Difference Between String Interpolation and String Initializer in Swift

Difference between String interpolation and String initializer in Swift

String interpolation "\(item)" gives you the result of calling description on the item. String(item) calls a String initializer and returns a String value, which frequently is the same as the String you would get from string interpolation, but it is not guaranteed.

Consider the following contrived example:

class MyClass: CustomStringConvertible {
    var str: String

    var description: String { return "MyClass - \(str)" }

    init(str: String) {
        self.str = str
    }
}

extension String {
    init(_ myclass: MyClass) {
        self = myclass.str
    }
}

let mc = MyClass(str: "Hello")
String(mc)  // "Hello"
"\(mc)"     // "MyClass - Hello"

Difference between String interpolation and String concatenation

From a speed point of view, to differentiate concatenation (value1 + "-" + value2) and interpolation ("\(value1)-\(value2)"), results may depend on the number of operations done to obtain the final string.

My results on an iPhone 8 show that:

if there is roughly < 30 substrings to attach together, then concatenation is faster
if there is roughly > 30 substrings to attach together, then interpolation is faster

Thank you Sirens for figuring out that one wasn't always faster than the other!

Try it yourself (and don't forget to adapt the tested character set and iterations for your needs):

import UIKit

class ViewController: UIViewController {
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        DispatchQueue.global(qos: .default).async {
            ViewController.buildDataAndTest()
        }
    }

    private static func buildDataAndTest(times: Int = 1_000) {
        let characterSet = CharacterSet.alphanumerics
        characterSet.cacheAllCharacters()
        let data: [(String, String)] = (0 ..< times).map { _ in
             (characterSet.randomString(length: 50), characterSet.randomString(length: 20))
        }
        _ = testCIA(data)
        _ = testInterpol(data)
        print("concatenation: " + String(resultConcatenation))
        print("interpolation: \(resultInterpolation)")
    }

    /// concatenation in array
    static var resultConcatenation: CFTimeInterval = 0
    private static func testCIA(_ array: [(String, String)]) -> String {
        var foo = ""
        let start = CACurrentMediaTime()
        for (a, b) in array {
            foo = foo + " " + a + "+" + b
        }
        resultConcatenation = CACurrentMediaTime() - start
        return foo
    }

    /// interpolation
    static var resultInterpolation: CFTimeInterval = 0
    private static func testInterpol(_ array: [(String, String)]) -> String {
        var foo = ""
        let start = CACurrentMediaTime()
        for (a, b) in array {
            foo = "\(foo) \(a)+\(b)"
        }
        resultInterpolation = CACurrentMediaTime() - start
        return foo
    }
}

extension CharacterSet {
    static var cachedCharacters: [Character] = []

    public func cacheAllCharacters() {
        CharacterSet.cachedCharacters = characters()
    }

    /// extracting characters
    /// https://stackoverflow.com/a/52133647/1033581
    public func characters() -> [Character] {
        return codePoints().compactMap { UnicodeScalar($0) }.map { Character($0) }
    }
    public func codePoints() -> [Int] {
        var result: [Int] = []
        var plane = 0
        for (i, w) in bitmapRepresentation.enumerated() {
            let k = i % 8193
            if k == 8192 {
                plane = Int(w) << 13
                continue
            }
            let base = (plane + k) << 3
            for j in 0 ..< 8 where w & 1 << j != 0 {
                result.append(base + j)
            }
        }
        return result
    }

    // http://stackoverflow.com/a/42895178/1033581
    public func randomString(length: Int) -> String {
        let charArray = CharacterSet.cachedCharacters
        let charArrayCount = UInt32(charArray.count)
        var randomString = ""
        for _ in 0 ..< length {
            randomString += String(charArray[Int(arc4random_uniform(charArrayCount))])
        }
        return randomString
    }
}

Using stringInterpolationSegment string initializer in swift

From https://developer.apple.com/reference/swift/string/1539185-init, Apple says:

Creates a string containing the given value’s textual representation.
Do not call this initializer directly. It is used by the compiler when interpreting string interpolations.

(emphasis mine)

And they show you an example in https://developer.apple.com/reference/swift/stringinterpolationconvertible, where we see that indeed we should use String interpolation with "\()".

What are the supported Swift String format specifiers?

The format specifiers for String formatting in Swift are the same as those in Objective-C NSString format, itself identical to those for CFString format and are buried deep in the archives of Apple Documentation (same content for both pages, both originally from year 2002 or older):

https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFStrings/formatSpecifiers.html
https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/Strings/Articles/formatSpecifiers.html

But this documentation page itself is incomplete, for instance the flags, the precision specifiers and the width specifiers aren't mentioned. Actually, it claims to follow IEEE printf specifications (Issue 6, 2004 Edition), itself aligned with the ISO C standard. So those specifiers should be identical to what we have with C printf, with the addition of the %@ specifier for Objective-C objects, and the addition of the poorly documented %D, %U, %O specifiers and q length modifier.

