问题
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?
回答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.
回答2:
How about
NSString *test1 = [NSString stringWithFormat:@"%.15f", d1];
Or simply go for the double as
NSString *test1 = [NSString stringWithFormat:@"%lf", d1];
来源：https://stackoverflow.com/questions/8495076/objective-c-stringvalue-from-double