cannot subscript a value of type 'inout' [String :Double] (aka inout Dictionary<String, Double>)

问题

How can if i try to add three menus in the dictionary i wont let me and throws an error when i try to force unwrap the 3rd item in the menu. However if i force unwrap two of them, i can get them sum of two

var menu = ["fish": 10.99, "chips": 5.99, "kebab": 6.99]
var totalCost = menu["fish"]! + menu["chips"]! + menu["kebab"]!
print("The total cost of the three items is \(totalCost)")

But when i tried it this way it worked

var menu = ["fish": 10.99, "chips": 5.99, "kebab": 6.99]
var totalCost = menu["fish"]! + menu["chips"]! 
var thisCost = totalCost + menu["kebab"]!
print("The total cost of the three items is \(thisCost)"

I am using swift 3. Could it be that can no longer be supported in swift 3?

回答1:

You can alway iterate through and add to total, much simpler that a line of long addition.

var totalCost: Double = 0
for each in menu {
    totalCost += each.value
}
print(totalCost)

来源：https://stackoverflow.com/questions/39913093/cannot-subscript-a-value-of-type-inout-string-double-aka-inout-dictionary