Question

Possible Duplicate:
Java float division precision

I’m trying to fix a bug in one of my games, and need to calculate the optimum tile size for that. To do that, I get the device’s screen resolution, number of rows and columns for that level, and then calculate the tile size which allows my game to use the maximum screen space. However, Android seems to be failing at basic math, losing precision on a simple division operation. Here’s the appropriate code:

while(!foundIt)
    {
        int tileArea = boxes*i*i; //Boxes = 20*26
        float ratio = displayArea/tileArea;  //displayArea = width*height
        Log.d("Balance", displayArea + "/" + tileArea);
        Log.d("Balance", boxes + ", " + i);
        Log.d("Balance", "Ratio: " + ratio);
        int last = 0;
        if(ratio>1)
        {
            if(last==LAST_WAS_LESS_THAN_ONE)
            {
                i--;
                foundIt=true;
            }
            last = LAST_WAS_MORE_THAN_ONE;
        }
        else if(ratio==1)
        {
            Log.d("Balance", "Found a perfect fit!!");
            foundIt=true;
        }
        else if(ratio<1)
        {
            if(last==LAST_WAS_MORE_THAN_ONE)
            {
                i--;
                foundIt=true;
            }
            last = LAST_WAS_LESS_THAN_ONE;
        }
        if(!foundIt)
        {
            i++;
        }
    }

Here is a sample run on my Nexus S (Resolution: 480 by 800):

06-18 13:20:45.808: D/Balance(1069): Width: 480 Height: 800
06-18 13:20:45.808: D/Balance(1069): 384000/520
06-18 13:20:45.808: D/Balance(1069): 520, 1
06-18 13:20:45.808: D/Balance(1069): Ratio: 738.0
06-18 13:20:45.808: D/Balance(1069): 384000/2080
06-18 13:20:45.808: D/Balance(1069): 520, 2
06-18 13:20:45.808: D/Balance(1069): Ratio: 184.0
06-18 13:20:45.808: D/Balance(1069): 384000/4680
06-18 13:20:45.808: D/Balance(1069): 520, 3
06-18 13:20:45.808: D/Balance(1069): Ratio: 82.0
06-18 13:20:45.808: D/Balance(1069): 384000/8320
06-18 13:20:45.808: D/Balance(1069): 520, 4
06-18 13:20:45.808: D/Balance(1069): Ratio: 46.0
06-18 13:20:45.808: D/Balance(1069): 384000/13000
06-18 13:20:45.808: D/Balance(1069): 520, 5
06-18 13:20:45.808: D/Balance(1069): Ratio: 29.0
06-18 13:20:45.808: D/Balance(1069): 384000/18720
06-18 13:20:45.808: D/Balance(1069): 520, 6
06-18 13:20:45.808: D/Balance(1069): Ratio: 20.0
06-18 13:20:45.808: D/Balance(1069): 384000/25480
06-18 13:20:45.808: D/Balance(1069): 520, 7
06-18 13:20:45.808: D/Balance(1069): Ratio: 15.0
06-18 13:20:45.808: D/Balance(1069): 384000/33280
06-18 13:20:45.808: D/Balance(1069): 520, 8
06-18 13:20:45.808: D/Balance(1069): Ratio: 11.0
06-18 13:20:45.808: D/Balance(1069): 384000/42120
06-18 13:20:45.808: D/Balance(1069): 520, 9
06-18 13:20:45.808: D/Balance(1069): Ratio: 9.0
06-18 13:20:45.808: D/Balance(1069): 384000/52000
06-18 13:20:45.808: D/Balance(1069): 520, 10
06-18 13:20:45.808: D/Balance(1069): Ratio: 7.0
06-18 13:20:45.808: D/Balance(1069): 384000/62920
06-18 13:20:45.808: D/Balance(1069): 520, 11
06-18 13:20:45.808: D/Balance(1069): Ratio: 6.0
06-18 13:20:45.808: D/Balance(1069): 384000/74880
06-18 13:20:45.808: D/Balance(1069): 520, 12
06-18 13:20:45.808: D/Balance(1069): Ratio: 5.0
06-18 13:20:45.808: D/Balance(1069): 384000/87880
06-18 13:20:45.808: D/Balance(1069): 520, 13
06-18 13:20:45.808: D/Balance(1069): Ratio: 4.0
06-18 13:20:45.808: D/Balance(1069): 384000/101920
06-18 13:20:45.808: D/Balance(1069): 520, 14
06-18 13:20:45.808: D/Balance(1069): Ratio: 3.0
06-18 13:20:45.808: D/Balance(1069): 384000/117000
06-18 13:20:45.808: D/Balance(1069): 520, 15
06-18 13:20:45.808: D/Balance(1069): Ratio: 3.0
06-18 13:20:45.808: D/Balance(1069): 384000/133120
06-18 13:20:45.812: D/Balance(1069): 520, 16
06-18 13:20:45.812: D/Balance(1069): Ratio: 2.0
06-18 13:20:45.816: D/Balance(1069): 384000/150280
06-18 13:20:45.816: D/Balance(1069): 520, 17
06-18 13:20:45.816: D/Balance(1069): Ratio: 2.0
06-18 13:20:45.816: D/Balance(1069): 384000/168480
06-18 13:20:45.816: D/Balance(1069): 520, 18
06-18 13:20:45.816: D/Balance(1069): Ratio: 2.0
06-18 13:20:45.816: D/Balance(1069): 384000/187720
06-18 13:20:45.816: D/Balance(1069): 520, 19
06-18 13:20:45.816: D/Balance(1069): Ratio: 2.0
06-18 13:20:45.816: D/Balance(1069): 384000/208000
06-18 13:20:45.816: D/Balance(1069): 520, 20
06-18 13:20:45.816: D/Balance(1069): Ratio: 1.0
06-18 13:20:45.816: D/Balance(1069): Found a perfect fit!!

Calculating the ratio on a calculator reveals a loss of precision. Does anybody know why this is? Is something in my implementation wrong?

Answer 1

From the print out, it seems that displayArea and tileArea are declared as int or long. And you are doing integer division (displayArea/tileArea) before the result is promoted to float type and assigned to ratio.

Since you are comparing whether one is larger than the other, why not doing the direct comparison between displayArea and tileArea? It is much better than fooling around with floating point operations.