How to check if input is double or string in Objective-C?

Question

How do you check if a given input is a string or a double? I tried doing [self.display.text doubleValue] and if that's not a valid double, it returns 0, but the problem is that 0 is actually a valid input for an actual double, so the program won't know if it's the default error fallback or an actual valid input.

How do you go around this?

@Eimantas So just use `NSPredicate` with a regex such as: `^[0-9](\.[0-9]+)?$` and test against it? — swiftcode, Jul 06 '12 at 14:31
Yes, but I'd use simpler POSIX `[:number:]` template to check if it matches a number. — Eimantas, Jul 06 '12 at 14:49

score 2 · Answer 1 · edited May 23 '17 at 12:12

2

-[NSFormatter getObjectValue:forString:range:error:] does what you want.

More details here.

edited May 23 '17 at 12:12

Community

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answered Jul 06 '12 at 15:46

N_A

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trojanfoe · Answer 2 · 2012-07-06T15:04:58.463

1

Do it the 'C' way using strtod (reference) as that provides you with the last character parsed:

NSString *input = @"123.456";
char *endptr;
double value = strtod([input UTF8String], &endptr);
if (*endptr != '\0')
{
    NSLog(@"input is a string");
}

edited Jul 06 '12 at 15:04

answered Jul 06 '12 at 14:48

trojanfoe

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score 1 · Accepted Answer · answered Jul 06 '12 at 15:43

You might use Regular Expressions and NSPredicate to test if it is a double value.

NSPredicate *predicate;
predicate = [NSPredicate predicateWithFormat:@"SELF MATCHES '^[-+]?[0-9]*\.?[0-9]+$'"];
BOOL result = [predicate evaluateWithObject:@"yourdoublevaluehere"];

You might need to adjust the Regular expression as I wroteit off the top of my head.

How to check if input is double or string in Objective-C?

3 Answers3