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?

PengOne · Answer 1 · 2011-12-13T19:46:09.630

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.

score 6 · Answer 2 · edited Dec 13 '11 at 19:48

6

How about

NSString *test1 = [NSString stringWithFormat:@"%.15f", d1];

Or simply go for the double as

NSString *test1 = [NSString stringWithFormat:@"%lf", d1];

edited Dec 13 '11 at 19:48

Richard J. Ross III

55,009
24
135
201

answered Dec 13 '11 at 19:39

A Salcedo

6,378
8
31
42

@PengOne I fixed that for him, converted it to `%lf`, long floating point. – Richard J. Ross III Dec 13 '11 at 19:48
@RichardJ.RossIII My understanding is that `%lf` is only necessary when `scan`ing. – PengOne Dec 13 '11 at 19:51
@PengOne actually, no, `%f` expects a 32-bit floating number, contrary to what the docs say, and `%lf` expects a 64-bit floating number. – Richard J. Ross III Dec 13 '11 at 19:52
%lf returns string "12.123457" – Shield Dec 13 '11 at 20:01
All you have to do is specify the number of decimals you want displayed by the .# before the type specifier. – A Salcedo Dec 13 '11 at 20:34

objective-c stringvalue from double

2 Answers2

Linked