NSNumberFormatter's inconsistent behaviour

Question

When I set the usesSignificantDigits property to true, the NSNumberFormatter won't obey the maximumFractionDigits. I think this also applies to minimumFractionDigits.

For example:

let formatter: NSNumberFormatter = NSNumberFormatter()
formatter.locale = NSLocale.currentLocale()
formatter.maximumFractionDigits = 2
formatter.allowsFloats = true
formatter.numberStyle = NSNumberFormatterStyle.DecimalStyle
formatter.usesSignificantDigits = true

let result = formatter.stringFromNumber(1.2345)

The above code returns "1.2345" as result. The intended result is "1.23". When I set the usesSignificantDigits to false, it works.

This is contradicting the statement in the developer reference for usesSignificantDigits:

Determines whether the receiver uses minimum and maximum significant digits properties.

What am I doing wrong?

Answer 1

The property usesSignificantDigits controls the use of minimumSignificantDigits and maximumSignificantDigits, not minimumFractionDigits and maximumFractionDigits. You did not set maximumSignificantDigits in your code so it will use its default value (which seems to be 6).

Consider this enhancement of your original code:

let formatter: NSNumberFormatter = NSNumberFormatter()
formatter.locale = NSLocale.currentLocale()
formatter.maximumFractionDigits = 2
formatter.allowsFloats = true
formatter.numberStyle = NSNumberFormatterStyle.DecimalStyle

// Makes formatter use maximumSignificantDigits
formatter.usesSignificantDigits = true

println(formatter.maximumSignificantDigits) // Prints 6

// Will use 6 significant digits, hence "1.2345"
let result = formatter.stringFromNumber(1.2345)

If you don't want to use the value of maximumSignificantDigits, you should not set usesSignificantDigits to true. This behavior matches the description in the official documentation.