java integer rounding (division related)
I'm expecting integer math to round all fractions down.
First of all, you're not doing integer division: You're doing floating point division. (float)-8
is a floating point expression. Therefore, the s
in (float)-8/s
gets promoted to (float) before the division happens.
Then the floating point result is converted to an int. Reuseman said that integer division rounds toward zero. Well, float->int conversion also rounds toward zero.
Keybounce
Updated on July 09, 2022Comments
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Keybounce almost 2 years
I have run into a surprise with integer division not rounding down as expected.
Simple code:
public class HelloMath { public static void main(String[] args) { for (int s=1; s< 20; s++) { int div = 1<<s; int res = (int) ((float)-8/ s); System.out.printf("Bit %d, result %d\n", s, res); } } }
Even with the explicit (float) casts, the output is:
Bit 1, result -8 Bit 2, result -4 Bit 3, result -2 Bit 4, result -2 Bit 5, result -1 Bit 6, result -1 Bit 7, result -1 Bit 8, result -1 Bit 9, result 0 Bit 10, result 0 Bit 11, result 0 Bit 12, result 0 Bit 13, result 0 Bit 14, result 0 Bit 15, result 0 Bit 16, result 0 Bit 17, result 0 Bit 18, result 0 Bit 19, result 0
I was expecting -1 all the way down.
The real code where this is happening does this:
public static int fluidTo8th(int fluid) { if (0 == fluid) return 0; // Technically, this isn't needed :-). int wholePart = (fluid-1) * 8 / RealisticFluids.MAX_FLUID; // -1 gets rounding correct; // consider fluid of exactly 1/8th. return 1+wholePart; }
RealisticFluids.MAX_FLUID has the value (1<<20). The code is supposed to take an input that is 1 to MAX_FLUID (0 should only happen if the input block is air), and return a number from 0 to 7 -- 1 to 1/8th max is 0, 1/8th +1 to 2/8th is 2, up to 7/8th + 1 to 8/8th is 7.
I'm expecting integer math to round all fractions down. But that's not happening -- I wind up with off-by-one errors all over the place as 5.999 winds up becoming 6 instead of 5.
- Where is this behavior documented?
- What's the easiest work-around to get the rounding down that I expected?
(full code context: https://github.com/keybounce/Finite-Fluids/blob/master/src/main/java/com/mcfht/realisticfluids/Util.java)
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Andreas almost 8 yearsJava specification backing your statement: JLS §5.1.3 ... the floating-point value is rounded to an integer value V, rounding toward zero using IEEE 754 round-toward-zero mode (§4.2.3).