java整数舍入（除法相关）
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java integer rounding (division related)
提问by Keybounce
I have run into a surprise with integer division notrounding down as expected.
我遇到了一个惊喜，整数除法没有按预期四舍五入。
Simple code:
简单代码：
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:
即使使用显式（浮点）强制转换，输出也是：
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.
我一直期待-1。
The real code where this is happening does this:
发生这种情况的真实代码是这样的：
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.
RealisticFluids.MAX_FLUID 具有值 (1<<20)。该代码应该采用 1 到 MAX_FLUID 的输入（只有当输入块是空气时才会出现 0），并返回一个从 0 到 7 的数字——1 到 1/8 的最大值是 0、1/8+1到 2/8th 是 2，到 7/8th + 1 到 8/8th 是 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.
我期待整数数学将所有分数四舍五入。但这并没有发生——因为 5.999 最终变成了 6 而不是 5，所以我在整个地方都出现了一对一的错误。
- Where is this behavior documented?
- What's the easiest work-around to get the rounding down that I expected?
- 这种行为记录在哪里？
- 达到我预期的四舍五入的最简单的解决方法是什么？
(full code context: https://github.com/keybounce/Finite-Fluids/blob/master/src/main/java/com/mcfht/realisticfluids/Util.java)
（完整代码上下文：https: //github.com/keybounce/Finite-Fluids/blob/master/src/main/java/com/mcfht/realisticfluids/Util.java）
回答by Solomon Slow
I'm expecting integer math to round all fractions down.
我期待整数数学将所有分数四舍五入。
First of all, you're not doing integer division: You're doing floating point division. (float)-8is a floating point expression. Therefore, the sin (float)-8/sgets promoted to (float) before the division happens.
首先，你不是在做整数除法：你在做浮点除法。 (float)-8是一个浮点表达式。因此，s在(float)-8/s除法发生之前in被提升为 (float)。
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.
然后将浮点结果转换为 int。Reuseman 说整数除法会向零舍入。好吧，float->int 转换也会向零舍入。
