Rounding Number to 2 Decimal Places in C

Question

How can I round a float (such as 37.777779) to two decimal places (37.78) in C?

Answer 1

printf("%.2f", 37.777779);

Answer 2

printf("%.2f", 37.777779);

If you want to write to C-string:

char number[24]; // dummy size, you should take care of the size!
sprintf(number, "%.2f", 37.777779);

Answer 3

There isn't a way to round a float to another float because the rounded float may not be representable (a limitation of floating-point numbers). For instance, say you round 37.777779 to 37.78, but the nearest representable number is 37.781.

However, you can "round" a float by using a format string function.

Answer 4

Assuming you're talking about round the value for printing, then Andrew Coleson and AraK's answer are correct:

printf("%.2f", 37.777779);

But note that if you're aiming to round the number to exactly 37.78 for internal use (eg to compare against another value), then this isn't possible, due to the way floating point numbers work.

See the link in Greg Hewgill's answer to a related question, which also covers why you shouldn't use floating point for financial calculations.

Answer 5

You can still use:

float ceilf(float x); // don't forget #include <math.h> and link with -lm.

example:

float valueToRound = 37.777779;
float roundedValue = ceilf(valueToRound * 100) / 100;

Answer 6

How about this:

float value = 37.777779;
float rounded = ((int)(value * 100 + .5) / 100.0);

Answer 7

If you just want to round the number for output purposes, then the "%.2f" format string is indeed the correct answer. However, if you actually want to round the floating point value for further computation, something like the following works:

#include <math.h>
/* ... */
float val = 37.777779;

float rounded_down = floorf(val * 100) / 100;
float nearest = floorf(val * 100 +  0.5) / 100;
float rounded_up = ceilf(val * 100) / 100;

Notice that are three different rounding rules you might want to choose: round down (ie, truncate after two decimal places), rounded to nearest, and round up. Usually, you want round to nearest. In this case, the three values are 37.77, 37.78, and 37.78, respectively.

As several others have pointed out, due to the quirks of floating point representation, these rounded values may not be exactly the "obvious" decimal values, but they will be very very close.