Empty Strings in Swift

How do you tell if a string is empty in Swift? That depends on what you mean by “empty”. You might mean a string with zero length, or maybe also an optional string that is nil. What about a “blank” string that only contains whitespace. Let’s see how to test for each of those conditions with Swift.

Using isEmpty

A Swift String is a collection of characters and the Collection protocol already has a test for an empty collection:

var isEmpty: Bool { get }

We have access to the source code for Collection.swift in the standard library so we can see what this does:

public var isEmpty: Bool {
  return startIndex == endIndex
}

If the startIndex and endIndex of the collection are the same the collection is empty. Using this for a String:

"Hello".isEmpty  // false
"".isEmpty       // true

Note: Use isEmpty rather than comparing count to zero which requires iterating over the entire string:

// Don't do this to test for empty
myString.count == 0

What about whitespace?

Sometimes I want to test not only for an empty string but for a blank string. For example, I want a test that also returns true for each of these strings:

" "        // space
"\t\r\n"   // tab, return, newline
"\u{00a0}" // Unicode non-breaking space
"\u{2002}" // Unicode en space
"\u{2003}" // Unicode em space

I’ve seen people do this by first trimming whitespace from the string and then testing for empty. With Swift 5, we can make use of character properties to directly test for whitespace. We could write the test like this:

func isBlank(_ string: String) -> Bool {
  for character in string {
    if !character.isWhitespace {
        return false
    }
  }
  return true
}

That works but a simpler way to test all elements in a sequence is to use allSatisfy. Rewriting as an extension of String:

extension String {
  var isBlank: Bool {
    return allSatisfy({ $0.isWhitespace })
  }
}

This is looking promising:

"Hello".isBlank        // false
"   Hello   ".isBlank  // false
"".isBlank             // true
" ".isBlank            // true
"\t\r\n".isBlank       // true
"\u{00a0}".isBlank     // true
"\u{2002}".isBlank     // true
"\u{2003}".isBlank     // true

What about optional strings?

We can extend the solution to allow for optional strings. Here’s an extension to Optional where the wrapped element is a String:

extension Optional where Wrapped == String {
  var isBlank: Bool {
    return self?.isBlank ?? true
  }
}

Using optional chaining with a default value we return true if the optional string is nil else we use the isBlank property we previously added to String to test as before. We can now also write:

var title: String? = nil
title.isBlank            // true
title = ""               
title.isBlank            // true
title = "  \t  "               
title.isBlank            // true
title = "Hello"
title.isBlank            // false

Testing for a “blank” string iterates over the string so don’t use it when isEmpty is all you need.

Never Miss A Post

Sign up to get my iOS posts and news direct to your inbox and I'll also send you a PDF of my WWDC Viewing Guide

    Unsubscribe at any time. See privacy policy.

    Archives Categories