What is the Swift equivalent of isEqualToString in Objective-C?

Question

I am trying to run the code below:

import UIKit

class LoginViewController: UIViewController {

@IBOutlet var username : UITextField = UITextField()
@IBOutlet var password : UITextField = UITextField()

@IBAction func loginButton(sender : AnyObject) {

    if username .isEqual("") || password.isEqual(""))
    {
        println("Sign in failed. Empty character")
    }
}

My previous code was in Objective-C, which was working fine:

 if([[self.username text] isEqualToString: @""] ||
    [[self.password text] isEqualToString: @""] ) {

I assume I cannot use isEqualToString in Swift.

Answer 1

One important point is that Swift's == on strings might not be equivalent to Objective-C's -isEqualToString:. The peculiarity lies in differences in how strings are represented between Swift and Objective-C.

Just look on this example:

let composed = "Ö" // U+00D6 LATIN CAPITAL LETTER O WITH DIAERESIS
let decomposed = composed.decomposedStringWithCanonicalMapping // (U+004F LATIN CAPITAL LETTER O) + (U+0308 COMBINING DIAERESIS)

composed.utf16.count // 1
decomposed.utf16.count // 2

let composedNSString = composed as NSString
let decomposedNSString = decomposed as NSString

decomposed == composed // true, Strings are equal
decomposedNSString == composedNSString // false, NSStrings are not

NSString's are represented as a sequence of UTF–16 code units (roughly read as an array of UTF-16 (fixed-width) code units). Whereas Swift Strings are conceptually sequences of "Characters", where "Character" is something that abstracts extended grapheme cluster (read Character = any amount of Unicode code points, usually something that the user sees as a character and text input cursor jumps around).

The next thing to mention is Unicode. There is a lot to write about it, but here we are interested in something called "canonical equivalence". Using Unicode code points, visually the same "character" can be encoded in more than one way. For example, "Á" can be represented as a precomposed "Á" or as decomposed A + ◌́ (that's why in example composed.utf16 and decomposed.utf16 had different lengths). A good thing to read on that is this great article.

-[NSString isEqualToString:], according to the documentation, compares NSStrings code unit by code unit, so:

[Á] != [A, ◌́]

Swift's String == compares characters by canonical equivalence.

[ [Á] ] == [ [A, ◌́] ]

In swift the above example will return true for Strings. That's why -[NSString isEqualToString:] is not equivalent to Swift's String ==. Equivalent pure Swift comparison could be done by comparing String's UTF-16 Views:

decomposed.utf16.elementsEqual(composed.utf16) // false, UTF-16 code units are not the same
decomposedNSString == composedNSString // false, UTF-16 code units are not the same
decomposedNSString.isEqual(to: composedNSString as String) // false, UTF-16 code units are not the same

Also, there is a difference between NSString == NSString and String == String in Swift. The NSString == will cause isEqual and UTF-16 code unit by code unit comparison, where as String == will use canonical equivalence:

decomposed == composed // true, Strings are equal
decomposed as NSString == composed as NSString // false, UTF-16 code units are not the same

And the whole example:

let composed = "Ö" // U+00D6 LATIN CAPITAL LETTER O WITH DIAERESIS
let decomposed = composed.decomposedStringWithCanonicalMapping // (U+004F LATIN CAPITAL LETTER O) + (U+0308 COMBINING DIAERESIS)

composed.utf16.count // 1
decomposed.utf16.count // 2

let composedNSString = composed as NSString
let decomposedNSString = decomposed as NSString

decomposed == composed // true, Strings are equal
decomposedNSString == composedNSString // false, NSStrings are not

decomposed.utf16.elementsEqual(composed.utf16) // false, UTF-16 code units are not the same
decomposedNSString == composedNSString // false, UTF-16 code units are not the same
decomposedNSString.isEqual(to: composedNSString as String) // false, UTF-16 code units are not the same

Answer 2

With Swift you don't need anymore to check the equality with isEqualToString

You can now use ==

Example:

let x = "hello"
let y = "hello"
let isEqual = (x == y)

now isEqual is true.

Answer 3

For the UITextField text comparison I am using below code and working fine for me, let me know if you find any error.

if(txtUsername.text.isEmpty || txtPassword.text.isEmpty)
{
    //Do some stuff
}
else if(txtUsername.text == "****" && txtPassword.text == "****")
{
    //Do some stuff
}

Answer 4

Actually, it feels like swift is trying to promote strings to be treated less like objects and more like values. However this doesn't mean under the hood swift doesn't treat strings as objects, as am sure you all noticed that you can still invoke methods on strings and use their properties.

For example:-

//example of calling method (String to Int conversion)
let intValue = ("12".toInt())
println("This is a intValue now \(intValue)")


//example of using properties (fetching uppercase value of string)
let caUpperValue = "ca".uppercaseString
println("This is the uppercase of ca \(caUpperValue)")

In objectC you could pass the reference to a string object through a variable, on top of calling methods on it, which pretty much establishes the fact that strings are pure objects.

Here is the catch when you try to look at String as objects, in swift you cannot pass a string object by reference through a variable. Swift will always pass a brand new copy of the string. Hence, strings are more commonly known as value types in swift. In fact, two string literals will not be identical (===). They are treated as two different copies.

let curious = ("ca" === "ca")
println("This will be false.. and the answer is..\(curious)")

As you can see we are starting to break aways from the conventional way of thinking of strings as objects and treating them more like values. Hence .isEqualToString which was treated as an identity operator for string objects is no more a valid as you can never get two identical string objects in Swift. You can only compare its value, or in other words check for equality(==).

 let NotSoCuriousAnyMore = ("ca" == "ca")
 println("This will be true.. and the answer is..\(NotSoCuriousAnyMore)")

This gets more interesting when you look at the mutability of string objects in swift. But thats for another question, another day. Something you should probably look into, cause its really interesting. :) Hope that clears up some confusion. Cheers!

Answer 5

I addition to @JJSaccolo answer, you can create custom equals method as new String extension like:

extension String {
     func isEqualToString(find: String) -> Bool {
        return String(format: self) == find
    }
}

And usage:

let a = "abc"
let b = "abc"

if a.isEqualToString(b) {
     println("Equals")
}

For sure original operator == might be better (works like in Javascript) but for me isEqual method gives some code clearness that we compare Strings

Hope it will help to someone,

Answer 6

In Swift the isEmpty function it will check if the string is empty.

 if username.isEmpty || password.isEmpty {
      println("Sign in failed. Empty character")
 }

Answer 7

Use == operator instead of isEqual

Comparing Strings

Swift provides three ways to compare String values: string equality, prefix equality, and suffix equality.

String Equality

Two String values are considered equal if they contain exactly the same characters in the same order:

let quotation = "We're a lot alike, you and I."
let sameQuotation = "We're a lot alike, you and I."
if quotation == sameQuotation {
    println("These two strings are considered equal")
}
// prints "These two strings are considered equal"
.
.
.

For more read official documentation of Swift (search Comparing Strings).

Answer 8

In Swift, the == operator is equivalent to Objective C's isEqual: method (it calls the isEqual method instead of just comparing pointers, and there's a new === method for testing that the pointers are the same), so you can just write this as:

if username == "" || password == ""
{
    println("Sign in failed. Empty character")
}