Ios 无法分配给属性：“项”是“let”常量（Swift 4）

我正在尝试在用于处理调用的函数中创建一个循环，但遇到无法分配给属性的错误：“item”是一个“let”常量

 func didReceiveResponse(response:NearbyCars?, error : Error?) -> Void {
        dount += 1
        if let error = error {
            let alertController = UIAlertController(title: "Error", message: error.localizedDescription, preferredStyle: .alert)
            let actionDismiss = UIAlertAction(title: "Dismiss", style: .cancel, handler: nil)
            let actionRetry = UIAlertAction(title: "Retry", style: .default, handler: { (action) in
                DispatchQueue.main.async {
                    self.loadPlaces(true)

                }
            })
            alertController.addAction(actionRetry)
            alertController.addAction(actionDismiss)
            DispatchQueue.main.async {
                self.present(viewController: alertController)

            }
        }
        if let response = response {

        self.response = response
        if response.status == "OK" {
            if let carsDownloaded = response.cars {
                 var number = numberCars / (categories?.count)!
                let quotient = numberCars / (categories?.count)!
                let remainder = numberCars % (categories?.count)!

                for item in (categories?.enumerated())! {
                    item.element?.availability = quotient + (item.offset < remainder ? 1 : 0)
                }      // HERE THE ERROR



            cars.append(contentsOf: carsDownloaded.prefix(number))

        if dount == numberCars { return }
        sortedArray = cars.sorted {
            distance(from: currentLocation!, to: $0.location!) < distance(from: currentLocation!, to: $1.location!)

        }
    }


             self.tableView?.reloadData()
           } else {
                let alert = UIAlertController.init(title: "Error", message: response.status, preferredStyle: .alert)
                alert.addAction(UIAlertAction.init(title: "Cancel", style: .cancel, handler: nil))
                alert.addAction(UIAlertAction.init(title: "Retry", style: .default, handler: { (action) in
                    DispatchQueue.main.async {
                        self.loadPlaces(true)

                    }
                }))
                 self.present(viewController: alert)
             }
            isLoading = false

        } else {
            print("response is nil")
        }
    }

---

在该行，item.element？.availability=商+item.offset<余数？1:0，而我正在尝试创建一个分配其余数字/类别？的循环。计数！到现在起的类别var number=numberCars/categories？计数！只有在没有休息的时候才工作。如何修复它？

您可以在一个范围内进行循环，然后通过索引访问类别以获得可变对象：

for i in 0..<(categories?.count)! {
    var item = categories[i]
    item.element?.availability = quotient + (item.offset < remainder ? 1 : 0)
} 

---

这可能是重复的没有帮助@4kman这个技巧在swift4中不再有效。@dasblinkenlight虽然这是真的，但问题仍然是重复的。

guard let categories = categories else {
   //Handle a nil categories array here
   return
}