为什么我在C ++中用^计算指数时得不到正确的结果？ [英] Why am I not getting correct result when I calculate exponent with ^ in C++?

问题描述

我使用Bode的formuala计算第n个行星距太阳的距离

  dist =（4 + 3 * （n-2）））/ 10 
  

如果我以这种方式计算距离，值：

  dist [2] =（（4 + 3 * 1）/10.0）; 
 dist [3] =（（4 + 3 * 2）/10.0）; 
 dist [4] =（（4 + 3 * 4）/10.0）; 
  

但这样做会给我不正确的值：

 矢量< double> dist（5）; 
 
 for（unsigned int i = 2; i <5; i ++）
 {
 dist [i] =（（4 + 3 * ）））/ 10.0）; 
} 
  

为什么如此？

^ 字符表示按位异或，而不是您期望的指数函数。

由于你已经在一个循环中计算这个，你可以很容易地在方程中生成你需要的2的幂，简单的（接近你的代码）将是：

 矢量< double> dist（5）; 
 unsigned int j = 1; 
 for（unsigned int i = 2; i <5; i ++）{
 dist [i] =（4 + 3 * j）/10.0; 
j = j * 2; 
} 
  

在这个特定的实例中，我们初始化 j 到你需要的第一个值 n-2 ，然后继续乘以2，得到你需要的下一个2的幂。 p>


I am using Bode's formuala to calculate distance of nth planet from sun

dist = (4 + 3*(2^(n-2)))/10

If I calculate the distance this way, I get the right values:

dist[2] = ((4 + 3*1)/10.0) ;  
dist[3] = ((4 + 3*2)/10.0) ;  
dist[4] = ((4 + 3*4)/10.0) ;

But doing it this way, gives me incorrect values:

vector <double> dist(5);  

for (unsigned int i = 2; i < 5; i++)  
{  
    dist[i] = ((4 + 3*(2^(3-2)))/10.0) ;  
}

Why so?

解决方案

The ^ character represents a bitwise exclusive or, not the exponential function that you expect.

Since you're calculating this in a loop already you can easily generate the powers of 2 that you need in the equation, something simple (and close to your code) would be:

vector<double> dist(5);
unsigned int j = 1;
for(unsigned int i = 2; i < 5; i++){
    dist[i] = (4+3*j)/10.0;
    j = j * 2;
}

In this particular instance, we initialize j to the first value of n-2 that you require, then proceed to multiply it by 2 to get the next power of 2 that you require.

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