log

接口定义

C interface:

float logf(float x);

double log(double x);

double log_18(double x); // 仅在高精度版本提供

Fortran interface:

RES = LOGF(X);

RES = LOG(X);

参数

返回值

• 返回以e为底x的对数函数值y，y ∈ (-INF, +INF)。
• 输入x为±0，返回-∞。
• 输入x为1，返回0。
• 输入x为负数，返回nan。
• 输入x为+∞，返回+∞。
• 输入x为nan，返回nan。

依赖

C: "km.h"

Fortran: "km.f03"

示例

C interface:

    double pi = acos(-1); 
    // typical usage 
    double x1 = pi/2, x2 = 2.0, x3 = 125, x4 = 5; 
    // special handing 
    double a = 0.0, b = INFINITY, c = -INFINITY, d = NAN, e = 1.0; 
    // print result 
    printf("base-5 logarithm of 125 = %.15f\n", log(x3)/log(x4)); 
    printf("log(1.0) = %.15f\n", log(e)); 
    printf("log(pi/2) = %.15f\n", log(x1)); 
    printf("log(2.0) = %.15f\n", log(x2)); 
    printf("log(0.0) = %.15f\n", log(a)); 
    printf("log(INFINITY) = %.15f\n", log(b)); 
    printf("log(-INFINITY) = %.15f\n", log(c)); 
    printf("log(NAN) = %.15f\n", log(d)); 
    /* 
     *  base-5 logarithm of 125 = 3.000000000000000 
     *  log(1.0) = 0.000000000000000 
     *  log(pi/2) = 0.451582705289455 
     *  log(2.0) = 0.693147180559945 
     *  log(+0.0) = -inf 
     *  log(INFINITY) = inf 
     *  log(-INFINITY) = nan 
     *  log(NAN) = nan 
     * 
     * */

Fortran interface:

    REAL(8) :: X = 2 
    PRINT*,  LOG(X) 
    ! 
    ! OUTPUT 
    !     0.693147180559945 
    !