Double initialized as Decimal returns number with trailing fractional digits

Question

sen-mik OP

Created Nov ’21

Replies 4

Boosts 0

Participants 3

Hello there!

I'm using Swift 5, you can check the code below in online playground: https://swiftfiddle.com/x3xli3a3avby7pf6tzuqwaebfa

In the example below you can see that a double is initialized as Decimal but instead of returning 0.94 it returns different number

let x:Double = 0.94
print(Decimal(x))
> 0.9399999999999997952 // expected: 0.94

At the same time if I try to do the same with numbers like 0.93 or 0.95 Decimal conversion returns exactly those numbers.

The problem it creates is that when similar operation is done with formatter it returns an unexpected number:

let formatter = NumberFormatter()
formatter.numberStyle = .decimal

let ynumber = formatter.number(from: y)
print("formatted number: ", ynumber)
print("formatted decimal value: ", ynumber?.decimalValue)
> formatted number:  Optional(0.94)
> formatted decimal value: Optional(0.9399999999999997952) // expected Optional(0.94)

Is there a way to make Decimal always return the same number as the value it is initialized with?

Thanks!

Answered by Claude31 in 696171022

This will work.

let formatter = NumberFormatter()
formatter.numberStyle = .decimal

let x : Double = 0.94
let string = formatter.string(for: x) ?? "?"
print("x as string =", string)

let num = try Decimal(string, format: .number)
print("As decimal =", num)

let numPlus10 = num + 10
print("numPlus10 =", numPlus10) // To show it is effectively a number, not a String

You get:

x as string = 0.94
As decimal = 0.94
numPlus10 = 10.94

Boost

Answer 1

MobileTen OP

Nov ’21

Hence the reason for visual formatters. Don't the Decimal type unless you really need to.

0

Answer 2

Claude31 OP

Nov ’21

This should work:

let x : Double = 0.94
print(Decimal(x))

let formatter = NumberFormatter()
formatter.numberStyle = .decimal

let string = formatter.string(for: x) ?? "?"
print(string)

giving:

0.9399999999999997952
0.94

That's for display. Of course, you should keep the raw value x if you need to reuse.

Note: how is y defined in

let ynumber = formatter.number(from: y)

But

let ynumber = formatter.number(from: string) ?? 0.0

gives:

0.9399999999999999

Because of precision of conversion.

0

Answer 3

MobileTen OP

Nov ’21

Hence the reason for visual formatters. Don't use the Decimal type unless you really need to.

var x:Double = 0.94
var result = ""
print(x, terminator: "", to: &result)
print(result) // 0.94

0

Answer 4

Claude31 OP

Nov ’21

Accepted Answer

This will work.

let formatter = NumberFormatter()
formatter.numberStyle = .decimal

let x : Double = 0.94
let string = formatter.string(for: x) ?? "?"
print("x as string =", string)

let num = try Decimal(string, format: .number)
print("As decimal =", num)

let numPlus10 = num + 10
print("numPlus10 =", numPlus10) // To show it is effectively a number, not a String

You get:

x as string = 0.94
As decimal = 0.94
numPlus10 = 10.94

1