ipow.c File Reference

Detailed Description

This file provides the ipow function for computing integer powers of a double.

Copyright

• (c) 2002 The University of Chicago, as Operator of Argonne National Laboratory.
• (c) 2002 The Regents of the University of California, as Operator of Los Alamos National Laboratory.

License

This file is distributed under the terms of the Software License Agreement found in the file LICENSE included with this distribution.

Author

M. Borland, C. Saunders, R. Soliday

Definition in file ipow.c.

#include "mdb.h"

Go to the source code of this file.

Functions
double	ipow (const double x, const int64_t p)
	Compute x raised to the power p (x^p).

Function Documentation

ipow()

double ipow	(	const double	x,
		const int64_t	p )

Compute x raised to the power p (x^p).

Uses straightforward repeated multiplication for efficiency, which can be faster than using pow() for integer exponents.

Special cases:

• If x = 0 and p < 0, an error is reported since division by zero occurs.
• If p = 0, the result is 1.0.
• If p < 0, the result is 1/(x^(-p)).

Parameters

[in]	x	The base value.
[in]	p	The integer exponent.

Returns

The computed value of x raised to the power p.

Definition at line 33 of file ipow.c.

                                             {
  double hp;
  int64_t n;
 
  if (x == 0) {
    if (p < 0)
      bomb("Floating divide by zero in ipow().", NULL);
    return (p == 0 ? 1. : 0.);
  }
 
  if (p < 0)
    return (1. / ipow(x, -p));
 
  switch (p) {
  case 0:
    return (1.);
  case 1:
    return (x);
  case 2:
    return (x * x);
  case 3:
    hp = x * x;
    return (hp * x);
  case 4:
    hp = x * x;
    return (hp * hp);
  case 5:
    hp = x * x;
    return (hp * hp * x);
  case 6:
    hp = x * x;
    return (hp * hp * hp);
  case 7:
    hp = x * x * x;
    return (hp * hp * x);
  case 8:
    hp = x * x;
    hp = hp * hp;
    return (hp * hp);
  default:
    n = p / 2;
    hp = ipow(x, n);
    switch (p - 2 * n) {
    case 0:
      return (hp * hp);
    case 1:
      return (hp * hp * x);
    }
    break;
  }
  return (0.); /* never actually executed--keeps compiler happy */
}