Round positive value half-up to 2 decimal places in C

Question

Typically, Rounding to 2 decimal places is very easy with

printf("%.2lf",<variable>);

However, the rounding system will usually rounds to the nearest even. For example,

2.554 -> 2.55
2.555 -> 2.56
2.565 -> 2.56
2.566 -> 2.57

And what I want to achieve is that

2.555 -> 2.56
2.565 -> 2.57

In fact, rounding half-up is doable in C, but for Integer only;

int a = (int)(b+0.5)

So, I'm asking for how to do the same thing as above with 2 decimal places on positive values instead of Integer to achieve what I said earlier for printing.

Answer 1

It is not clear whether you actually want to "round half-up", or rather "round half away from zero", which requires different treatment for negative values.

Single precision binary float is precise to at least 6 decimal places, and 20 for double, so nudging a FP value by DBL_EPSILON (defined in float.h) will cause a round-up to the next 100th by printf( "%.2lf", x ) for n.nn5 values. without affecting the displayed value for values not n.nn5

double x2  = x * (1 + DBL_EPSILON) ; // round half-away from zero
printf( "%.2lf", x2 ) ;

For different rounding behaviours:

double x2  = x * (1 - DBL_EPSILON) ;  // round half-toward zero
double x2  = x + DBL_EPSILON ;        // round half-up
double x2  = x - DBL_EPSILON ;        // round half-down