expm1, expm1f, expm1l

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Common mathematical functions
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(C99)
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(C99)
(C99)(C99)(C99)(C23)
Maximum/minimum operations
(C99)
(C23)    
Exponential functions
(C23)
(C99)
expm1
(C99)
(C23)
(C23)
(C99)
(C99)(C23)
(C23)
(C23)
Power functions
(C99)
(C23)
(C23)
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Trigonometric and hyperbolic functions
(C23)
(C23)
(C23)
(C23)
(C99)
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(C99)
Error and gamma functions
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(C99)
(C99)
Nearest integer floating-point operations
(C99)(C99)(C99)
(C99)
(C99)(C99)(C99)
(C23)(C23)(C23)(C23)
Floating-point manipulation functions
(C99)(C99)
(C99)(C23)
(C99)
Narrowing operations
(C23)
(C23)
(C23)
(C23)
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Quantum and quantum exponent functions
Decimal re-encoding functions
Total order and payload functions
Classification
(C99)
(C99)
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(C23)
Types
(C99)(C99)    

(C99)(C99)    

Macro constants
Special floating-point values
(C99)(C23)
Arguments and return values
(C99)(C99)(C99)(C99)(C99)    
Error handling
 
Defined in header <math.h>
float       expm1f( float arg );
(1) (since C99)
double      expm1( double arg );
(2) (since C99)
long double expm1l( long double arg );
(3) (since C99)
Defined in header <tgmath.h>
#define expm1( arg )
(4) (since C99)
1-3) Computes the e (Euler's number, 2.7182818) raised to the given power arg, minus 1.0. This function is more accurate than the expression exp(arg)-1.0 if arg is close to zero.
4) Type-generic macro: If arg has type long double, expm1l is called. Otherwise, if arg has integer type or the type double, expm1 is called. Otherwise, expm1f is called.

Parameters

arg - floating point value

Return value

If no errors occur earg
-1
is returned.

If a range error due to overflow occurs, +HUGE_VAL, +HUGE_VALF, or +HUGE_VALL is returned.

If a range error occurs due to underflow, the correct result (after rounding) is returned.

Error handling

Errors are reported as specified in math_errhandling.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

  • If the argument is ±0, it is returned, unmodified
  • If the argument is -∞, -1 is returned
  • If the argument is +∞, +∞ is returned
  • If the argument is NaN, NaN is returned

Notes

The functions expm1 and log1p are useful for financial calculations, for example, when calculating small daily interest rates: (1+x)n
-1
can be expressed as expm1(n * log1p(x)). These functions also simplify writing accurate inverse hyperbolic functions.

For IEEE-compatible type double, overflow is guaranteed if 709.8 < arg.

Example

#include <stdio.h>
#include <math.h>
#include <float.h>
#include <errno.h>
#include <fenv.h>
// #pragma STDC FENV_ACCESS ON
int main(void)
{
    printf("expm1(1) = %f\n", expm1(1));
    printf("Interest earned in 2 days on $100, compounded daily at 1%%\n"
           " on a 30/360 calendar = %f\n",
           100*expm1(2*log1p(0.01/360)));
    printf("exp(1e-16)-1 = %g, but expm1(1e-16) = %g\n",
           exp(1e-16)-1, expm1(1e-16));
    // special values
    printf("expm1(-0) = %f\n", expm1(-0.0));
    printf("expm1(-Inf) = %f\n", expm1(-INFINITY));
    //error handling
    errno = 0; feclearexcept(FE_ALL_EXCEPT);
    printf("expm1(710) = %f\n", expm1(710));
    if(errno == ERANGE) perror("    errno == ERANGE");
    if(fetestexcept(FE_OVERFLOW)) puts("    FE_OVERFLOW raised");
}

Possible output:

expm1(1) = 1.718282
Interest earned in 2 days on $100, compounded daily at 1%
 on a 30/360 calendar = 0.005556
exp(1e-16)-1 = 0, but expm1(1e-16) = 1e-16
expm1(-0) = -0.000000
expm1(-Inf) = -1.000000
expm1(710) = inf
    errno == ERANGE: Result too large
    FE_OVERFLOW raised

References

  • C17 standard (ISO/IEC 9899:2018):
  • 7.12.6.3 The expm1 functions (p: 177)
  • 7.25 Type-generic math <tgmath.h> (p: 272-273)
  • F.10.3.3 The expm1 functions (p: 379)
  • C11 standard (ISO/IEC 9899:2011):
  • 7.12.6.3 The expm1 functions (p: 243)
  • 7.25 Type-generic math <tgmath.h> (p: 373-375)
  • F.10.3.3 The expm1 functions (p: 521)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.12.6.3 The expm1 functions (p: 223-224)
  • 7.22 Type-generic math <tgmath.h> (p: 335-337)
  • F.9.3.3 The expm1 functions (p: 458)

See also

(C99)(C99)
computes e raised to the given power (ex)
(function)
(C99)(C99)(C99)
computes 2 raised to the given power (2x)
(function)
(C99)(C99)(C99)
computes natural (base-e) logarithm of 1 plus the given number (ln(1+x))
(function)