28.16 Scaling by Powers of the Base

We have discussed rounding errors several times in this chapter, but it is important to remember that when results require no more bits than the exponent and significand bits can represent, those results are exact.

One particularly useful exact operation is scaling by a power of the base. While one, in principle, could do that with code like this:

y = x * pow (2.0, (double)k);   /* Undesirable scaling: avoid! */

that is not advisable, because it relies on the quality of the math-library power function, and that happens to be one of the most difficult functions in the C math library to make accurate. What is likely to happen on many systems is that the returned value from pow will be close to a power of two, but slightly different, so the subsequent multiplication introduces rounding error.

The correct, and fastest, way to do the scaling is either via the traditional C library function, or with its C99 equivalent:

y = ldexp (x, k);            /* Traditional pre-C99 style. */
y = scalbn (x, k);           /* C99 style. */

Both functions return x * 2**k. See Normalization Functions in The GNU C Library Reference Manual.