计算年金现值系数，输入参数为实数。

接口定义

C interface：

float annuityf(float x, float y);

double annuity(double x, double y);

参数

参数名	类型	描述	输入/输出
x	在annuityf中，x是单精度浮点类型。在annuity中，x是双精度浮点类型。	表示输入数据的浮点值。	输入
y	在annuityf中，y是单精度浮点类型。在annuity中，y是双精度浮点类型。	表示输入数据的浮点值。	输入

返回值

x大于-1且y ∈(-∞, +∞)，返回计算结果。
输入x为任意值，y为±0，返回0。
输入x为nan，y为任意值，返回nan。
输入x为任意值，y为nan，返回nan。
输入x为+∞，y为-1，则返回-1；y < -1，则返回-∞；y ∈(-1, 0)，则返回-0。
输入x为0，y != 0，则返回y。
输入x为-1，y < 0，则返回-1；y > 0，则返回+∞。
输入x < -1，y != 0，则返回nan。
输入y为-∞，x ∈(-1， 0)，则返回1.0 / x；x > 0，则返回-∞。
输入y为+∞，x ∈(-1， 0)，则返回+∞；x > 0，则返回1.0 / x。

依赖

C: "km.h"

示例

C interface：

    // print result
    printf("annuity(1.0, 10.0) = %.15f\n", annuity(1.0, 10.0));
    printf("annuity(0.5, -3.0) = %.15f\n", annuity(0.5, -3.0));
    printf("annuity(0.3, -10.0) = %.15f\n", annuity(1.0, 10.0));
    printf("annuity(4, 2.0) = %.15f\n", annuity(0.5, -3.0));
    printf("annuity(INFINITY, 0) = %.15f\n", annuity(INFINITY, 0));
    printf("annuity(INFINITY, 0) = %.15f\n", annuity(INFINITY, 0));
    printf("annuity(1.0, NAN) = %.15f\n", annuity(1.0, NAN));
    printf("annuity(NAN, 1.0) = %.15f\n", annuity(NAN, 1.0));
    printf("annuity(INFINITY, -1.0) = %.15f\n", annuity(INFINITY, -1.0));
    printf("annuity(INFINITY, -10.0) = %.15f\n", annuity(INFINITY, -10.0));
    printf("annuity(INFINITY, 10.0) = %.15f\n", annuity(INFINITY, 10.0));
    printf("annuity(0, 1.0) = %.15f\n", annuity(0, 1.0));
    printf("annuity(-1.0, -1.0) = %.15f\n", annuity(-1.0, -1.0));
    printf("annuity(-1.0, 1.0) = %.15f\n", annuity(-1.0, 1.0));
    printf("annuity(-10.0, 1.0) = %.15f\n", annuity(-10.0, 1.0));
    printf("annuity(-0.5, -INFINITY) = %.15f\n", annuity(-0.5, -INFINITY));
    printf("annuity(0.5, -INFINITY) = %.15f\n", annuity(0.5, -INFINITY));
    printf("annuity(-0.5, INFINITY) = %.15f\n", annuity(-0.5, INFINITY));
    printf("annuity(0.5, INFINITY) = %.15f\n", annuity(0.5, INFINITY));
    /* 
     * annuity(1.0, 10.0) = 0.999023437500000
     * annuity(0.5, -3.0) = -4.750000000000000
     * annuity(0.3, -10.0) = 0.999023437500000
     * annuity(4, 2.0) = -4.750000000000000
     * annuity(INFINITY, 0) = 0.000000000000000
     * annuity(INFINITY, 0) = 0.000000000000000
     * annuity(1.0, NAN) = nan
     * annuity(NAN, 1.0) = nan
     * annuity(INFINITY, -1.0) = -1.000000000000000
     * annuity(INFINITY, -10.0) = -inf
     * annuity(INFINITY, 10.0) = 0.000000000000000
     * annuity(0, 1.0) = 1.000000000000000
     * annuity(-1.0, -1.0) = -1.000000000000000
     * annuity(-1.0, 1.0) = inf
     * annuity(-10.0, 1.0) = nan
     * annuity(-0.5, -INFINITY) = -2.000000000000000
     * annuity(0.5, -INFINITY) = -inf
     * annuity(-0.5, INFINITY) = inf
     * annuity(0.5, INFINITY) = 2.000000000000000
     * 
     * */