fast_exp_test.c

来自「ns2下用于802.11的无线传输模型的代码 增加了基于信噪比计算差错率等功能」· C语言 代码 · 共 57 行

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <sys/time.h>

int main()
{
  int i,j,k;
  long period;
  double pre[30];
  double base,r;
  struct timeval before, after;
  
  base = 0.99999;
  pre[0] = base;
  for(i=1;i<30;i++)
  {
    pre[i] = pre[i-1]*pre[i-1];
  }

  
  gettimeofday(&before,NULL);
  for(i=0;i<10000000;i++)
  {
    r = pow(base, 120000);
  }
  printf("%12lf \n", r);
  gettimeofday(&after,NULL);

  period = (after.tv_sec-before.tv_sec)*1000*1000;
  period = period+after.tv_usec-before.tv_usec;
  printf("Time needed for original pow function is %d ms \n",period/1000);
  

  gettimeofday(&before,NULL);
  for(i=0;i<10000000;i++)
  {
    j=0;
    k=120000;
    r=1;
    while(k>0)
    {
      if(k%2==1) r= r*pre[j];
      j++;
      k=k/2;
    }
  }
  printf("%12lf \n", r);
  gettimeofday(&after,NULL);

  period = (after.tv_sec-before.tv_sec)*1000*1000;
  period = period+after.tv_usec-before.tv_usec;
  printf("Time needed for fast pow function is %d ms \n",period/1000);

  return 0;
}