arctan查表法_查表法

查表法是将一些事先计算好的结果，存储在常量数组中，运行时节省计算开销。

例如，

计算字节中位1的个数，

int countBits( unsigned char dat )

{

static char nBitTab[256] = { 0, 1, 1, 2, 1, 2, 2, 3, 1, ...... };

return nBitTab[dat];

}

或将字节按位逆序，

unsigned char reverse( unsigned char dat )

{

static char revTab[256] = { 0x0, 0x80, 0x40, ......};

return nBitTab[dat];

}

或计算0~360度的正弦值，

int sin( int ang )

{

static const int sinV[] = {

/*sin00=*/0, 1144,2287,3430,4572,5712,6850,7987,9121,10252,

/*sin10=*/11380,12505,13626,14742,15855,16962,18064,19161,20252,21336,

/*sin20=*/22415,23486,24550,25607,26656,27697,28729,29753,30767,31772,

/*sin30=*/32768,33754,34729,35693,36647,37590,38521,39441,40348,41243,

/*sin40=*/42126,42995,43852,44695,45525,46341,47143,47930,48703,49461,

/*sin50=*/50203,50931,51643,52339,53020,53684,54332,54963,55578,56175,

/*sin60=*/56756,57319,57865,58393,58903,59396,59870,60326,60764,61183,

/*sin70=*/61584,61966,62328,62672,62997,63303,63589,63856,64104,64332,

/*sin80=*/64540,64729,64898,65048,65177,65287,65376,65446,65496,65526,

/*sin90=*/65536,

};

if ( 0 <= ang && ang <= 90 )

{

return sinV[ang];

}

if ( 90 < ang && ang <= 180 )

{

return sinV[180-ang];

}

if ( 180 < ang && ang <= 270 )

{

return -sinV[ang-180];

}

if ( 270 < ang && ang < 360 )

{

return -sinV[360-ang];

}

return 0;

}

理论上，y=f(x),x的取值范围为有限种状态，且f(x)有一定运算量时，都可以考虑查表法。

如，前两个例子中x的范围是字节的256个值，第三个例子是0~90度。

/**

* 数组的操作————查表法

* eg : 数组查表法(根据键盘录入索引,查找对应星期)

*/

import java.util.Scanner;

public class arraynum {

public static void main(String[] args) {

Scanner sc = new Scanner(System.in);

System.out.println("请输入对应的星期范围在1-7");

int week = sc.nextInt();

//sc.close();

System.out.println("星期" + getWeek(week));

}

/*

根据索引返回对应的星期

1,返回值类型char

2,参数列表int week

*/

//数组第0个元素为null，这样1就对应的一，以此类推

public static char getWeek(int week) {

char[] arr = {' ','一','二','三','四','五','六','日'};//定义了一张星期表

return arr[week];//通过索引获取表中的元素

}

}

另外，查表法还用于简化switch/case语句，还可将数据与代码分离。