How to restrict number of fraction digits when parsing number from string? - objective-c

Question

I want to restrict the number of fraction digits a user is allowed to enter into a UITextField that only accepts (localized) numeric input.
Example with 4 fraction digits allowed:
Good: 42, 10.123, 12345.2345
Bad: 0.123456, 6.54321
Right now, I'm using NSNumberFormatter's numberFromString: in the UITextField delegate's textField:shouldChangeCharactersInRange:replacementString: to determine whether it's a legal numeric value.
Unfortunately, NSNumberFormatter seems to ignore maximumFractionDigits in numberFromString:. In tests using getObjectValue:forString:range:error: I had the same problem, and range also was the full length of the string afterwards (unless I start entering letters; then range indicates only the part of the string with digits):
NSNumberFormatter* formatter = [[NSNumberFormatter alloc] init];
formatter.maximumFractionDigits = 3;
formatter.roundingMode = NSNumberFormatterRoundHalfUp;
formatter.generatesDecimalNumbers = YES;
NSDecimalNumber* n = (NSDecimalNumber*)[formatter numberFromString:#"10.12345"];
NSLog(#"Number: %#", n.description); // expected: 10.123, but is: 10.12345
How to best restrict the number of fraction digits in user input?

Answer 1

after you get the unrestricted number, you can use stringWithFormat on that number to create a string with a certain number of decimal places.
eg.
double number = myTextField.text.doubleValue;
NSString *restrictedString = [NSString stringWithFormat:#"%.4f", number];

Answer 2

There are a few ways to do this, but the easiest is probably to split the string into two parts (you will have to localize the '.') and check the length of the second part, like this:
- (BOOL)LNNumberIsValid:(NSString *)string
{
NSArray *numArray = [string componentsSeparatedByString:#"."];
if ([numArray count] == 2)
if ([[numArray objectAtIndex:1] length] > 4)
return NO;
return YES;
}
// Tests
NSLog(#"42: %i", [self LNNumberIsValid:#"42"]); // 1
NSLog(#"10.123: %i", [self LNNumberIsValid:#"10.123"]); // 1
NSLog(#"12345.2345: %i", [self LNNumberIsValid:#"12345.2345"]); // 1
NSLog(#"0.123456: %i", [self LNNumberIsValid:#"0.123456"]); // 0
NSLog(#"6.54321: %i", [self LNNumberIsValid:#"6.54321"]); // 0
EDIT:
The problem with the code that you added to your question is that you are printing the description of the NSDecimalNumber, which is not localized or limited to the number of digits. The NSDecimalNumber itself stores everything that you give it, so you need to change the original string (like my example above) if you want to change that. However, once you have your NSDecimalNumber, you can use the same number formatter to convert it back to a string in the format that you like:
NSNumberFormatter* formatter = [[NSNumberFormatter alloc] init];
formatter.maximumFractionDigits = 3;
formatter.roundingMode = NSNumberFormatterRoundHalfUp;
formatter.generatesDecimalNumbers = YES;
NSDecimalNumber* n = (NSDecimalNumber*)[formatter numberFromString:#"10.12345"];
NSString *s = [formatter stringFromNumber:n];
NSLog(#"Number: %#", s); // expected: 10.123, and is: 10.123

Answer 3

The way I solved this is by checking the position of the decimal separator and making sure that the insertion either is before that position or the insertion would not exceed the maximum number of fraction digits.
Also, I check that the input of a new separator does not occur at a place that would lead to more then the allowed fraction digits and that not more than one separators can be inserted
-(BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
NSString *separator = self.numberFormatter.decimalSeparator;
if(string.length == 0) {
// Empty String means deletion, always possible
return YES;
}
// Check for valid characters (0123456789 + decimal Separator)
for (int i = 0; i < [string length]; ++i) {
unichar c = [string characterAtIndex:i];
if (![self.legalCharSet characterIsMember:c])
{
return NO;
}
}
// Checks if input is separator
if([string isEqualToString:separator]) {
// Check that separator insertion would not lead to more than 2 fraction digits and that not more than one separators are inserted
// (the MIN() makes sure that length - kMaxFractionDigits won’t be below 0 as length and location are NSUIntegers)
return range.location >= self.valueField.text.length - MIN(self.valueField.text.length,kMaxFractionDigits) && [self.valueField.text containsString:separator] == NO;
} else {
// Check if a separator is already included in the string
NSRange separatorPos = [self.valueField.text rangeOfString: separator];
if(separatorPos.location != NSNotFound) {
// Make sure that either the input is before the decimal separator or that the fraction digits would not exceed the maximum fraction digits.
NSInteger fractionDigits = self.valueField.text.length - (separatorPos.location + 1);
return fractionDigits + string.length <= kMaxFractionDigits || range.location <= separatorPos.location;
}
}
return YES;
}
The method may not be bullet proof but it should be sufficient for common text insertions.