LAPACK  3.4.2 LAPACK: Linear Algebra PACKage
dorgrq.f
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1 *> \brief \b DORGRQ
2 *
3 * =========== DOCUMENTATION ===========
4 *
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17 *
18 * Definition:
19 * ===========
20 *
21 * SUBROUTINE DORGRQ( M, N, K, A, LDA, TAU, WORK, LWORK, INFO )
22 *
23 * .. Scalar Arguments ..
24 * INTEGER INFO, K, LDA, LWORK, M, N
25 * ..
26 * .. Array Arguments ..
27 * DOUBLE PRECISION A( LDA, * ), TAU( * ), WORK( * )
28 * ..
29 *
30 *
31 *> \par Purpose:
32 * =============
33 *>
34 *> \verbatim
35 *>
36 *> DORGRQ generates an M-by-N real matrix Q with orthonormal rows,
37 *> which is defined as the last M rows of a product of K elementary
38 *> reflectors of order N
39 *>
40 *> Q = H(1) H(2) . . . H(k)
41 *>
42 *> as returned by DGERQF.
43 *> \endverbatim
44 *
45 * Arguments:
46 * ==========
47 *
48 *> \param[in] M
49 *> \verbatim
50 *> M is INTEGER
51 *> The number of rows of the matrix Q. M >= 0.
52 *> \endverbatim
53 *>
54 *> \param[in] N
55 *> \verbatim
56 *> N is INTEGER
57 *> The number of columns of the matrix Q. N >= M.
58 *> \endverbatim
59 *>
60 *> \param[in] K
61 *> \verbatim
62 *> K is INTEGER
63 *> The number of elementary reflectors whose product defines the
64 *> matrix Q. M >= K >= 0.
65 *> \endverbatim
66 *>
67 *> \param[in,out] A
68 *> \verbatim
69 *> A is DOUBLE PRECISION array, dimension (LDA,N)
70 *> On entry, the (m-k+i)-th row must contain the vector which
71 *> defines the elementary reflector H(i), for i = 1,2,...,k, as
72 *> returned by DGERQF in the last k rows of its array argument
73 *> A.
74 *> On exit, the M-by-N matrix Q.
75 *> \endverbatim
76 *>
77 *> \param[in] LDA
78 *> \verbatim
79 *> LDA is INTEGER
80 *> The first dimension of the array A. LDA >= max(1,M).
81 *> \endverbatim
82 *>
83 *> \param[in] TAU
84 *> \verbatim
85 *> TAU is DOUBLE PRECISION array, dimension (K)
86 *> TAU(i) must contain the scalar factor of the elementary
87 *> reflector H(i), as returned by DGERQF.
88 *> \endverbatim
89 *>
90 *> \param[out] WORK
91 *> \verbatim
92 *> WORK is DOUBLE PRECISION array, dimension (MAX(1,LWORK))
93 *> On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
94 *> \endverbatim
95 *>
96 *> \param[in] LWORK
97 *> \verbatim
98 *> LWORK is INTEGER
99 *> The dimension of the array WORK. LWORK >= max(1,M).
100 *> For optimum performance LWORK >= M*NB, where NB is the
101 *> optimal blocksize.
102 *>
103 *> If LWORK = -1, then a workspace query is assumed; the routine
104 *> only calculates the optimal size of the WORK array, returns
105 *> this value as the first entry of the WORK array, and no error
106 *> message related to LWORK is issued by XERBLA.
107 *> \endverbatim
108 *>
109 *> \param[out] INFO
110 *> \verbatim
111 *> INFO is INTEGER
112 *> = 0: successful exit
113 *> < 0: if INFO = -i, the i-th argument has an illegal value
114 *> \endverbatim
115 *
116 * Authors:
117 * ========
118 *
119 *> \author Univ. of Tennessee
120 *> \author Univ. of California Berkeley
121 *> \author Univ. of Colorado Denver
122 *> \author NAG Ltd.
123 *
124 *> \date November 2011
125 *
126 *> \ingroup doubleOTHERcomputational
127 *
128 * =====================================================================
129  SUBROUTINE dorgrq( M, N, K, A, LDA, TAU, WORK, LWORK, INFO )
130 *
131 * -- LAPACK computational routine (version 3.4.0) --
132 * -- LAPACK is a software package provided by Univ. of Tennessee, --
133 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
134 * November 2011
135 *
136 * .. Scalar Arguments ..
137  INTEGER info, k, lda, lwork, m, n
138 * ..
139 * .. Array Arguments ..
140  DOUBLE PRECISION a( lda, * ), tau( * ), work( * )
141 * ..
142 *
143 * =====================================================================
144 *
145 * .. Parameters ..
146  DOUBLE PRECISION zero
147  parameter( zero = 0.0d+0 )
148 * ..
149 * .. Local Scalars ..
150  LOGICAL lquery
151  INTEGER i, ib, ii, iinfo, iws, j, kk, l, ldwork,
152  \$ lwkopt, nb, nbmin, nx
153 * ..
