LAPACK  3.6.1
LAPACK: Linear Algebra PACKage
double precision function dlaran ( integer, dimension( 4 )  ISEED)

DLARAN

Purpose: 
 DLARAN returns a random real number from a uniform (0,1)
 distribution.
Parameters
[in,out]ISEED
          ISEED is INTEGER array, dimension (4)
          On entry, the seed of the random number generator; the array
          elements must be between 0 and 4095, and ISEED(4) must be
          odd.
          On exit, the seed is updated.
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date
November 2011
Further Details: 
  This routine uses a multiplicative congruential method with modulus
  2**48 and multiplier 33952834046453 (see G.S.Fishman,
  'Multiplicative congruential random number generators with modulus
  2**b: an exhaustive analysis for b = 32 and a partial analysis for
  b = 48', Math. Comp. 189, pp 331-344, 1990).

  48-bit integers are stored in 4 integer array elements with 12 bits
  per element. Hence the routine is portable across machines with
  integers of 32 bits or more.

Definition at line 69 of file dlaran.f.

69 *
70 * -- LAPACK auxiliary routine (version 3.4.0) --
71 * -- LAPACK is a software package provided by Univ. of Tennessee, --
72 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
73 * November 2011
74 *
75 * .. Array Arguments ..
76  INTEGER iseed( 4 )
77 * ..
78 *
79 * =====================================================================
80 *
81 * .. Parameters ..
82  INTEGER m1, m2, m3, m4
83  parameter ( m1 = 494, m2 = 322, m3 = 2508, m4 = 2549 )
84  DOUBLE PRECISION one
85  parameter ( one = 1.0d+0 )
86  INTEGER ipw2
87  DOUBLE PRECISION r
88  parameter ( ipw2 = 4096, r = one / ipw2 )
89 * ..
90 * .. Local Scalars ..
91  INTEGER it1, it2, it3, it4
92  DOUBLE PRECISION rndout
93 * ..
94 * .. Intrinsic Functions ..
95  INTRINSIC dble, mod
96 * ..
97 * .. Executable Statements ..
98  10 CONTINUE
99 *
100 * multiply the seed by the multiplier modulo 2**48
101 *
102  it4 = iseed( 4 )*m4
103  it3 = it4 / ipw2
104  it4 = it4 - ipw2*it3
105  it3 = it3 + iseed( 3 )*m4 + iseed( 4 )*m3
106  it2 = it3 / ipw2
107  it3 = it3 - ipw2*it2
108  it2 = it2 + iseed( 2 )*m4 + iseed( 3 )*m3 + iseed( 4 )*m2
109  it1 = it2 / ipw2
110  it2 = it2 - ipw2*it1
111  it1 = it1 + iseed( 1 )*m4 + iseed( 2 )*m3 + iseed( 3 )*m2 +
112  $ iseed( 4 )*m1
113  it1 = mod( it1, ipw2 )
114 *
115 * return updated seed
116 *
117  iseed( 1 ) = it1
118  iseed( 2 ) = it2
119  iseed( 3 ) = it3
120  iseed( 4 ) = it4
121 *
122 * convert 48-bit integer to a real number in the interval (0,1)
123 *
124  rndout = r*( dble( it1 )+r*( dble( it2 )+r*( dble( it3 )+r*
125  $ ( dble( it4 ) ) ) ) )
126 *
127  IF (rndout.EQ.1.0d+0) THEN
128 * If a real number has n bits of precision, and the first
129 * n bits of the 48-bit integer above happen to be all 1 (which
130 * will occur about once every 2**n calls), then DLARAN will
131 * be rounded to exactly 1.0.
132 * Since DLARAN is not supposed to return exactly 0.0 or 1.0
133 * (and some callers of DLARAN, such as CLARND, depend on that),
134 * the statistically correct thing to do in this situation is
135 * simply to iterate again.
136 * N.B. the case DLARAN = 0.0 should not be possible.
137 *
138  GOTO 10
139  END IF
140 *
141  dlaran = rndout
142  RETURN
143 *
144 * End of DLARAN
145 *
dlaran
double precision function dlaran(ISEED)
DLARAN
Definition: dlaran.f:69