重新排列方程 [英] Rearranging an equation

问题描述

我有以下的公式在我的C code

I have the following equation in my C code

k * dl * (1.0 + pHold / centre
       + (pHold * pHold) / (2.0 * centre * centre)
       - square / (2.0 * centre))

我知道浮点部门是很多比乘法贵，而且我一直在这个搏斗了一会儿。有什么办法重新安排该切出一个部门？

I know that floating point divisions are a lot more expensive than multiplications, and I have been wrestling with this for a while. Is there any way to rearrange this to cut out a division?

感谢

推荐答案

如果你看一下分母你可以看到，做一个常用的单位将允许你做师只有一次（以更乘法为代价的分数）：

If you look at the denominators for the fractions you can see that making a common denomination would allow you to do the division just once (at the expense of more multiplications):

k * dl * (1.0
  + pHold                  / (centre)
  - square                 / (2.0 * centre)
  + (pHold * pHold)        / (2.0 * centre * centre)
)

如果您确定浮点乘法比浮点除法则更好：

If you are sure that a floating point multiplication is better than a floating point division then:

k * dl * (1.0
  + (pHold * 2.0 * centre) / (2.0 * centre * centre)
  - (square * centre)      / (2.0 * centre * centre)
  + (pHold * pHold)        / (2.0 * centre * centre)
)

这已经成为：

k * dl * (1.0
  + ( (pHold * 2.0 * centre)
    - (square * centre)
    + (pHold * pHold) )     / (2.0 * centre * centre)
)

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