Padding a swift String for printing - swift

Question

I'm trying to print a list of Strings all padded to the same width.
In C, I would use something like printf("%40s", cstr), where cstr is a C string.
In Swift, the best I could come up is this:
line += String(format: "%40s",string.cStringUsingEncoding(<someEncoding>))
Is there a better way ?

Answer 1

In Swift 3 you can use:
let str = "Test string"
let paddedStr = str.padding(toLength: 20, withPad: " ", startingAt: 0)
Result string: "Test string "
If you need to pad to the left the text (right justify), you can write the following function as an extension to String:
extension String {
func leftPadding(toLength: Int, withPad character: Character) -> String {
let newLength = self.characters.count
if newLength < toLength {
return String(repeatElement(character, count: toLength - newLength)) + self
} else {
return self.substring(from: index(self.startIndex, offsetBy: newLength - toLength))
}
}
}
So if you write:
let str = "Test string"
let paddedStr = str.leftPadding(toLength: 20, withPad: " ")
Result string: " Test string"
In Swift 4.1 the substring method is deprecated and there are a number of new methods to obtain a substring. Either prefix, suffix or subscripting the String with a Range<String.Index>.
For the previous extension we can use the suffix method to accomplish the same result. Since the suffix method returns a String.SubSequence, it needs to be converted into a String before being returned.
extension String {
func leftPadding(toLength: Int, withPad character: Character) -> String {
let stringLength = self.count
if stringLength < toLength {
return String(repeatElement(character, count: toLength - stringLength)) + self
} else {
return String(self.suffix(toLength))
}
}
}

Answer 2

For Swift >= 3
line += string.padding(toLength: 40, withPad: " ", startingAt: 0)
For Swift < 3
NSString has the stringByPaddingToLength: method:
line += string.stringByPaddingToLength(40, withString: " ", startingAtIndex: 0)

Answer 3

extension RangeReplaceableCollection where Self: StringProtocol {
func paddingToLeft(upTo length: Int, using element: Element = " ") -> SubSequence {
return repeatElement(element, count: Swift.max(0, length-count)) + suffix(Swift.max(count, count-length))
}
}
"123".paddingToLeft(upTo: 5) // " 123"
"123".paddingToLeft(upTo: 5, using: "0") // "00123"
"123".paddingToLeft(upTo: 3, using: "0") // "123"
"$199.99".dropLast(3).paddingToLeft(upTo: 10, using: "_") // "______$199"
To replicate the same behaviour as padding(toLength:, withPad:, startingAt:) we can add rotateTo left functionality to RangeReplaceableCollection
extension RangeReplaceableCollection {
func rotatingLeft(positions: Int) -> SubSequence {
let index = self.index(startIndex, offsetBy: positions, limitedBy: endIndex) ?? endIndex
return self[index...] + self[..<index]
}
}
And implement it as follow:
extension RangeReplaceableCollection where Self: StringProtocol {
func paddingToLeft<S: StringProtocol & RangeReplaceableCollection>(upTo length: Int, with string: S, startingAt index: Int = 0) -> SubSequence {
let string = string.rotatingLeft(positions: index)
return repeatElement(string, count: length-count/string.count)
.joined().prefix(length-count) + suffix(Swift.max(count, count-length))
}
}
"123".paddingToLeft(upTo: 10, with: "abc", startingAt: 2) // "cabcabc123"
"123".padding(toLength: 10, withPad: "abc", startingAt: 2) // "123cabcabc"

Answer 4

Put all string-format-code into extension and reuse it wherever you want.
extension String {
func padding(length: Int) -> String {
return self.stringByPaddingToLength(length, withString: " ", startingAtIndex: 0)
}
func padding(length: Int, paddingString: String) -> String {
return self.stringByPaddingToLength(length, withString: paddingString, startingAtIndex: 0)
}
}
var str = "str"
print(str.padding(10)) // "str "
print(str.padding(10, paddingString: "+")) // "str+++++++"

Answer 5

The following two functions return a string padded to the given width, either left or right justified. It is pure Swift 4, no NSString, and no C string either. You may choose if a string longer than the padding width will be truncated or not.
extension String {
func rightJustified(width: Int, truncate: Bool = false) -> String {
guard width > count else {
return truncate ? String(suffix(width)) : self
}
return String(repeating: " ", count: width - count) + self
}
func leftJustified(width: Int, truncate: Bool = false) -> String {
guard width > count else {
return truncate ? String(prefix(width)) : self
}
return self + String(repeating: " ", count: width - count)
}
}

Answer 6

For left padding, you can use double-reverse trick:
String(String(s.reversed()).padding(toLength: 5, withPad: "0", startingAt: 0).reversed())
Of course, you can wrap it as an extension:
extension String {
func leftPadding(toLength: Int, withPad: String) -> String {
String(String(reversed()).padding(toLength: toLength, withPad: withPad, startingAt: 0).reversed())
}
}

Answer 7

Here's my solution, specific to String, but I'm sure someone smarter than I could make it more generic.
extension String {
func frontPadding(toLength length: Int, withPad pad: String, startingAt index: Int) -> String {
return String(String(self.reversed()).padding(toLength: length, withPad: pad, startingAt: index).reversed())
}
}

Answer 8

Swift 5:
I just spent an embarrassing amount of time playing with this same problem.
let short = "ab"
String(repeating: "0", count: 8 - short.count).appending(short)
// 000000ab

Answer 9

import Foundation // for NSString.padding()
/**
* Custom Extension's API
* ------------------------------
* • str.padEnd(_:_:)
* • str.padStart(_:_:)
* ------------------------------
* • int.padStart(_:_:forceSign:)
*/
extension String {
// str.padEnd(8, "-")
func padEnd(_ length: Int, _ pad: String) -> String {
return padding(toLength: length, withPad: pad, startingAt: 0)
}
// str.padStart(8, "*")
func padStart(_ length: Int, _ pad: String) -> String {
let str = String(self.reversed())
return String(str.padEnd(length, pad).reversed())
}
}
extension Int {
// int.padStart(8)
func padStart(
_ length: Int, // total length
_ pad: String = "0", // pad character
forceSign: Bool = false // force + sign if positive
) -> String {
let isNegative = self < 0
let n = abs(self)
let str = String(n).padStart(length, pad)
return
isNegative ? "- " + str :
forceSign ? "+ " + str :
str
}
}
// test run
let s1 = "abc"
[
s1.padEnd(15, "*"), // abc************
s1.padStart(15, "*"), // ************abc
3.padStart(8, forceSign: true), // + 00000003
(-125).padStart(8) // - 00000125
].forEach{ print($0) }

Answer 10

left padding is here
extension String {
func leftPadding(toLength: Int, withPad character: Character) -> String {
if count < toLength {
return String(repeating: character, count: toLength - count) + self
} else {
return self
}
}
}
result
"234".leftPadding(toLength: 1, withPad: "0") // 234
"234".leftPadding(toLength: 2, withPad: "0") // 234
"234".leftPadding(toLength: 3, withPad: "0") // 234
"234".leftPadding(toLength: 4, withPad: "0") // 0234
"234".leftPadding(toLength: 5, withPad: "0") // 00234