问题
I am trying to convert a given double into scientific notation, and running into some problems. I cant seem to find much documentation on how to do it either. Currently I am using:
var val = 500
var numberFormatter = NSNumberFormatter()
numberFormatter.numberStyle = NSNumberFormatterStyle.ScientificStyle
let number = numberFormatter.numberFromString("\(val)")
println(number as Double?)
// Prints optional(500) instead of optional(5e+2)
What am I doing wrong?
回答1:
You can set NumberFormatter properties positiveFormat and exponent Symbol to format your string as you want as follow:
let val = 500
let formatter = NumberFormatter()
formatter.numberStyle = .scientific
formatter.positiveFormat = "0.###E+0"
formatter.exponentSymbol = "e"
if let scientificFormatted = formatter.string(for: val) {
print(scientificFormatted) // "5e+2"
}
update: Xcode 9 • Swift 4
You can also create an extension to get a scientific formatted description from Numeric types as follow:
extension Formatter {
static let scientific: NumberFormatter = {
let formatter = NumberFormatter()
formatter.numberStyle = .scientific
formatter.positiveFormat = "0.###E+0"
formatter.exponentSymbol = "e"
return formatter
}()
}
extension Numeric {
var scientificFormatted: String {
return Formatter.scientific.string(for: self) ?? ""
}
}
print(500.scientificFormatted) // "5e+2"
回答2:
The issue is that you are printing the number... not the formatted number. You are calling numberForString
instead of stringForNumber
var val = 500
var numberFormatter = NSNumberFormatter()
numberFormatter.numberStyle = NSNumberFormatterStyle.ScientificStyle
let numberString = numberFormatter.stringFromNumber(val)
println(numberString)
回答3:
Slight modification to the answer by leo-dabus to Xcode 9 Swift 4:
extension Double {
struct Number {
static var formatter = NumberFormatter()
}
var scientificStyle: String {
Number.formatter.numberStyle = .scientific
Number.formatter.positiveFormat = "0.###E+0"
Number.formatter.exponentSymbol = "e"
let number = NSNumber(value: self)
return Number.formatter.string(from :number) ?? description
}
}
来源:https://stackoverflow.com/questions/31934088/convert-double-to-scientific-notation-in-swift