indices smoothly when a removal happens. This is the only safe way to walk and modify a list; RemoveAll with LINQ is an alternative, but here we learn the explicit loop.

Exercise Write UpdateQuantity and RemoveProduct

Add two methods to the Cart class. public void UpdateQuantity(string name, int quantity): walk products with foreach and update the Quantity of the matching one. public void RemoveProduct(string name): walk backward with for (int i = items.Count - 1; i >= 0; i--) and remove matching products with items.RemoveAt(i). Main adds a Notebook, sets its quantity to 5 and tries to remove a Pen; GetSubtotal should return 25*5 = 125.

Show example solution

public void UpdateQuantity(string name, int quantity) { foreach (var p in items) { if (p.Name == name) p.Quantity = quantity; } }
public void RemoveProduct(string name) { for (int i = items.Count - 1; i >= 0; i--) { if (items[i].Name == name) items.RemoveAt(i); } }
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;
    }

    public void UpdateQuantity(string name, int quantity)
    {
        foreach (var p in items)
        {
            if (p.Name == name) p.Quantity = quantity;
        }
    }

    public void RemoveProduct(string name)
    {
        for (int i = items.Count - 1; i >= 0; i--)
        {
            if (items[i].Name == name) items.RemoveAt(i);
        }
    }
}

class Program
{
    static void Main()
    {
        var cart = new Cart();
        cart.AddProduct(new Product { Name = "Notebook", Price = 25, Quantity = 2 });
        cart.UpdateQuantity("Notebook", 5);
        cart.RemoveProduct("Pen");
        Console.WriteLine(cart.GetSubtotal());
    }
}

Test yourself

Match each List member to its job

items.Count

Element count
items.RemoveAt

Remove by index
items.Add

Append to end
items[i]

Access by index
What does cart.UpdateQuantity("Notebook", 5) do?
```
cart.UpdateQuantity("Notebook", 5);
```
- Sets the Notebook quantity to 5
- Removes the Notebook
- Adds a new Notebook
- Sets the total to 5
Show answer

Sets the Notebook quantity to 5 — Correct. UpdateQuantity sets the Quantity property of the matching product.

Write the items.RemoveAt call

for (int i = items.Count - 1; i >= 0; i--)
{
    if (items[i].Name == name) ____(i);
}

Show answer

for (int i = items.Count - 1; i >= 0; i--)
{
    if (items[i].Name == name) items.RemoveAt(i);
}

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Updating quantity