Leading zeros for Int in Swift

Question

I'd like to convert an Int in Swift to a String with leading zeros. For example consider this code:

for myInt in 1 ... 3 {
    print("\(myInt)")
}

Currently the result of it is:

1
2
3

But I want it to be:

01
02
03

Is there a clean way of doing this within the Swift standard libraries?

Solution 1

Assuming you want a field length of 2 with leading zeros you'd do this:

import Foundation

for myInt in 1 ... 3 {
    print(String(format: "%02d", myInt))
}

output:

01
02
03

This requires import Foundation so technically it is not a part of the Swift language but a capability provided by the Foundation framework. Note that both import UIKit and import Cocoa include Foundation so it isn't necessary to import it again if you've already imported Cocoa or UIKit.

---

The format string can specify the format of multiple items. For instance, if you are trying to format 3 hours, 15 minutes and 7 seconds into 03:15:07 you could do it like this:

let hours = 3
let minutes = 15
let seconds = 7
print(String(format: "%02d:%02d:%02d", hours, minutes, seconds))

output:

03:15:07

Solution 2

With Swift 5, you may choose one of the three examples shown below in order to solve your problem.

---

#1. Using String's init(format:_:) initializer

Foundation provides Swift String a init(format:_:) initializer. init(format:_:) has the following declaration:

init(format: String, _ arguments: CVarArg...)

Returns a String object initialized by using a given format string as a template into which the remaining argument values are substituted.

The following Playground code shows how to create a String formatted from Int with at least two integer digits by using init(format:_:):

import Foundation

let string0 = String(format: "%02d", 0) // returns "00"
let string1 = String(format: "%02d", 1) // returns "01"
let string2 = String(format: "%02d", 10) // returns "10"
let string3 = String(format: "%02d", 100) // returns "100"

---

#2. Using String's init(format:arguments:) initializer

Foundation provides Swift String a init(format:arguments:) initializer. init(format:arguments:) has the following declaration:

init(format: String, arguments: [CVarArg])

Returns a String object initialized by using a given format string as a template into which the remaining argument values are substituted according to the user’s default locale.

The following Playground code shows how to create a String formatted from Int with at least two integer digits by using init(format:arguments:):

import Foundation

let string0 = String(format: "%02d", arguments: [0]) // returns "00"
let string1 = String(format: "%02d", arguments: [1]) // returns "01"
let string2 = String(format: "%02d", arguments: [10]) // returns "10"
let string3 = String(format: "%02d", arguments: [100]) // returns "100"

---

#3. Using NumberFormatter

Foundation provides NumberFormatter. Apple states about it:

Instances of NSNumberFormatter format the textual representation of cells that contain NSNumber objects and convert textual representations of numeric values into NSNumber objects. The representation encompasses integers, floats, and doubles; floats and doubles can be formatted to a specified decimal position.

The following Playground code shows how to create a NumberFormatter that returns String? from a Int with at least two integer digits:

import Foundation

let formatter = NumberFormatter()
formatter.minimumIntegerDigits = 2

let optionalString0 = formatter.string(from: 0) // returns Optional("00")
let optionalString1 = formatter.string(from: 1) // returns Optional("01")
let optionalString2 = formatter.string(from: 10) // returns Optional("10")
let optionalString3 = formatter.string(from: 100) // returns Optional("100")

Solution 3

For left padding add a string extension like this:

Swift 5.0 +

extension String {
    func PadLeft( totalWidth: Int,byString:String) -> String {
    let toPad = totalWidth - self.count
    if toPad < 1 {
        return self
    }
    
    return "".padding(toLength: toPad, withPad: byString, startingAt: 0) + self
}
}

Using this method:

for myInt in 1...3 {
    print("\(myInt)".padLeft(totalWidth: 2, with: "0"))
}

Solution 4

Swift 3.0+

Left padding String extension similar to padding(toLength:withPad:startingAt:) in Foundation

extension String {
    func leftPadding(toLength: Int, withPad: String = " ") -> String {

        guard toLength > self.characters.count else { return self }

        let padding = String(repeating: withPad, count: toLength - self.characters.count)
        return padding + self
    }
}

Usage:

let s = String(123)
s.leftPadding(toLength: 8, withPad: "0") // "00000123"

Solution 5

Using Swift 5’s fancy new extendible interpolation:

extension DefaultStringInterpolation {
    mutating func appendInterpolation(pad value: Int, toWidth width: Int, using paddingCharacter: Character = "0") {
        appendInterpolation(String(format: "%\(paddingCharacter)\(width)d", value))
    }
}

let pieCount = 3
print("I ate \(pad: pieCount, toWidth: 3, using: "0") pies")  // => `I ate 003 pies`
print("I ate \(pad: 1205, toWidth: 3, using: "0") pies")  // => `I ate 1205 pies`

View more solutions

Comments

[Jeehut]

Although this is not part of Swift it actually looks very clean. I think there just isn't a Swift native way of doing this so this might be the closest for now. Thank you, vacawama. :)

[Jeehut]

I think your answer is right when you want to format numbers the same way at multiple places. As I didn't ask for that I've chosen vacawama's answer as the correct but. But thank you for the answer! :)

[Jeehut]

By the way: If I just wanted a single zero in front of my number I'd chose println("0\(myInt)") over your suggestion. That would use Swift native String class instead going through NSString formatting.

[Jose M Pan]

Is useful until you get to "10", hehe

[codecowboy]

How about two leading zeros for anything between 0 > 100 ?

[codecowboy]

@ImanouPetit. FYI, I used this with minimum number digits 3. If I don't explicitly unwrap i.e. let formattedNumber = formatter.stringFromNumber(counter)! then the strings contain Optional("001") so my code to dynamically choose an image path fails. Unwrapping with '!" solves the problem

[vacawama]

String(format: "%03d", myInt) will give you "000", "001", ... , "099", "100".

[frank]

Why don't use this String(format: "%02d:%02d", arguments:[1,2]) ?

[nhgrif]

This may or may not work as the user expects if the withPad argument passed is more than a single character.

[possen]

Like your repeatElement approach, see my answer for padding strings.

[Codetard]

These's issue if value like -3, -9 occurs, It still returns the same without a leading zero.

[vacawama]

@SatnamSync, indeed negative numbers need extra care because you need to make space for the - sign. You can extend the field width by 1 for negative numbers, or use NumberFormatter as described in part #3 of this answer.

[Codetard]

Yes, I implemented a negative integer check with if else statement. Your answer is very helpful. Thanks.

[Mike Glukhov]

why?! what is the advantage of this?

[cleverbit]

These will not work for two digit ints (eg. 10 becomes 010). Also the original question asked specifically for solutions involving Standard Libraries. The answer above by @ImanouPetit is preferred.

[Nick N]

extensions FTW..!! This also worked fine in Swift 4. For those of us struggling to move older code from 3, to 4, and eventually to Swift 5... :)