Check if NSNumber is fraction

Question

What is the best way of checking if a NSNumber is a fraction?

NumberIsFraction(@(0)); // NO;
NumberIsFraction(@(0.5)); // YES;
NumberIsFraction(@(1.0)); // NO;

"Best" in terms of border case handling and performance.

Answer 1

Avoiding conversions to types with a smaller domain:

BOOL NumberIsFraction(NSNumber *number) {
    double dValue = [number doubleValue];
    if (dValue < 0.0)
        return (dValue != ceil(dValue));
    else
        return (dValue != floor(dValue));
}

Answer 2

The solutions from Jonathan, hpique, and prashant all have the same bug: they involve casting NSNumber to fixed-precision types (variously double, long long, etc.), which breaks if the value is a very large NSDecimalNumber. For example, the accepted answer fails on NSDecimalNumber(string: "1000000000000000.1").

A more correct implementation (as an extension, in Swift):

extension NSNumber {
    var isFraction: Bool {
        var decimal = decimalValue, decimalRounded = decimal
        NSDecimalRound(&decimalRounded, &decimal, 0, .down)
        return NSDecimalNumber(decimal: decimalRounded) != self
    }
}

Or in Objective-C, as a global function:

BOOL NumberIsFraction(NSNumber *number) {
    NSDecimal rounded = number.decimalValue;
    NSDecimalRound(&rounded, &rounded, 0, NSRoundDown);
    return ![number isEqualToNumber:
        [[NSDecimalNumber alloc] initWithDecimal:rounded]];
}