如何比较Swift中的范围？

Question

I'm not sure if it's a bug in XCode or I didn't understand ranges in Swift. Here is some code to show how range works:

let range = 0..<5
contains(range, 0) // true
contains(range, 5) // false
range.startIndex == 0 // true
range.endIndex == 5 // true

let str = "Hello, playground"
str.rangeOfString("Hello")! // 0..<5

Great! Now let's use it real code:

let str = "Hello, playground"
if let range = str.rangeOfString("Hello") {
    if range.startIndex == 0 {
        print("str starts with 'Hello'")
    }
}

I'm getting following error in line that reads if range.startIndex == 0 {

Cannot invoke '==' with an argument lis of type (String.index, IntegerLiteralConvertible)'

Answer 1

rangeOfString actually returns a range of String.Index s and not Int s; the two aren't comparable (even though the playground shows the range as 0..<5 ). You can read about the reasons for that in the answers to this question .

If you want your example to work then, instead of comparing range.startIndex with 0 , you should compare it with str.startIndex :

let str = "Hello, playground"
if let range = str.rangeOfString("Hello") {
    if range.startIndex == str.startIndex {
        print("str starts with 'Hello'")
    }
}

If you need to compare to the range's startIndex with, say, the second character index in the string, then you can use the advance function to increment str.startIndex by 1 and compare range.startIndex to that:

let str = "Hello, playground"
if let range = str.rangeOfString("e") {
    if range.startIndex == advance(str.startIndex, 1) {
        print("str's 2nd character is 'e'")
    }
}

If you need to know the length of the range, then you can use the distance function:

let str = "Hello, playground"
if let range = str.rangeOfString("play") {
    let length = distance(range.startIndex, range.endIndex)
    print("found a range \(length) characters long")
}

Answer 2

The ~= operator in Swift

Sometimes, we have to check if a number is between a range, and as usual, we do something like :

Check if number is between 0 and 100 include

if number >=0 && number <= 100 {  
   // TODO:
}

It works, but we can do better and swiftier. The Swift Standard library have an ~= operator, so we can do instead :

if 0...100 ~= number {  
   // TODO:
 }