ldexp, ldexpf, ldexpl - cppreference.com

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# ldexp, ldexpf, ldexpl

From cppreference.com
< c‎ | numeric‎ | math

Defined in header `<math.h>` |  |
float ldexpf( float arg, int exp ); | (1) | (since C99)
double ldexp( double arg, int exp ); | (2) |
long double ldexpl( long double arg, int exp ); | (3) | (since C99)
Defined in header `<tgmath.h>` |  |
#define ldexp( arg, exp ) | (4) | (since C99)

1-3) Multiplies a floating point value `arg` by the number 2 raised to the `exp` power.
4) Type-generic macro: If `arg` has type long double, `ldexpl` is called. Otherwise, if `arg` has integer type or the type double, `ldexp` is called. Otherwise, `ldexpf` is called, respectively.

### [edit] Parameters

arg | - | floating point value
exp | - | integer value

### [edit] Return value

If no errors occur, `arg` multiplied by 2 to the power of `exp` (arg×2exp
) is returned.

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

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

### [edit] Error handling

Errors are reported as specified in math_errhandling.

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

- Unless a range error occurs, FE_INEXACT is never raised (the result is exact)
- Unless a range error occurs, the current rounding mode is ignored
- If `arg` is ±0, it is returned, unmodified
- If `arg` is ±∞, it is returned, unmodified
- If `exp` is 0, then `arg` is returned, unmodified
- If `arg` is NaN, NaN is returned

### [edit] Notes

On binary systems (where FLT_RADIX is `2`), `ldexp` is equivalent to scalbn.

The function `ldexp` ("load exponent"), together with its dual, frexp, can be used to manipulate the representation of a floating-point number without direct bit manipulations.

On many implementations, `ldexp` is less efficient than multiplication or division by a power of two using arithmetic operators.

### [edit] Example

Run this code

    #include <stdio.h>
    #include <math.h>
    #include <float.h>
    #include <errno.h>
    #include <fenv.h>
    #pragma STDC FENV_ACCESS ON
    int main(void)
    {
        printf("ldexp(7, -4) = %f\n", ldexp(7, -4));
        printf("ldexp(1, -1074) = %g (minimum positive subnormal double)\n",
                ldexp(1, -1074));
        printf("ldexp(nextafter(1,0), 1024) = %g (largest finite double)\n",
                ldexp(nextafter(1,0), 1024));
        // special values
        printf("ldexp(-0, 10) = %f\n", ldexp(-0.0, 10));
        printf("ldexp(-Inf, -1) = %f\n", ldexp(-INFINITY, -1));
        //error handling
        errno = 0; feclearexcept(FE_ALL_EXCEPT);
        printf("ldexp(1, 1024) = %f\n", ldexp(1, 1024));
        if(errno == ERANGE) perror("    errno == ERANGE");
        if(fetestexcept(FE_OVERFLOW)) puts("    FE_OVERFLOW raised");
    }

Possible output:

    ldexp(7, -4) = 0.437500
    ldexp(1, -1074) = 4.94066e-324 (minimum positive subnormal double)
    ldexp(nextafter(1,0), 1024) = 1.79769e+308 (largest finite double)
    ldexp(-0, 10) = -0.000000
    ldexp(-Inf, -1) = -inf
    ldexp(1, 1024) = inf
        errno == ERANGE: Numerical result out of range
        FE_OVERFLOW raised

### [edit] References

- C11 standard (ISO/IEC 9899:2011):

  - 7.12.6.6 The ldexp functions (p: 244)

  - 7.25 Type-generic math <tgmath.h> (p: 373-375)

  - F.10.3.6 The ldexp functions (p: 522)

- C99 standard (ISO/IEC 9899:1999):

  - 7.12.6.6 The ldexp functions (p: 225)

  - 7.22 Type-generic math <tgmath.h> (p: 335-337)

  - F.9.3.6 The ldexp functions (p: 459)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.5.4.3 The ldexp function

### [edit] See also

frexpfrexpffrexpl (C99)(C99) | breaks a number into significand and a power of 2 (function) [edit]
scalbnscalbnfscalbnlscalblnscalblnfscalblnl (C99)(C99)(C99)(C99)(C99)(C99) | computes efficiently a number times FLT_RADIX raised to a power (function) [edit]
C++ documentation for ldexp

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