Swift Error on mutable state as let constant

I have simple codeable json. I am using for each loop for iteration, here I comparing the product based on id. If the id exists, then it should increase the quantity otherwise add the item and I am using this function in view as well. But I am getting following error. Left side of mutating operator isn’t mutable; ‘quantity’ is a ‘let’ constant. I have tried to use mutating key word before the function and also change the struct let to var but it did not help.

Here is my codeable json.

import Foundation
// MARK: - Welcome
struct ProductData: Codable, Hashable {
    let carts: [Cart]
}

// MARK: - Cart
struct Cart: Codable, Hashable {
    let id: Int
    let products: [Product]
}

// MARK: - Product
struct Product: Codable, Hashable, Identifiable {
    let id: Int
    let title: String
    let price, quantity, total: Int
    let discountPercentage: Double
    let discountedPrice: Int
    let thumbnail: String
}

Here is my function.

class Order: ObservableObject {
    
    @Published var product = [Product]()
    @Published private(set) var productTotal: Int = 0
    
    func add(item: Product) {
        var exists = false
        product.forEach { product in
            if product.id == item.id {
                exists = true
                product.quantity += 1 // error on this line
            }
        }
        if !exists {
            product.append(item)
        }
    }
}

Here is the screenshot of the error.

>Solution :

First of all please name product in plural form products.

@Published var products = [Product]()

There are two problems:

1. You have to declare quantity in Product as variable

    var quantity: Int
   

2. Index variables/elements in loops are immutable. In fact they are immutable for a good reason: Due to value semantics the index element is a copy of the array item which won’t modify the original item in the array.

But you don’t need a loop at all. Try to get the index of the item in the array. If the item exists increase quantity otherwise append the item. products[index] modifies the array in place

func add(item: Product) {
   if let index = products.firstIndex(where: {$0.id == item.id }) {
       products[index].quantity += 1
   } else {
       products.append(item)
   }
}