Is there an IOS framework for calculations with SI units - ios

Question

I am taking masses (as g, mg, µg, ng & kg) and volumes (as ml, µl & l) as input to a chemistry app.
Currently I convert all masses to grams and volumes to litres, save in core data and perform any calculations as doubles.
Finally results are then converted back into a meaningful unit
ie 0.000034litres is more useful expressed as 34µl for my customers
What is best practice for working between different units?

Answer 1

There may be a few libraries out there but what you are doing is specific so I doubt there is a specific "best practice".
You may want to investigate the properties of NSDouble, CGFloat however since they might be more suited across devices and give you more options them the primitive double.
There are no unit data types or many built in native converter functions. A number is a number its up to the programmer to give that number meaning in the app's context.

Answer 2

I found a suitable engineering formatter at https://github.com/dhoerl/EngineeringNotationFormatter
In addition I created a simple version:
-(NSString *)engineeringFormat:(double)value digits:(int)digits {
//calculate exponent in step of 3
int perMill = trunc(log10(value)/3);
if (value<1) {
perMill -= 1;
}
//calculate mantissa format range of 1 to 1000
double corrected = value;
while (corrected<1) {
corrected = corrected*1000;
}
while (corrected>=1000) {
corrected=corrected/1000;
}
//format number of significant digits
NSNumberFormatter *numberFormatDigits = [[NSNumberFormatter alloc] init];
numberFormatDigits.usesSignificantDigits = YES;
numberFormatDigits.maximumSignificantDigits = digits;
NSString *mantissa = [numberFormatDigits stringFromNumber:[NSNumber numberWithDouble:corrected]];
//select engineering notation prefix
NSArray *engSuffix = #[#"T",#"G",#"M",#"k",#"",#"m",#"µ",#"n",#"p"];
int index = 4 - perMill;
NSString *result;
if ((index > engSuffix.count-1) || (index<0)) {
result = #"Out of range";
} else {
result = [NSString stringWithFormat:#"%# %#",mantissa,[engSuffix objectAtIndex:index]];
}
return result;
}