Pure Swift: A one-line method to add insert functionality to String instances (Xcode 6.3; update for Xcode 7, iOS 9)

This is the third blogpost focusing on writing pure swift methods and extensions. And here's a really simple one to add insert functionality to String instances:

extension String {
    func insert(string:String,ind:Int) -> String {
        return  prefix(self,ind) + string + suffix(self,count(self)-ind)
    }
}

"Hello World!".insert("Swift ", ind: 6)

Enjoy!

Update: 14 April 2015

I wrote this post in the early days of Swift and in fact there's a much easier way of inserting a string called splice().

var aString = "Hello World!"
aString.splice("Swift ", atIndex: advance(str.startIndex,6))

So while there are any number of ways that we can approach the insertion of a String without splice, for example using subscripting (inspired by a StackOverflow post):

extension String {
    mutating func insert(str:String, atIndex:String.Index) {
        self = self[Range(start: self.startIndex,end: atIndex)] + str + self[Range(start: atIndex,end: self.endIndex)]
    }
}

var hi = "Hello World!"
hi.insert("Swift ", ind: advance(hi.startIndex,6))

Or using loops, perhaps reversing the string first if we felt so inclined so that the insert keeps happening at the same index:

extension String {
    mutating func insert(str:String, atIndex:String.Index) {
        for c in reverse(str) {
            self.insert(c, atIndex: atIndex)
        }
    }
}

var hi = "Hello World!"
hi.insert("Swift ", ind: advance(hi.startIndex,6))

Or going for an even more traditional looping approach:

extension String {
    mutating func insert(string:String, atIndex:String.Index) {
        var index = string.startIndex
        for c in string {
            self.insert(c, atIndex: index)
            index = advance(index, 1)
        }
    }
}
var hi = "Hello World!"
hi.insert("Swift ", ind: advance(hi.startIndex,6))

All we really need is to become accustomed to using splice().

Update: 3 November 2015 (Xcode 7, iOS 9)

As identified by EL Houcine Az in the comments below, splice() is no longer available in Xcode 7, iOS 9. The code becomes instead:

aString.insertContentsOf("Swift ".characters, at: aString.startIndex.advancedBy(6))  // "Hello Swift World!"

And we can write the following string extension to simply our code:

extension String {
    mutating func insert(string:String,ind:Int) {
        self.insertContentsOf(string.characters, at: string.startIndex.advancedBy(ind))
    }
}

var aStr = "Hello World!"
aStr.insert("Swift ", ind: 6) // "Hello Swift World!"

which means it's goodbye splice(), hello insertContentsOf() as Apple moves away from the sleek and functional naming conventions of early Swift back to more descriptive ones à la Objective-C.

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