Specifiers

Each conversion specification is introduced by the '%' character or by the character sequence "%n$".

n is the index of the parameter, like in:

String(format: "%2$@ %1$@", "world", "Hello")

Format Specifiers

%@ Objective-C object, printed as the string returned by descriptionWithLocale: if available, or description otherwise.

Actually, you may also use some Swift types, but they must be defined inside the standard library in order to conform to the CVarArg protocol, and I believe they need to support bridging to Objective-C objects: https://developer.apple.com/documentation/foundation/object_runtime/classes_bridged_to_swift_standard_library_value_types.

String(format: "%@", ["Hello", "world"])

%% '%' character.

String(format: "100%% %@", true.description)

%d, %i Signed 32-bit integer (int).

String(format: "from %d to %d", Int32.min, Int32.max)

%u, %U, %D Unsigned 32-bit integer (unsigned int).

String(format: "from %u to %u", UInt32.min, UInt32.max)

%x Unsigned 32-bit integer (unsigned int), printed in hexadecimal using the digits 0–9 and lowercase a–f.

String(format: "from %x to %x", UInt32.min, UInt32.max)

%X Unsigned 32-bit integer (unsigned int), printed in hexadecimal using the digits 0–9 and uppercase A–F.

String(format: "from %X to %X", UInt32.min, UInt32.max)

%o, %O Unsigned 32-bit integer (unsigned int), printed in octal.

String(format: "from %o to %o", UInt32.min, UInt32.max)

%f 64-bit floating-point number (double), printed in decimal notation. Produces "inf", "infinity", or "nan".

String(format: "from %f to %f", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%F 64-bit floating-point number (double), printed in decimal notation. Produces "INF", "INFINITY", or "NAN".

String(format: "from %F to %F", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%e 64-bit floating-point number (double), printed in scientific notation using a lowercase e to introduce the exponent.

String(format: "from %e to %e", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%E 64-bit floating-point number (double), printed in scientific notation using an uppercase E to introduce the exponent.

String(format: "from %E to %E", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%g 64-bit floating-point number (double), printed in the style of %e if the exponent is less than –4 or greater than or equal to the precision, in the style of %f otherwise.

String(format: "from %g to %g", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%G 64-bit floating-point number (double), printed in the style of %E if the exponent is less than –4 or greater than or equal to the precision, in the style of %f otherwise.

String(format: "from %G to %G", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%c 8-bit unsigned character (unsigned char).

String(format: "from %c to %c", "a".utf8.first!, "z".utf8.first!)

%C 16-bit UTF-16 code unit (unichar).

String(format: "from %C to %C", "爱".utf16.first!, "终".utf16.first!)

%s Null-terminated array of 8-bit unsigned characters.

"Hello world".withCString {
    String(format: "%s", $0)
}

%S Null-terminated array of 16-bit UTF-16 code units.

"Hello world".withCString(encodedAs: UTF16.self) {
    String(format: "%S", $0)
}

%p Void pointer (void *), printed in hexadecimal with the digits 0–9 and lowercase a–f, with a leading 0x.

var hello = "world"
withUnsafePointer(to: &hello) {
    String(format: "%p", $0)
}

%n The argument shall be a pointer to an integer into which is written the number of bytes written to the output so far by this call to one of the fprintf() functions.

The n format specifier seems unsupported in Swift 4+

%a 64-bit floating-point number (double), printed in scientific notation with a leading 0x and one hexadecimal digit before the decimal point using a lowercase p to introduce the exponent.

String(format: "from %a to %a", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

%A 64-bit floating-point number (double), printed in scientific notation with a leading 0X and one hexadecimal digit before the decimal point using a uppercase P to introduce the exponent.

String(format: "from %A to %A", Double.leastNonzeroMagnitude, Double.greatestFiniteMagnitude)

Flags

' The integer portion of the result of a
decimal conversion ( %i, %d, %u, %f, %F, %g, or %G ) shall be
formatted with thousands' grouping characters. For other conversions
the behavior is undefined. The non-monetary grouping character is
used.

The ' flag seems unsupported in Swift 4+

- The result of the conversion shall be left-justified within the field. The conversion is right-justified if
this flag is not specified.

String(format: "from %-12f to %-12d.", Double.leastNonzeroMagnitude, Int32.max)

+ The result of a signed conversion shall always begin with a sign ( '+' or '-' ). The conversion shall begin
with a sign only when a negative value is converted if this flag is
not specified.

