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log1p

Compute the base-e logarithm of (1+x), that is, ln(1+x).

Interface Definition

C interface:

float log1pf(float x);

double log1p(double x);

long double log1pl(long double x);

Parameters

Parameter

Type

Description

Input/Output

x
  • For log1pf, x is of single-precision floating-point type.
  • For log1p, x is of double-precision floating-point type.
  • For log1pl, x is of long double-precision floating-point type.

Floating-point value of the input data.

Input

Return Value

  • The base-e logarithm y of 1+x is returned. y ∈ (-INF, +INF)
  • If input x is +0, the return value is +0.
  • If input x is -0, the return value is -0.
  • If input x is -1, the return value is -∞.
  • If input x is +∞, the return value is +∞.
  • If input x is less than -1, the return value is NaN.
  • If input x is NaN, the return value is NaN.

Dependency

C: "km.h"

Example

C interface:

    // typical usage
    double x1 = 0.0, x2 = -0.0, x3 = -1, x4 = -2;
    // special handing
    double a = INFINITY, b = -INFINITY, c = NAN;
    // print result
    printf("log1p(0.0) = %.15f\n", log1p(x1));
    printf("log1p(-0.0) = %.15f\n", log1p(x2));
    printf("log1p(-1) = %.15f\n", log1p(x3));
    printf("log1p(-2) = %.15f\n", log1p(x4));
    printf("log1p(INFINITY) = %.15f\n", log1p(a));
    printf("log1p(-INFINITY) = %.15f\n", log1p(b));
    printf("log1p(NAN) = %.15f\n", log1p(c));

    /* 
     * log1p(0.0) = 0.000000000000000
     * log1p(-0.0) = -0.000000000000000
     * log1p(-1) = -inf
     * log1p(-2) = nan
     * log1p(INFINITY) = inf
     * log1p(-INFINITY) = nan
     * log1p(NAN) = nan
     * 
     * */