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Working with strings in Swift

We’ve used strings in lots of the projects so far, and I’ve tried to introduce you to a handful of important properties and methods as we go. Here, though, I’m going to run through some of those, plus a few more, while also looking at how we can write extensions to make strings a little more useful.

First, there are methods for checking whether a string starts with or ends with a substring: hasPrefix() and hasSuffix(). They look like this:

let password = "12345"

We can add extension methods to String to extend the way prefixing and suffixing works:

extension String {
    // remove a prefix if it exists
    func deletingPrefix(_ prefix: String) -> String {
        guard self.hasPrefix(prefix) else { return self }
        return String(self.dropFirst(prefix.count))

    // remove a suffix if it exists
    func deletingSuffix(_ suffix: String) -> String {
        guard self.hasSuffix(suffix) else { return self }
        return String(self.dropLast(suffix.count))

That uses the dropFirst() and dropLast() method of String, which removes a certain number of letters from either end of the string.

We’ve used lowercased() and uppercased() in previous projects, but there’s also the capitalized property that gives the first letter of each word a capital letter. For example:

let weather = "it's going to rain"

That will print “It’s Going To Rain”.

We could add our own specialized capitalization that uppercases only the first letter in our string:

extension String {
    var capitalizedFirst: String {
        guard let firstLetter = self.first else { return "" }
        return firstLetter.uppercased() + self.dropFirst()

One thing you can’t see in that is an interesting subtlety of working with strings: individual letters in strings aren’t instances of String, but are instead instances of Character – a dedicated type for holding single-letters of a string.

So, that uppercased() method is actually a method on Character rather than String. However, where things get really interesting is that Character.uppercased() actually returns a string, not an uppercased Character. The reason is simple: language is complicated, and although many languages have one-to-one mappings between lowercase and uppercase characters, some do not.

For example, in English “a” maps to “A”, “b” to “B”, and so on, but in German “ß” becomes “SS” when uppercased. “SS” is clearly two separate letters, so uppercased() has no choice but to return a string.

One last useful method of strings is contains(), which returns true if it contains another string. So, this will return true:

let input = "Swift is like Objective-C without the C"

So, contains() takes a string parameter and returns true or false depending on whether that parameter exists in the string. Keep that in your head for a moment.

Now look at this code:

let languages = ["Python", "Ruby", "Swift"]

That will also return true, because arrays have a contains() method that returns true or false depending on whether they contain the element you were looking for.

Now for the part that confuses people – brace yourself!

We have an array of strings (["Python", "Ruby", "Swift"]) and we have an input string ("Swift is like Objective-C without the C"). How can we check whether any string in our array exists in our input string?

Well, we might start writing an extension on String like this:

extension String {
    func containsAny(of array: [String]) -> Bool {
        for item in array {
            if self.contains(item) {
                return true

        return false

We can now run our check like this:

input.containsAny(of: languages)

That certainly works, but it’s not elegant – and Swift has a better solution built right in.

You see, arrays have a second contains() method called contains(where:). This lets us provide a closure that accepts an element from the array as its only parameter and returns true or false depending on whatever condition we decide we want. This closure gets run on all the items in the array until one returns true, at which point it stops.

Now let’s put together the pieces:

  • When used with an array of strings, the contains(where:) method wants to call a closure that accepts a string and returns true or false.
  • The contains() method of String accepts a string as its parameter and returns true or false.
  • Swift massively blurs the lines between functions, methods, closures, and more.

So, what we can actually do is pass one function directly into the other, like this:

languages.contains(where: input.contains)

Don’t feel bad if you need to read that single line of code several times – it’s not easy! Let’s break it down.

contains(where:) will call its closure once for every element in the languages array until it finds one that returns true, at which point it stops.

In that code we’re passing input.contains as the closure that contains(where:) should run. This means Swift will call input.contains("Python") and get back false, then it will call input.contains("Ruby") and get back false again, and finally call input.contains("Swift") and get back true – then stop there.

So, because the contains() method of strings has the exact same signature that contains(where:) expects (take a string and return a Boolean), this works perfectly – do you see what I mean about how Swift blurs the lines between these things?

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