creating a generalized rounding function in C - c

Question

Most the rounding questions on Stack Overflow are about specific cases and, as I just made a generic rounding function, I thought I would share it.
I have a number f, a starting value a, and an increment b, and I want to “round” f to the nearest element of the set {a + b•i | i is an integer}. For example:
a = 0.0, b = 0.5: round to one of the values 0.0, 0.5, 1.0, 1.5, etc.
a = 0.25, b = 0.5: round to one of the values 0.25, 0.75, 1.25, 1.75, etc.
a = 0.21, b = 0.23: round to one of the values 0.21, 0.44, 0.67, 0.80, etc.
The standard C library has roundf() but it only rounds to the nearest integer.
How do I go about doing this?

Answer 1

#include <stdio.h>
#include <math.h>
float round_float(float x, float inc, float start_val)
{
return ( roundf( (x - start_val) / inc ) * inc + start_val );
}
int main(void)
{
printf("%f\n", round_float(12.522, 0.5, 0));
printf("%f\n", round_float(12.522, -0.5, -0.1));
printf("%f\n", round_float(5.318, 0.23, 125));
printf("%f\n", round_float(-12.522, 12, 5));
printf("%f\n", round_float(-12.522, 3.6, -2));
}
output:
12.500000
12.400000
5.400002
-7.000000
-12.799999
first example: round_float(10.1521, 0.5, 0)
second example: round_float(10.1521, 0.5, 0.25)
final example: round_float(10.1521, 0.23, 0.21)
Extended example
This code shows how random sample values are rounded to the nearest entry in each of three sequences specified by the starting value and the increment — using the values outlined in the question.
#include <stdio.h>
#include <math.h>
static inline float round_float(float x, float inc, float start_val)
{
return roundf((x - start_val) / inc) * inc + start_val;
}
int main(void)
{
printf("%f\n", round_float(12.522, 0.5, 0));
printf("%f\n", round_float(12.522, -0.5, -0.1));
printf("%f\n", round_float(5.318, 0.23, 125));
printf("%f\n", round_float(-12.522, 12, 5));
printf("%f\n", round_float(-12.522, 3.6, -2));
static const float samples[] =
{
-14.2751, -12.3080, -10.5320, -6.4804, -1.0859,
0.1999, 0.2099, 5.2980, 5.7819, 11.7052,
};
enum { NUM_SAMPLES = sizeof(samples) / sizeof(samples[0]) };
static const float control[][2] =
{
{ 0.00, 0.50 },
{ 0.25, 0.50 },
{ 0.21, 0.23 },
};
enum { NUM_CONTROL = sizeof(control) / sizeof(control[0]) };
for (int i = 0; i < NUM_CONTROL; i++)
{
float a = control[i][0];
float b = control[i][1];
printf("Start: %8.4f; increment %8.4f\n", a, b);
for (int j = 0; j < NUM_SAMPLES; j++)
{
printf(" Sample: %8.4f rounds to %8.4f\n",
samples[j], round_float(samples[j], b, a));
}
}
return 0;
}
Example output:
12.500000
12.400000
5.400002
-7.000000
-12.799999
Start: 0.0000; increment 0.5000
Sample: -14.2751 rounds to -14.5000
Sample: -12.3080 rounds to -12.5000
Sample: -10.5320 rounds to -10.5000
Sample: -6.4804 rounds to -6.5000
Sample: -1.0859 rounds to -1.0000
Sample: 0.1999 rounds to 0.0000
Sample: 0.2099 rounds to 0.0000
Sample: 5.2980 rounds to 5.5000
Sample: 5.7819 rounds to 6.0000
Sample: 11.7052 rounds to 11.5000
Start: 0.2500; increment 0.5000
Sample: -14.2751 rounds to -14.2500
Sample: -12.3080 rounds to -12.2500
Sample: -10.5320 rounds to -10.7500
Sample: -6.4804 rounds to -6.2500
Sample: -1.0859 rounds to -1.2500
Sample: 0.1999 rounds to 0.2500
Sample: 0.2099 rounds to 0.2500
Sample: 5.2980 rounds to 5.2500
Sample: 5.7819 rounds to 5.7500
Sample: 11.7052 rounds to 11.7500
Start: 0.2100; increment 0.2300
Sample: -14.2751 rounds to -14.2800
Sample: -12.3080 rounds to -12.2100
Sample: -10.5320 rounds to -10.6000
Sample: -6.4804 rounds to -6.4600
Sample: -1.0859 rounds to -1.1700
Sample: 0.1999 rounds to 0.2100
Sample: 0.2099 rounds to 0.2100
Sample: 5.2980 rounds to 5.2700
Sample: 5.7819 rounds to 5.7300
Sample: 11.7052 rounds to 11.7100
I'm sure a more judicious choice of sample values would make the point even clearer; those are just a collection of random numbers in the range -20 .. +20 in sorted order.
Apologies for the extra level of indent in the last example.
The preview was not showing 'code' when it was not indented two levels, and I've no idea why. (Configuration: Firefox Quantum 57.0.1 (restart pending) on macOS High Sierra 10.13.2 and SO 2017.12.22.28257)