Swift中的精确字符串格式说明符

Question

下面是我之前将浮点数截断为两位小数的方法

NSLog(@" %.02f %.02f %.02f", r, g, b);

我检查了文档和电子书,但还没弄清楚.谢谢!

Answer 1

一个简单的方法是:

import Foundation // required for String(format: _, _)

print(String(format: "hex string: %X", 123456))
print(String(format: "a float number: %.5f", 1.0321))

Answer 2

到目前为止,我的最佳解决方案是David的回应:

import Foundation

extension Int {
    func format(f: String) -> String {
        return String(format: "%\(f)d", self)
    }
}

extension Double {
    func format(f: String) -> String {
        return String(format: "%\(f)f", self)
    }
}

let someInt = 4, someIntFormat = "03"
println("The integer number \(someInt) formatted with \"\(someIntFormat)\" looks like \(someInt.format(someIntFormat))")
// The integer number 4 formatted with "03" looks like 004

let someDouble = 3.14159265359, someDoubleFormat = ".3"
println("The floating point number \(someDouble) formatted with \"\(someDoubleFormat)\" looks like \(someDouble.format(someDoubleFormat))")
// The floating point number 3.14159265359 formatted with ".3" looks like 3.142

我认为这是最像Swift的解决方案,将格式化操作直接绑定到数据类型.很可能某个地方有一个内置的格式化操作库,或者它很快就会发布.请注意,该语言仍处于测试阶段.

Answer 3

我发现String.localizedStringWithFormat工作得很好:

例:

let value: Float = 0.33333
let unit: String = "mph"

yourUILabel.text = String.localizedStringWithFormat("%.2f %@", value, unit)

Answer 4

这是一种非常快速和简单的方法,不需要复杂的解决方案.

let duration = String(format: "%.01f", 3.32323242)
// result = 3.3

Answer 5

这里的大多数答案都有效.但是,如果您经常格式化数字,请考虑扩展Float类以添加返回格式化字符串的方法.请参阅下面的示例代码 这个通过使用数字格式化器和扩展来实现相同的目标.

extension Float {
    func string(fractionDigits:Int) -> String {
        let formatter = NSNumberFormatter()
        formatter.minimumFractionDigits = fractionDigits
        formatter.maximumFractionDigits = fractionDigits
        return formatter.stringFromNumber(self) ?? "\(self)"
    }
}

let myVelocity:Float = 12.32982342034

println("The velocity is \(myVelocity.string(2))")
println("The velocity is \(myVelocity.string(1))")

控制台显示:

The velocity is 12.33
The velocity is 12.3

SWIFT 3.1更新

extension Float {
    func string(fractionDigits:Int) -> String {
        let formatter = NumberFormatter()
        formatter.minimumFractionDigits = fractionDigits
        formatter.maximumFractionDigits = fractionDigits
        return formatter.string(from: NSNumber(value: self)) ?? "\(self)"
    }
}

Answer 6

你不能用(或)字符串插值来做.你最好的选择仍然是NSString格式:

println(NSString(format:"%.2f", sqrt(2.0)))

从python推断,似乎合理的语法可能是:

@infix func % (value:Double, format:String) -> String {
    return NSString(format:format, value)
}

然后,您可以将它们用作:

M_PI % "%5.3f"                // "3.142"

您可以为所有数字类型定义类似的运算符,遗憾的是我还没有找到使用泛型的方法.

Answer 7

iOS 15+ 之后建议使用此解决方案：

2.31234.formatted(.number.precision(.fractionLength(1)))

Answer 8

import Foundation

extension CGFloat {
    var string1: String {
        return String(format: "%.1f", self)
    }
    var string2: String {
        return String(format: "%.2f", self)
    }
}

用法

let offset = CGPoint(1.23, 4.56)
print("offset: \(offset.x.string1) x \(offset.y.string1)")
// offset: 1.2 x 4.6

Answer 9

为什么要这么复杂？您可以使用此代替:

import UIKit

let PI = 3.14159265359

round( PI ) // 3.0 rounded to the nearest decimal
round( PI * 100 ) / 100 //3.14 rounded to the nearest hundredth
round( PI * 1000 ) / 1000 // 3.142 rounded to the nearest thousandth

看它在Playground工作.

PS:解决方案来自:http://rrike.sh/xcode/rounding-various-decimal-places-swift/

Answer 10

更优雅和通用的解决方案是重写ruby/python %运算符:

// Updated for beta 5
func %(format:String, args:[CVarArgType]) -> String {
    return NSString(format:format, arguments:getVaList(args))
}

"Hello %@, This is pi : %.2f" % ["World", M_PI]

Answer 11

斯威夫特4

let string = String(format: "%.2f", locale: Locale.current, arguments: 15.123)

Answer 12

细节

Xcode 9.3,Swift 4.1

解

(5.2).rounded()
// 5.0
(5.5).rounded()
// 6.0
(-5.2).rounded()
// -5.0
(-5.5).rounded()
// -6.0

用法

let x = 6.5

// Equivalent to the C 'round' function:
print(x.rounded(.toNearestOrAwayFromZero))
// Prints "7.0"

// Equivalent to the C 'trunc' function:
print(x.rounded(.towardZero))
// Prints "6.0"

// Equivalent to the C 'ceil' function:
print(x.rounded(.up))
// Prints "7.0"

// Equivalent to the C 'floor' function:
print(x.rounded(.down))
// Prints "6.0"

结果

Answer 13

您仍然可以在Objective-C中使用Swift中的NSLog,而不使用@符号.

NSLog("%.02f %.02f %.02f", r, g, b)

编辑:在使用Swift一段时间之后,我想添加这个变体

    var r=1.2
    var g=1.3
    var b=1.4
    NSLog("\(r) \(g) \(b)")

输出:

2014-12-07 21:00:42.128 MyApp[1626:60b] 1.2 1.3 1.4

Answer 14

extension Double {
  func formatWithDecimalPlaces(decimalPlaces: Int) -> Double {
     let formattedString = NSString(format: "%.\(decimalPlaces)f", self) as String
     return Double(formattedString)!
     }
 }

 1.3333.formatWithDecimalPlaces(2)