菜鸡笔记之——二分查找

菜鸡笔记之——二分查找+文件操作

二分查找：
前提是所有数据都已经排好序，将目标值每次跟中间的数mid比较，如果相等可以直接返回，如果比mid大则继续查找大的一边，否则继续查找小的一边。

代码实现（二分查找+读取文件）：
文件arctan.txt存放arctan(i)的值，i = 1，2，3,…360,输入一个arctani 的值查出i为多少。

# include <stdio.h>
# include <stdlib.h>
# include <math.h>
# define LEN 361

int main(void)
{
    void file_read(char * filename,float *array,int array_len);
    int binary_search(float *array,int array_len,float search_number);
    float array[361] = {0},y;
    char filename[30] = {0};

    //read data from the file
    printf("which file do you want to read:");
    scanf("%s",filename);
    file_read(filename,array,LEN);

    //binary search
    printf("please enter the a float number y (ps:y = arctan(x),x = 1,2,...):\n");
    scanf("%f",&y);
    int x = binary_search(array,LEN,y);

    if (x!=0)
    {
        printf("arctan%d = %.4f\n",x,y);
    }
    else
    {
        printf("can not find this number!\n");
    }

    return 0;
}
//read file function
void file_read(char * filename,float * array,int array_len)
{
    FILE * fp = NULL;
    fp = fopen(filename,"r");
    if (fp==NULL)
    {
        printf("file open error!\n");
        return;
    }
    for (int i = 1;i<array_len;i++)
    {
        fscanf(fp,"%f",array+i);
    }
    fclose(fp);
}
//binary search
int binary_search(float * array,int array_len,float y)
{
    int low = 1,high = array_len-1,mid;
    while (low<high)
    {
        mid = (high+low)/2;
        if (fabs(array[mid]-y)<1e-3)
        {
            return (mid);
        }
        else if (y<array[mid])
        {
            high = mid-1;
        }
        else if (y>array[mid])
        {
            low = mid+1;
        }
    }
    return 0;       //return 0 if can not find the target
}

附：arctan.txt

运行结果：