String(format: "from %+f to %+d", Double.leastNonzeroMagnitude, Int32.max)

<space> If the first character of a signed
conversion is not a sign or if a signed conversion results in no
characters, a <space> shall be prefixed to the result. This means
that if the <space> and '+' flags both appear, the <space> flag
shall be ignored.

String(format: "from % d to % d.", Int32.min, Int32.max)

# Specifies that the value is to be converted to an alternative form. For o conversion, it increases the precision
(if necessary) to force the first digit of the result to be zero. For
x or X conversion specifiers, a non-zero result shall have 0x (or 0X)
prefixed to it. For a, A, e, E, f, F, g , and G conversion specifiers,
the result shall always contain a radix character, even if no digits
follow the radix character. Without this flag, a radix character
appears in the result of these conversions only if a digit follows it.
For g and G conversion specifiers, trailing zeros shall not be removed
from the result as they normally are. For other conversion specifiers,
the behavior is undefined.

String(format: "from %#a to %#x.", Double.leastNonzeroMagnitude, UInt32.max)

0 For d, i, o, u, x, X, a, A, e, E, f, F, g,
and G conversion specifiers, leading zeros (following any indication
of sign or base) are used to pad to the field width; no space padding
is performed. If the '0' and '-' flags both appear, the '0' flag is
ignored. For d, i, o, u, x, and X conversion specifiers, if a
precision is specified, the '0' flag is ignored. If the '0' and '"
flags both appear, the grouping characters are inserted before zero
padding. For other conversions, the behavior is undefined.

String(format: "from %012f to %012d.", Double.leastNonzeroMagnitude, Int32.max)

Width modifiers

If the converted value has fewer bytes than the field width, it shall be padded with spaces by default on the left; it shall be padded on the right if the left-adjustment flag ( '-' ) is given to the field width. The field width takes the form of an asterisk ( '*' ) or a decimal integer.

String(format: "from %12f to %*d.", Double.leastNonzeroMagnitude, 12, Int32.max)

Precision modifiers

An optional precision that gives the minimum number of digits to appear for the d, i, o, u, x, and X conversion specifiers; the number of digits to appear after the radix character for the a, A, e, E, f, and F conversion specifiers; the maximum number of significant digits for the g and G conversion specifiers; or the maximum number of bytes to be printed from a string in the s and S conversion specifiers. The precision takes the form of a period ( '.' ) followed either by an asterisk ( '*' ) or an optional decimal digit string, where a null digit string is treated as zero. If a precision appears with any other conversion specifier, the behavior is undefined.

String(format: "from %.12f to %.*d.", Double.leastNonzeroMagnitude, 12, Int32.max)

Length modifiers

h Length modifier specifying that a following
d, o, u, x, or X conversion specifier applies to a short or unsigned
short argument.

String(format: "from %hd to %hu", CShort.min, CUnsignedShort.max)

hh Length modifier specifying that a following
d, o, u, x, or X conversion specifier applies to a signed char or
unsigned char argument.

String(format: "from %hhd to %hhu", CChar.min, CUnsignedChar.max)

l Length modifier specifying that a following
d, o, u, x, or X conversion specifier applies to a long or unsigned
long argument.

String(format: "from %ld to %lu", CLong.min, CUnsignedLong.max)

ll, q Length modifiers specifying that a
following d, o, u, x, or X conversion specifier applies to a long long
or unsigned long long argument.

String(format: "from %lld to %llu", CLongLong.min, CUnsignedLongLong.max)

L Length modifier specifying that a following
a, A, e, E, f, F, g, or G conversion specifier applies to a long
double argument.

I wasn't able to pass a CLongDouble argument to format in Swift 4+

z Length modifier specifying that a following
d, o, u, x, or X conversion specifier applies to a size_t.

String(format: "from %zd to %zu", size_t.min, size_t.max)

t Length modifier specifying that a following
d, o, u, x, or X conversion specifier applies to a ptrdiff_t.

String(format: "from %td to %tu", ptrdiff_t.min, ptrdiff_t.max)

j Length modifier specifying that a following
d, o, u, x, or X conversion specifier applies to a intmax_t or
uintmax_t argument.

String(format: "from %jd to %ju", intmax_t.min, uintmax_t.max)

String Interpolation in Swift URL Initializer Throwing Error

Try building the string first then printing it to the console before you hand it off to your initializer. Does it look like a valid URL?

swift default string interpolation for object

The property description you have seen belongs to the protocol CustomStringConvertible and is used when you want to convert an object to a string representation. So all you need to do is to conform to the protocol and description will be used in your tests

struct Blah: CustomStringConvertible {
    let value: String

    init(_ value: String) {
        self.value = value
    }

    var description: String {
        value
    }
}

Difference Between String Interpolation and String Initializer in Swift