objective-c stringvalue from double

Question

I have the following code:

double d1 = 12.123456789012345;

NSString *test1 = [NSString stringWithFormat:@"%f", d1]; // string is: 12.123457

NSString *test1 = [NSString stringWithFormat:@"%g", d1]; // string is: 12.1235

How do I get a string value that is exactly the same as d1?

Answer 1

It may help you to take a look at Apple's guide to String Format Specifiers.

%f  64-bit floating-point number 
%g  64-bit floating-point number (double), printed in the style of %e if the exponent is less than –4 or greater than or equal to the precision, in the style of %f otherwise

Also read up on floating point (in)accuracy, and, of course What Every Computer Scientist Should Know About Floating-Point Arithmetic.

If you really want the string to match the double exactly, then use NSString to encode it and call doubleValue when you want the value. Also take a look at NSNumberFormatter.