154 * .. External Subroutines ..
155  EXTERNAL dlarfb, dlarft, dorgr2, xerbla
156 * ..
157 * .. Intrinsic Functions ..
158  INTRINSIC max, min
159 * ..
160 * .. External Functions ..
161  INTEGER ilaenv
162  EXTERNAL ilaenv
163 * ..
164 * .. Executable Statements ..
165 *
166 * Test the input arguments
167 *
168  info = 0
169  lquery = ( lwork.EQ.-1 )
170  IF( m.LT.0 ) THEN
171  info = -1
172  ELSE IF( n.LT.m ) THEN
173  info = -2
174  ELSE IF( k.LT.0 .OR. k.GT.m ) THEN
175  info = -3
176  ELSE IF( lda.LT.max( 1, m ) ) THEN
177  info = -5
178  END IF
179 *
180  IF( info.EQ.0 ) THEN
181  IF( m.LE.0 ) THEN
182  lwkopt = 1
183  ELSE
184  nb = ilaenv( 1, 'DORGRQ', ' ', m, n, k, -1 )
185  lwkopt = m*nb
186  END IF
187  work( 1 ) = lwkopt
188 *
189  IF( lwork.LT.max( 1, m ) .AND. .NOT.lquery ) THEN
190  info = -8
191  END IF
192  END IF
193 *
194  IF( info.NE.0 ) THEN
195  CALL xerbla( 'DORGRQ', -info )
196  return
197  ELSE IF( lquery ) THEN
198  return
199  END IF
200 *
201 * Quick return if possible
202 *
203  IF( m.LE.0 ) THEN
204  return
205  END IF
206 *
207  nbmin = 2
208  nx = 0
209  iws = m
210  IF( nb.GT.1 .AND. nb.LT.k ) THEN
211 *
212 * Determine when to cross over from blocked to unblocked code.
213 *
214  nx = max( 0, ilaenv( 3, 'DORGRQ', ' ', m, n, k, -1 ) )
215  IF( nx.LT.k ) THEN
216 *
217 * Determine if workspace is large enough for blocked code.
218 *
219  ldwork = m
220  iws = ldwork*nb
221  IF( lwork.LT.iws ) THEN
222 *
223 * Not enough workspace to use optimal NB: reduce NB and
224 * determine the minimum value of NB.
225 *
226  nb = lwork / ldwork
227  nbmin = max( 2, ilaenv( 2, 'DORGRQ', ' ', m, n, k, -1 ) )
228  END IF
229  END IF
230  END IF
231 *
232  IF( nb.GE.nbmin .AND. nb.LT.k .AND. nx.LT.k ) THEN
233 *
234 * Use blocked code after the first block.
235 * The last kk rows are handled by the block method.
236 *
237  kk = min( k, ( ( k-nx+nb-1 ) / nb )*nb )
238 *
239 * Set A(1:m-kk,n-kk+1:n) to zero.
240 *
241  DO 20 j = n - kk + 1, n
242  DO 10 i = 1, m - kk
243  a( i, j ) = zero
244  10 continue
245  20 continue
246  ELSE
247  kk = 0
248  END IF
249 *
250 * Use unblocked code for the first or only block.
251 *
252  CALL dorgr2( m-kk, n-kk, k-kk, a, lda, tau, work, iinfo )
253 *
254  IF( kk.GT.0 ) THEN
255 *
256 * Use blocked code
257 *
258  DO 50 i = k - kk + 1, k, nb
259  ib = min( nb, k-i+1 )
260  ii = m - k + i
261  IF( ii.GT.1 ) THEN
262 *
263 * Form the triangular factor of the block reflector
264 * H = H(i+ib-1) . . . H(i+1) H(i)
265 *
266  CALL dlarft( 'Backward', 'Rowwise', n-k+i+ib-1, ib,
267  \$ a( ii, 1 ), lda, tau( i ), work, ldwork )
268 *
269 * Apply H**T to A(1:m-k+i-1,1:n-k+i+ib-1) from the right
270 *
271  CALL dlarfb( 'Right', 'Transpose', 'Backward', 'Rowwise',
272  \$ ii-1, n-k+i+ib-1, ib, a( ii, 1 ), lda, work,
273  \$ ldwork, a, lda, work( ib+1 ), ldwork )
274  END IF
275 *
276 * Apply H**T to columns 1:n-k+i+ib-1 of current block
277 *
278  CALL dorgr2( ib, n-k+i+ib-1, ib, a( ii, 1 ), lda, tau( i ),
279  \$ work, iinfo )
280 *
281 * Set columns n-k+i+ib:n of current block to zero
282 *
283  DO 40 l = n - k + i + ib, n
284  DO 30 j = ii, ii + ib - 1
285  a( j, l ) = zero
286  30 continue
287  40 continue
288  50 continue
289  END IF
290 *
291  work( 1 ) = iws
292  return
293 *
294 * End of DORGRQ
295 *
296  END