苹果Swift中的For Loop

Question

Apple's newly released language Swift has an example on the official documentation . Example is like this;

let interestingNumbers = [
    "Prime": [2, 3, 5, 7, 11, 13],
    "Fibonacci": [1, 1, 2, 3, 5, 8],
    "Square": [1, 4, 9, 16, 25],
]
var largest = 0
for (kind, numbers) in interestingNumbers {
    for number in numbers {
        if number > largest {
            largest = number
        }
    }
}
largest

This is pretty simple but as an extra exercise,it requires to add another variable in order to return what type is the largest number (ie Square is the case here)

However, I can't seem to figure out what is "(kind,numbers)" here represent and how should I make my for-loop to go through all Dictionary(interestingNumbers) keys and find which key has the largest number.

Thank you all for your help in advance

Answer 1

Swift allows you to loop over a dictionary with tuple-syntax (key, value) . So in every iteration of the for-loop Swift cares about reassigning the specified tuple-variables ( kind and number in your case) to the actual dictionary-record.

To figure out which Key includes the highest number in your example you can extend your code as follows:

let interestingNumbers = [
    "Prime": [2, 3, 5, 7, 11, 13],
    "Fibonacci": [1, 1, 2, 3, 5, 8],
    "Square": [1, 4, 9, 16, 25],
]

var largest = 0
var largestKey = ""
for (kind, numbers) in interestingNumbers {
    for number in numbers {
        if number > largest {
            largest = number
            largestKey = kind
        }
    }
}
largest     // =25
largestKey  // ="Square"

Or if you want to practice the tuple-syntax try that (with the same result):

var largest = 0
var largestKey = ""
for (kind, numbers) in interestingNumbers {
    for number in numbers {
        if number > largest {
            (largestKey, largest) = (kind, number)
        }
    }
}
largest     // =25
largestKey  // ="Square"

Answer 2

I can't seem to figure out what is "(kind,numbers)" here represents

It's a key-value pair (a tuple) containing the kind of the number. This syntax is called decomposition, basically, inside the loop you can access kind as the kind and numbers as the numbers that map for it.

For example, in some iteration:

kind  // "Prime"
numbers // [2, 3, 5, 7, 11, 13]

Quoting the guide:

You can also iterate over a dictionary to access its key-value pairs. Each item in the dictionary is returned as a (key, value) tuple when the dictionary is iterated, and you can decompose the (key, value) tuple's members as explicitly named constants for use within in the body of the for-in loop.

Answer 3

for (kind, numbers) in interestingNumbers{}

This for loop actually enumerating the key/value pairs of dictionary interestingNumbers . Where kind is the key and numbers is the correspoding value

kind:Prime       //Key
numbers: [2, 3, 5, 7, 11, 13] //Value  

Here the complete solution of the exercise

let interestingNumbers = [
    "Prime": [2, 3, 5, 7, 11, 13],
    "Fibonacci": [1, 1, 2, 3, 5, 8],
    "Square": [1, 4, 9, 16, 25],
]
var largest = 0
var type: String = ""
for (kind, numbers) in interestingNumbers {
    for number in numbers {
        if number > largest {
            largest = number
            type = kind
        }
    }
}
largest
type

Answer 4

However, I can't seem to figure out what is "(kind,numbers)" here represent

The loop iterates over the dictionary, and every iteration gives you a key and associated value. Those are called kind (key) and numbers (value) here. You can choose any name you want.

and how should I make my for-loop to go through all Dictionary(interestingNumbers) keys and find which key has the largest number.

You get each key in turn in the kind loop variable.

Once you find one that results in a new largest , you can assign that to a result variable, say largestKind .

At the end of the loop, largestKind will contain the key of the array with the largest number (that number being the largest you already have).

Answer 5

let interestingNumbers = [
    "Prime": [2, 3, 5, 7, 11, 13],
    "Fibonacci": [1, 1, 2, 3, 5, 8],
    "Square": [1, 4, 9, 16, 25],
]
var largest = 0
for (kind, numbers) in interestingNumbers {
    for number in numbers {
        if number > largest {
            largest = number
        }
    }
}
largest

This will return pair of (String,Int) which we have in Our Dictionary similar to function return multiple value as below,

   func refreshWebPage() -> (status:String,code:Int){
    //refresh logic
                return ("Success",200)
    }