How to treat a double 3.14567 as a 3.14 (i.e with a given percision)

Question

Hi

I was wondering if there is any way in c++ to treat a double with a given precision. for example, the number 3.1345 will be considered as 3.13 and the number 0.009 will be considered as 0 (precision of 2 after the dot).

I need that it will be applied on mathematical operations. for example:

double a = 0.009;
double b = 3.12345

//a should be considered as 0, b as 3.12

double c = a*b // should output 0.
double d = a+b // should output 3.12

since the function setprecision() is for std i was wondering if there is any other function to do that.

thanks

Answer 1

That's not going to work correctly with doubles, ever. Doubles are binary fractions, they don't have decimals or decimal precision. Read The Floating-Point Guide for details.

If you need specific decimal behaviour, you'll have to drop doubles and use a decimal data type. You may find double-based solutions that appear to work, but you'll always hit cases where it won't.

Answer 2

double a = 3.12345; double b = ((int) a*100) / 100.0;

Answer 3

The easiest way:

int a100 = int(a*100);
int b100 = int(b*100);
int c100 = a100 * b100; // Will be 0
int d100 = a100 + b100; // Will be 312

Answer 4

have a look at floor() and ceil()

http://www.cplusplus.com/reference/clibrary/cmath/floor/

a=floor(a); b=floor(b*100)/100;