了解 Swift 中的双精度和整数

Question

I thought I had a good understanding about Doubles and Ints until I accidentally ran into the following code.

To my surprise the following code works just fine.

let amounts = [50, 5.0, 10]
var total = 0.0

for i in 0..<amounts.count {
  total += amounts[i]

  print("Total: \(total)")
}

... but it stops working if I change the 5.0 to 10 or a 50 or even to 5 and generates the following error.

error: binary operator '+=' cannot be applied to operands of type 'Double' and 'Int'

Can someone tell me why is that the code doesn't break when mixing 50 , 5.0 and 10 ? I was under the impression that this wouldn't work.

Answer 1

As you know, Swift is very strict with types, but there's one area where it's not so strict - literals. Double conforms to ExpressibleByIntegerLiteral , so you could do:

let x: Double = 1 // "1" is "magically" converted to a Double!?

and have it compile. The same with arrays - the compiler thinks that the array literal that you have:

[50, 5.0, 10]

is a [Double] , because it can convert both 50 and 10 to Double . It can't be an [Int] because 5.0 can't be converted to an Int ( Int does not conform to ExpressibleByFloatLiteral )

The line:

total += amounts[i]

only works when both sides are of the same type. Note that here, the compiler will not try to convert from Int to Double because the expressions involved ( total and amounts[i] ) are not literals!

If you change the array literal to [50, 10, 10] , all elements are Int , so the compiler infers the array to be [Int] , and amount[i] becomes an Int , causing the line to fail compilation.

Answer 2

Arrays in Swift can hold elements of a single type, so when you mix 50, 5.0, 10 the compiler will infer the Array is of type Double

In the working code, the array is an array of [Doubles] when you change 5.0 to 10 the array is of [Int] that because of swift Type Inference.

Thus

Binary operator '+=' cannot be applied to operands of type 'Double' and 'Int'

Answer 3

Swift Compiler don't know operation about two different data-type like Int and Double.

So you can achieve this by type cast from Int to Double using this Code.

  let amounts = [50, 5, 10]
var total = 0.0

for i in 0..<amounts.count {
    total = total + Double(amounts[i])

  print("Total: \(total)")
}