LAPACK  3.6.1 LAPACK: Linear Algebra PACKage
 real function slaran ( integer, dimension( 4 ) ISEED )

SLARAN

Purpose:
SLARAN 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.
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 slaran.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  REAL one
85  parameter ( one = 1.0e+0 )
86  INTEGER ipw2
87  REAL r
88  parameter ( ipw2 = 4096, r = one / ipw2 )
89 * ..
90 * .. Local Scalars ..
91  INTEGER it1, it2, it3, it4
92  REAL rndout
93 * ..
94 * .. Intrinsic Functions ..
95  INTRINSIC mod, real
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*( REAL( it1 )+r*( REAL( it2 )+r*( REAL( it3 )+r*
125  \$ ( REAL( IT4 ) ) ) ) )
126 *
127  IF (rndout.EQ.1.0) 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 SLARAN will
131 * be rounded to exactly 1.0. In IEEE single precision arithmetic,
132 * this will happen relatively often since n = 24.
133 * Since SLARAN is not supposed to return exactly 0.0 or 1.0
134 * (and some callers of SLARAN, such as CLARND, depend on that),
135 * the statistically correct thing to do in this situation is
136 * simply to iterate again.
137 * N.B. the case SLARAN = 0.0 should not be possible.
138 *
139  GOTO 10
140  END IF
141 *
142  slaran = rndout
143  RETURN
144 *
145 * End of SLARAN
146 *
real function slaran(ISEED)
SLARAN
Definition: slaran.f:69