How to round a number to within a certain range?

Question

I have a value like this:

421.18834

And I have to round it mathematical correctly with a mask which can look like this:

0.05
0.04
0.1

For example, if the mask is 0.04, i have to get the value 421.20, because .18 is nearer at .20 than .16.

All functions that I found using Google didn't work.

Can you please help me?

Answer 1

double initial = 421.18834;
double range = 0.04;

int factor = Math.round(initial / range); // 10530 - will round to correct value

double result = factor * range; // 421.20

Answer 2

You don't need a special function. You multiply your original number by (1/mask), you round it to a decimal and you divide again by the same factor.

• Example with 0.05

  factor = 1/0.05 = 20
  421.18834 * 20 =  8423.7668
  int(  8423.7668 ) = 8424
  8424.0 / 20.0 = 421.20
  

• Example with 0.01

  factor = 1/0.1 = 10
  421.18834 * 10 = 4211.8834
  int( 4211.8834 ) = 4212
  4212.0 / 10.0 = 421.20
  

Answer 3

Both fredley and Matteo make the assumption that the rounding factor is itself a factor of 100. For factors like 0.06 or 0.07, this is an incorrect assumption.

Here's my Java routine:

public double rounded(double number, double factor) {
    long integer = (long) number;
    double fraction = number - integer;
    double multiple = (fraction / factor);
    multiple = Math.round(multiple);
    return factor * multiple + integer;
}