Having trouble using NSNumberFormatter for currency conversion in 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!