如何在C99中使数学对数函数? [英] How to make math logarithm functions in C99?

问题描述

我终于回到构建函数的过程，这是在制作实际的BPML语言之前要做的事情.在第3部分-数学中，我想做一些对数函数.

I'm finally back at building my functions, which is what I'm doing before making the actual BPML language. In Part 3 - Math, I want to make some logarithm functions.

我从一开始就不知道对数函数，但是随着我的深入，我学会了并做到这一点:

I never knew what logarithm functions were at the very beginning, but as I went deeper, I learned it and made this:

float log_num(int num) {
    int mult;
    float result = 0;
    for (int i = 0; ; i++) {
        mult = 10 ^ i;
        if (mult >= num) {
            result = i;
            break;
        }
    }
    return result;
}

log_num 仅支持 int ，并且 float 和 double 将具有各自的分隔符.

log_num only supports int and float and double will have their separate ones.

现在我在使用此功能时遇到两个问题:

Now I got 2 problems with this function:

1. 当我尝试运行它并使用 100 作为函数中的数字时，结果应该是 2.00 ，但它给了我 1.00 .
2. 因为要返回的值是 float ，所以我希望该函数给我不同的值(如果它不是10的幂).例如: 2 = 0.30102999566398119521521373889472449.

1. When I tried to run it and use 100 as the number in the function, the result should've been 2.00, but it gave me 1.00.
2. Since the value to be returned is a float, I want the function to actually give me different values if it is not a power of 10. An example of it is 2 = 0.30102999566398119521373889472449.

问:如何解决问题1，如何使功能按问题2中的解释工作?

我想从头开始制作该功能，而不要依赖其他功能.

I want to make the function from scratch and not relying on other functions.

推荐答案

问:如何解决问题1 ...?

Q: How do I fix problem 1 ... ?

10 ^ i 是10 独占或和 i .不是10 ⁱ .

10 ^ i is 10 exclusive-or'ed with i. It is not 10ⁱ.

查找log ₁₀ (num)

To find the integer portion of log₁₀(num)

int i_portion = 0;
if (num <= 0) Handle_elsewhere();
else {
  int i_portion = 0;
  int n = num;
  while (n >= 10) {
    n /= 10;
    i_portion++;
  } 
  printf("%d\n", i_portion);
} 

问:...以及如何使该功能按问题2中的解释工作?

Q: ... and how do I make the function work as how I explained in problem 2?

以下是自C99以来的快速解决方案:

Below is a quick solution since C99:

#include <math.h>
float log_num(int num) {
  return log10f(num);
}

要编写没有< math.h> 的代码是相当广泛的.为了高效施工，我们需要设计参数.

To code without <math.h> is fairly broad. For efficient construction, we need design parameters.

首先，要弄清楚基础.标准的 log()是 e ，而不是以10为底的问题暗示.

First, be clear about base. The standard log() is base e, not base 10 as the question implies.

转角/错误处理:您如何处理负输入?Log _{任何正基数}(0)通常以-∞形式返回.对于有限的正值，没有范围问题.+∞，也许是+∞应该返回什么?对于 NaN ，也许是NaN，应该返回什么?

Corner/Error handling: How do you want to handle negative inputs? Log_{any positive base}(0) is usually returned as -∞. For finite positive values, no range issues. What should be returned for +∞, perhaps +∞? What should be returned for NaN, perhaps NaN?

准确性/精确度:您是否希望获得最佳结果或愿意放弃速度或小代码占用量的准确性?为什么要返回 float 而不是更常见的 double ?

Accuracy/precision: Do you want the best result or willing to given up accuracy for speed or small code foot-print? Why return float versus the more common double?

性能:仅运行时性能差的代码好吗?只需按照对数计算进行编码.由于目标包括 my_log_i(int)，my_log_f(float)，my_log_d(double)，因此，现在只需编写代码 my_log_d(double)，然后让其他人调用它即可.

Performance: simply code that is of poor run-time performance OK? Just code per Logarithm Calculation. Since the goal includes my_log_i(int), my_log_f(float), my_log_d(double), for now, just code my_log_d(double) and have the the others call it.

可移植性-便携性如何?

Portability - how portable?

当然，我们可以编写一个简单的 float my_log_10(int)，但是如果没有设计细节，结果将在很多方面都缺乏.

Sure we can code up a simply float my_log_10(int), but without the design details, the result would be lacking in many ways.

这篇关于如何在C99中使数学对数函数?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！