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How do I round a number (Simple Java Calculator)


I have built a calculator that is pretty much complete but for the inability to round numbers

what I want to do is say input a number x (double) press the "round" key, enter another number(y double and converted to int)  and that number will be rounded to that number (y).


 9.4524 is entered (x)
 Press Round Button
 2 is entered (y)
 result = 9.45 (9.4524 to y decimal places)

I have been trying to implement this but with out success

Here is some "example Code"

public double Rounded(Double d, int i){
    BigDecimal rnd = new BigDecimal(d); /* the constructor is the only way I could find to convert */
    rnd.setScale(i);                                /* a double to BigDecimal format */
    return rnd.doubleValue();

Murray S

PS I have little in the way of knowlege re: BigDecimal so quick tutorial would be good
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1 Solution
There might be a better way, but this works :)

double round(double d, int i) {
   return ((int)((d + 5*Math.pow(10,-(i+1))) * Math.pow(10,i))/Math.pow(10,i));
Murray_SAuthor Commented:
Yep this works

But could you explain it to me please, how did you come up with this algorithm &c.

basically, it does this:

take the following number:

If you want 2 decimal places, you can multiply the number by 100 to get:

now if you cast that to an integer, you will get

Last of all, wel divide by 100 to get

That gives us the original number to two decimal places.  You also notice that first we add .005 to the number we are going to round, that way it rounds to the nearest integer when you cast it.  So if the original number is 1.45643, and you are rounding to two decimal places, you get 1.46 instead of 1.45.
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I'll write out the algorithm so it's easier to see

double d = 1.234567;
int i = 4;
d = d + 5*10^(-i - 1);  // that would give us d = d + .00005, or d = 1.234617;
d = d * 10^i; // that gives us d = d * 10000, or d = 12346.17;
d = (int)d;  // d = 12346
d = d / 10^i; // d = d / 10000, or d = 1.2346;  That is the answer we want!
just a question,
isn´t it to steps and intensive processing to get the result?
I want to use it to round the mouse move and place a component on the proper grid position. It wont slow down the operation?
If all you're doing is rounding mouse movement so that you can "snap" on to a grid, then why do you need be able to round to a variable number of decimal places?

If you knew the number of decimal places to round to before-hand, then you could make the algorithm much simpler by just doing:

d = Math.round(d * 100.0) / 100.0;  // This does two decimal places
As I can figure out it´s what I´m trying...

holdX and holdY are offsets from component origin to mouse positon.

    public void setSnapLocation( int x, int y, int gridX, int gridY ) {
        double ox, oy;
        double gx, gy;
        gx = gridX * 1.0;
        gy = gridY * 1.0;
        ox = Math.round( ( ( x - holdX ) * 1.0 + gx / 2  ) / gx ) * gx;
        oy = Math.round( ( ( y - holdY ) * 1.0 + gy / 2  ) / gy ) * gy;
        super.setLocation( (int)ox-GRABBERMARGIN, (int)oy-GRABBERMARGIN );

it´s a rubber move from mouse catch to every sides.
maybe I can give you some points to :)
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