Having trouble using NSNumberFormatter for currency conversion in iOS - ios

Question

I have a UITextField that receives numeric input from the user in my application. The values from this textfield then get converted into currency format using NSNumberFormatter within my shouldChangeCharactersInRange delegate method. When I enter the number "12345678", the number gets correctly converted to $123456.78 (the numbers are entered one digit at a time, and up to this point, everything works smoothly). However, when I enter another digit after this (e.g. 9), rather than displaying "1234567.89", the number "1234567.88" is displayed. If I enter another number after that, a totally different numbers after this (I'm using the number key pad in the application to enter the numbers. Here is the code that I have:
NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
[formatter setNumberStyle:NSNumberFormatterCurrencyStyle];
modifiedValue = [formatter stringFromNumber:[NSNumber numberWithFloat:[modifiedValue floatValue]]];
textField.text = modifiedValue;
The line that causes this unusual conversion is this one:
modifiedValue = [formatter stringFromNumber:[NSNumber numberWithFloat:[modifiedValue floatValue]]];
Can anyone see why this is?

Answer 1

It's likely to be a rounding error when doing the string->float conversion. You shouldn't use floats when dealing with currency. You could use a NSDecimalNumber instead.
NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
[formatter setNumberStyle:NSNumberFormatterCurrencyStyle];
// Below 2 lines if converting from a "currency" string
NSNumber *modifiedNumber = [formatter numberFromString:modifiedValue]; // To convert from the currency string to a number object
NSDecimalNumber *decimal = [NSDecimalNumber decimalNumberWithDecimal:[modifiedNumber decimalValue]];
// OR the below line if converting from a non-currency string
NSDecimalNumber *decimal = [NSDecimalNumber decimalNumberWithString:modifiedValue];
modifiedValue = [formatter stringFromNumber:decimal]; // Convert the new decimal back to a currency string
You may also consider making the number formatter lenient - often helps with user entered data.
[formatter setLenient:YES];

Answer 2

When I'm running number conversions to currency, I usually run this code:
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string
{
NSString *text = _textField.text;
NSString *decimalSeperator = #".";
NSCharacterSet *charSet = nil;
NSString *numberChars = #"0123456789";
// the number formatter will only be instantiated once ...
static NSNumberFormatter *numberFormatter;
if (!numberFormatter)
{
[numberFormatter setLocale:[NSLocale currentLocale]];
numberFormatter = [[NSNumberFormatter alloc] init];
numberFormatter.numberStyle = NSNumberFormatterCurrencyStyle;
numberFormatter.maximumFractionDigits = 10;
numberFormatter.minimumFractionDigits = 0;
numberFormatter.decimalSeparator = decimalSeperator;
numberFormatter.usesGroupingSeparator = NO;
}
// create a character set of valid chars (numbers and optionally a decimal sign) ...
NSRange decimalRange = [text rangeOfString:decimalSeperator];
BOOL isDecimalNumber = (decimalRange.location != NSNotFound);
if (isDecimalNumber)
{
charSet = [NSCharacterSet characterSetWithCharactersInString:numberChars];
}
else
{
numberChars = [numberChars stringByAppendingString:decimalSeperator];
charSet = [NSCharacterSet characterSetWithCharactersInString:numberChars];
}
// remove amy characters from the string that are not a number or decimal sign ...
NSCharacterSet *invertedCharSet = [charSet invertedSet];
NSString *trimmedString = [string stringByTrimmingCharactersInSet:invertedCharSet];
text = [text stringByReplacingCharactersInRange:range withString:trimmedString];
// whenever a decimalSeperator is entered, we'll just update the textField.
// whenever other chars are entered, we'll calculate the new number and update the textField accordingly.
if ([string isEqualToString:decimalSeperator] == YES)
{
textField.text = text;
}
else
{
NSNumber *number = [numberFormatter numberFromString:text];
if (number == nil)
{
number = [NSNumber numberWithInt:0];
}
textField.text = isDecimalNumber ? text : [numberFormatter stringFromNumber:number];
}
return NO; // we return NO because we have manually edited the textField contents.
}
The link explaining this is Re-Apply currency formatting to a UITextField on a change event
Hope this works!