Legacy Document

ldexp

You can use the ldexp function to perform efficient scaling by a power of 2.

double_t ldexp (double_t x, int n);

x

Any floating-point number.

n

An integer representing a power of 2 by which x should be multiplied.

DESCRIPTION

The ldexp function computes the value $x\times 2n$ without computing $2n$ . This is an ANSI standard C library function.

$ldexp(x,n)\; =\; x\times 2n$

The scalb function (described on page 10-19) performs the same operation as this function. The frexp function performs the inverse operation; that is, it splits x into its fraction field and exponent field.

EXCEPTIONS

When x is finite and nonzero, either the result of $ldexp(x,n)$ is exact or it raises one of the following exceptions:

• inexact (if an overflow or underflow occurs)
• overflow (if the result is outside the range of the data type)
• underflow (if the result is inexact and must be represented as a denormalized number or 0)

SPECIAL CASES

Table 10-10 shows the results when the floating-point argument to the ldexp function is a zero, a NaN, or an Infinity. In this table, n is any integer.

Special cases for the ldexp function

Operation	Result	Exceptions raised
$ldexp(+0,n)$	+0	None
$ldexp(-0,n)$	$-0$	None
$ldexp(NaN,n)$	NaN	None[31]
$ldexp(+$ ,n)	+	None
$ldexp(-$ ,n)	$-$	None

EXAMPLES

z = ldexp(3.0, 3);   /* z = 3  23 = 24 */
z = ldexp(0.0, 3);   /* z = 0  23 = 0 */

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[31] If the NaN is a signaling NaN, the invalid exception is raised.