Objective-C: Double to NSString with x-many decimals - objective-c

Question

I know that I can cast a double to a NSString with a specific amount of decimals like so:
double myDouble = 123.456789;
NSString *myString = [NSString stringWithFormat:#"%.4g", myDouble];
But how can I replace the number "4" in this example with a int variable?
Something like this doesn't work:
double myDouble = 123.456789;
int precision = 4;
NSString *myString = [NSString stringWithFormat:#"%.%dg", myDouble, precision];

Answer 1

Consider using NSNumberFormatter:
double myDouble = 123.456789;
int precision = 4;
NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
formatter.minimumFractionDigits = precision;
formatter.maximumFractionDigits = precision;
NSString *myString = [formatter stringFromNumber:#(myDouble)];
Or, if you want this in scientific notation, you can alternatively specify something like:
formatter.numberStyle = NSNumberFormatterScientificStyle;
formatter.usesSignificantDigits = YES;
formatter.minimumSignificantDigits = precision;
formatter.maximumSignificantDigits = precision;
It just depends upon what precisely you're looking for.
This also has the virtue of also honoring the user's regional settings (e.g. if in Germany, the decimal separator is a comma, not a period).
If you need to force the locale setting (e.g. this is for creating something that will be exchanged with a web service that expects the data in a specified format), you can set the formatter's locale (e.g. [NSLocale localeWithLocaleIdentifier:#"en_US"]). But when presenting results in the user interface, you always want to honor the device's locale settings.

Answer 2

You can use asterisk in place of optional width and precision specifiers. And set them as arguments
double myDouble = 123.456789;
int width = 10;
int precision = 6;
NSString *s = [NSString stringWithFormat:#"%*.*g", width, precision, myDouble];

Answer 3

If you follow the links in the documentation for stringWithFormat you will discover the IEEE printf specification which describes the formats supported. That tells you that a field width or precision can be an * to indicate the actual value is supplied an an int argument, so what you want is:
NSString *myString = [NSString stringWithFormat:#"%.*g", intPrecision, myDouble];

Answer 4

double myDouble = 123.456789;
int precision = 4;
NSString *myString = [NSString stringWithFormat:#"%.*f", precision, myDouble];
NSLog(#"myString: '%#'", myString);
NSLog output:
myString: '123.4567'

Answer 5

This is by no means an optimal way of doing it, but it is a way.
You can do it in multiple steps
NSString *val = [NSString stringWithFormat: #"%d", 4];
NSString *head = [#"%." stringByAppendingString: val];
NSString *format = [head stringByAppendingString: #"g"];
NSString *result = [NSString stringWithFormat: format, 123.456789];
Or if you're really adventurous (for the sake of readability, don't EVER do this):
NSString *result = [NSString stringWithFormat: [[#"%." stringByAppendingString: [NSString stringWithFormat: #"%d", 4]] stringByAppendingString: #"g"], 123.456789];