Rounding function in C - c

Question

So i am trying to write a code which can allow me to round UP any number to 3 decimal places. My code for rounding up a number was like this :
for (rowIndex = 0; rowIndex < MAX_ROWS; rowIndex++)
{
for (columnIndex = 0; columnIndex < MAX_COLUMNS; columnIndex++)
{
printf("%.3f ", ceil(rawData[rowIndex][columnIndex] * 1000.0) / 1000.0);
}
}
But yesterday my teacher told us to use a code which has a structure like this:
float roundValue(float value, int decimalPlaces)
{
// Place rounding code here
return value;
}
i am not quite sure how to write the code in this format! I am a beginner in coding so this might be so silly.
UPDATE:
so i just read all the comments below and tried to write the code but still has a problem. my code is :
double roundValue(double value, int decimalPlaces)
{
value = roundf( value * pow(10, decimalPlaces)) / pow(10, decimalPlaces);
return value;
}
int main(void)
{
int rowIndex = 0;
int columnIndex = 0;
double rawData[MAX_ROWS][MAX_COLUMNS]; // 2-dimensional array to store our
raw data
double value = rawData[MAX_ROWS][MAX_COLUMNS];
int decimalPlaces = 3;
// Print out the roundup data array
printf(" --- ROUNDED DATA ---\n");
for (rowIndex = 0; rowIndex < MAX_ROWS; rowIndex++)
{
for (columnIndex = 0; columnIndex < MAX_COLUMNS; columnIndex++)
{
printf("%.3f ", roundValue(value, 3));
}
printf("\n");
}
return 0;
}
it gives me only 0 for all the numbers.

Answer 1

Based on this answer, you could use the roundf function found in math.h:
#include <stdio.h>
#include <math.h>
/* function that rounds a float to the specified number of decimals */
float roundValue(float value, int decimalPlaces)
{
value = roundf(value * pow(10, decimalPlaces)) / pow(10, decimalPlaces);
return value;
}
/*to see the results: */
int main()
{
float value = 12.34567;
printf("%f", roundValue(value, 3));
return 0;
}
Compilation/run:
$ gcc -lm main.c
$ ./a.out
12.346000

Answer 2

He just told you to write your code in a function that you can call in the main() function.
So instead of rewriting your code every time you need a round value you can use a function, you give it the number that you want to calculate the round value for, and it will give you the result so your code won't be repetitive

Answer 3

Essentially it can't be done. The problem is that 0.1, or 0.001, cannot be represented in floating point format exactly. So you can only round to the nearest representation rounded = floor(x * 1000 + 0.5)/1000.0. It's best to use the full accuracy of the double, then round at the last moment, for display.
printf("%.3g", x);
will achieve this for you. In combination with strtod it's also another technique for rounding.

Answer 4

.. to round UP any number to 3 decimal places.
my teacher told us to use a code ... like float roundValue(float value, int decimalPlaces)
Without going to higher precision, it is very difficult to meet OP's goal with the best answer for all value.
Rounding a floating pointer value a) up or b) to the nearest representable 0.001 (or 10-n) is usually done in steps.
1) Multiply by 10n
2) Round a) up or b) to nearest
3) Divide by 10n
float roundValue(float value, int decimalPlaces) {
// form the power of 10
assert(decimalPlaces >= 0 && decimalPlaces <= 9);
int power_of_10 = 1;
while (decimalPlaces-- > 0) power_of_10 *= 10;
double fpower_of_10 = power_of_10; // or just use `pow(10, decimalPlaces);
Scaling by a power of 10 introduces imprecision. This slight error is magnified in the rounding step. A simple work-around it to use higher precision math. Fortunately the coding goal started with a float value and double often has higher precision.
Scaling by a power-of 10 can cause overflow, yet that is not likely when value is float and the product is double which has a wider range.
double y = value * fpower_of_10;
// round
double rounded_y = ceil(y); // round up
// or
double rounded_y = round(y); // round to nearest
The quotient will rarely provide an exact multiple of 0.001 (or whatever power-of-10) but a floating point value that is near a multiple of 0.001.
y = rounded_y / fpower_of_10;
return y;
}
Usage follows. Recall that unless your floating point types use a FLT_RADIX == 10 (very rare these days, usually 2), The result with only be near the desired "number to n decimal places.". If done well, the result will be the nearest possible float/double.
printf("%f\n", roundValue(123.456789, 3));
printf("%.10f\n", roundValue(123.456789, 3)); // to see more
More: an easy way to avoid overflow issues if higher precision is not available or used is to recognize that great C floating-points values have no fractional part and need no rounding.
float roundValue(float value, int decimalPlaces) {
float int_ptr;
float frac_part = modff(value, &int_ptr);
if (frac_part == 0) return value;
double pow10 = pow(10, decimalPlaces);
return round(value * pow10)/pow10; // or ceil()
}
There are other small subtle issues not noted here. double rounding, NaN, rounding mode, round() vs. rint(), nearbyint().