# Problem with rounding using lrint

**URL:** <https://community.unix.com/t/problem-with-rounding-using-lrint/363389>\
**Category:** Programming\
**Created:** [November 4, 2016, 3:52pm UTC](https://community.unix.com/t/problem-with-rounding-using-lrint/363389 "2016-11-04T15:52:17Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![migurus](https://community.unix.com/letter_avatar/migurus/32/5_5575768a8748004e209b776fc1b2916d.png) [@migurus](https://community.unix.com/u/migurus)\
**Post date:** [November 4, 2016, 3:52pm UTC](https://community.unix.com/t/problem-with-rounding-using-lrint/363389/1 "2016-11-04T15:52:17Z")

</div>

I run into a situation when integer result of 81 \* 0.5 is 40 (expected 41) when using lrint() function.

When I use a simple +0.5 approach results are right.

The code is:

```nohighlight
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
 int main(int argc, char *argv[])
{
        const int X = 81;
        double prc = (argc == 2) ? atof(argv[1]) : 0.5;
        double dval = X * prc;
        long ival = lrint(dval);
        printf("%i * %.16f = %.16f IVAL = %li ", X, prc, dval, ival);
        ival = (X * prc) + 0.5;
        printf("USING +0.5 IVAL = %li\n", ival);
        return(0);
}
 

```

results are:

```nohighlight
~$ c 0.5
81 * 0.5000000000000000 = 40.5000000000000000 IVAL = 40 USING +0.5 IVAL = 41

 

```

Using gcc ver 5.4.0 on Linux, running on 2 years old Xeon processor, if that matters. I checked floating point rounding defaults in compiler and it is a regular "round to the nearest integer" rule.

My question is - should I abandon rint-family functions and use +0.5 approach?

---

<div class="post-metadata">

**Author:** ![Don\_Cragun](https://community.unix.com/user_avatar/community.unix.com/don_cragun/32/2682_2.png) [@Don\_Cragun](https://community.unix.com/u/Don_Cragun)\
**Post date:** [November 4, 2016, 5:29pm UTC](https://community.unix.com/t/problem-with-rounding-using-lrint/363389/2 "2016-11-04T17:29:41Z")

</div>

You need to determine what rounding rules you want to use for tie-breaking cases (see [Wikipedia's discussion on rounding](https://en.wikipedia.org/wiki/Rounding))  
and you need to realize that even in double precision floating point, the result of a floating point calculation is not always exact (even when computing a value using decimal calculations would be exact). For example, the `awk` program (which uses double precision floating point for its calculations):

```plaintext
printf '.1 .5\n.3 .5\n' | awk '{printf("%.40f\n", $1 * $2)}'

```

produces the output:

```plaintext
0.0500000000000000027755575615628913510591
0.1499999999999999944488848768742172978818

```

not the output you might expect:

```plaintext
0.0500000000000000000000000000000000000000
0.1500000000000000000000000000000000000000

```

Which shows two examples where decimal arithmetic produces exact results, but binary arithmetic produced one result that was a little bit high and one result that was a little bit low.

---

<div class="post-metadata">

**Author:** ![migurus](https://community.unix.com/letter_avatar/migurus/32/5_5575768a8748004e209b776fc1b2916d.png) [@migurus](https://community.unix.com/u/migurus)\
**Post date:** [November 4, 2016, 7:43pm UTC](https://community.unix.com/t/problem-with-rounding-using-lrint/363389/3 "2016-11-04T19:43:11Z")

</div>

Thanks.  
I'd love to hear what others use in C/C++, that is rint() family functions? home grown solutions?

---

<div class="post-metadata">

**Author:** ![jim\_mcnamara](https://community.unix.com/letter_avatar/jim_mcnamara/32/5_5575768a8748004e209b776fc1b2916d.png) [@jim\_mcnamara](https://community.unix.com/u/jim_mcnamara)\
**Post date:** [November 5, 2016, 12:15pm UTC](https://community.unix.com/t/problem-with-rounding-using-lrint/363389/4 "2016-11-05T12:15:29Z")

</div>

Normally you call `fegetround()` to determine which rounding mode of the four possible ones is active.

`fesetround()` sets the mode. Only the fpu supported modes are available on a particular box.

For complete details try:  
[The GNU C Library: Rounding](https://www.gnu.org/software/libc/manual/html_node/Rounding.html)