round double in objective c

Question

I have double number in a format like 34.123456789. How can I change it to 34.123?

I want just 3 digits after the decimal point.

Answer 1

You can use:

#include <math.h>
:
dbl = round (dbl * 1000.0) / 1000.0;

Just keep in mind that floats and doubles are as close an approximation as the underlying type (usually IEEE754) can provide. It may not be exact.

Answer 2

You can print it to 3 decimal places with [NSString stringWithFormat:@".3f",d].

You can approximately round it with round(d*1000)/1000, but of course this isn't guaranteed to be exact since 1000 isn't a power of 2.