Using NSNumberFormatter to generate number with trailing zeros after decimal - ios

Question

In Swift, how would you create an NSNumberFormatter that would preserve trailing zeros after a decimal (12.000) while also generating a number appropriate for the current locale?
My current code and example output:
let formatter = NSNumberFormatter()
formatter.numberStyle = .DecimalStyle
formatter.locale = NSLocale.currentLocale()
formatter.maximumFractionDigits = 10
var doubleNumString = "12.0"
println(formatter.numberFromString(doubleNumString)) //in English it prints 12, want it to print 12.0
var doubleNumString = "12.000"
println(formatter.numberFromString(doubleNumString)) //prints 12, want it to print 12.000
var doubleNumString = "12.125"
println(formatter.numberFromString(doubleNumString)) //prints 12.125 as expected
var doubleNumString = "1234"
println(formatter.numberFromString(doubleNumString)) //prints 1,234 as expected
I've already coded it such that if the string ends in a decimal ("12.") then it won't use this formatter to generate the number and will instead just display the number then the decimal (but I will need to improve that because some languages read right to left).
One solution would be to check if the string contains a period and if so, check if all digits that follow it are 0, and if so then don't run it through the number formatter and instead run only the int value through the formatter then append/prepend the decimal followed by the appropriate number of 0's.
Is there a better/cleaner solution?

Answer 1

As mentioned by Martin R, you can set the minimumFractionDigits and maximumFractionDigits to the same number which will enforce that many fraction digits always be displayed. To know how many to display you need to take a substring after the decimal to the end and count its elements. To know whether or not all of the fraction digits are 0's, I created a helper method that converts that substring to a number and if it equals 0 then you know they were all 0's.
Unfortunately you need to convert the string to a localized number using a couple different NSNumberFormatters based on the original string number. So if it does contain a decimal and everything after it is a 0 then you need to create a different formatter, convert the string to a number, then convert that number to a string in order to display it respecting the user's locale. Otherwise you can just use your original number formatter.

Answer 2

This function takes care of your requirement. pass same for & from locale (e.g. en_US)
+ (NSString*) stringForString:(NSString*) string forLocale:(NSString*) toLocaleCode fromLocal:(NSString*) fromLocaleCode {
NSLocale *fromLocale = [[NSLocale alloc] initWithLocaleIdentifier:fromLocaleCode];
NSNumberFormatter *sourceFormatter = [[NSNumberFormatter alloc] init];
[sourceFormatter setNumberStyle:NSNumberFormatterDecimalStyle];
[sourceFormatter setUsesGroupingSeparator:NO];
[sourceFormatter setLocale:fromLocale];
NSNumber *localizedNumber = [sourceFormatter numberFromString:string];
if (!localizedNumber) {
return string;
}
NSLocale *toLocale = [[NSLocale alloc] initWithLocaleIdentifier:toLocaleCode];
NSNumberFormatter *destinationFormatter = [[NSNumberFormatter alloc] init];
[destinationFormatter setNumberStyle:NSNumberFormatterDecimalStyle];
[destinationFormatter setUsesGroupingSeparator:NO];
[destinationFormatter setLocale:toLocale];
NSString *localizedString = [destinationFormatter stringFromNumber:localizedNumber];
//add the zeros which were dropped because of the sourceDecimalString number conversion e.g. 0.20 is converted to 0.2
if (localizedString.length < string.length) {
NSRange rangeOfDecimal = [string rangeOfString:sourceFormatter.decimalSeparator];
if (rangeOfDecimal.location != NSNotFound) {
NSString* sourceDecimalString = [string substringFromIndex:rangeOfDecimal.location];
rangeOfDecimal = [localizedString rangeOfString:destinationFormatter.decimalSeparator];
if (rangeOfDecimal.location != NSNotFound) {
NSString* destinationDecimalString = [localizedString substringFromIndex:rangeOfDecimal.location];
if (destinationDecimalString.length < sourceDecimalString.length) {
int difference = sourceDecimalString.length - destinationDecimalString.length;
int toalDecimalDigits = (destinationDecimalString.length - 1) + difference; //-1 to remove '.'
destinationFormatter.minimumFractionDigits = toalDecimalDigits;
destinationFormatter.maximumFractionDigits = toalDecimalDigits;
localizedString = [destinationFormatter stringFromNumber:localizedNumber];
}
}
else{//this indicates no decimal separator in the return string
int toalDecimalDigits = (sourceDecimalString.length - 1); //-1 to remove '.'
destinationFormatter.minimumFractionDigits = toalDecimalDigits;
destinationFormatter.maximumFractionDigits = toalDecimalDigits;
localizedString = [destinationFormatter stringFromNumber:localizedNumber];
}
}
}
return localizedString;
}