 LAPACK 3.11.0 LAPACK: Linear Algebra PACKage
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## ◆ slaror()

 subroutine slaror ( character SIDE, character INIT, integer M, integer N, real, dimension( lda, * ) A, integer LDA, integer, dimension( 4 ) ISEED, real, dimension( * ) X, integer INFO )

SLAROR

Purpose:
``` SLAROR pre- or post-multiplies an M by N matrix A by a random
orthogonal matrix U, overwriting A.  A may optionally be initialized
to the identity matrix before multiplying by U.  U is generated using
the method of G.W. Stewart (SIAM J. Numer. Anal. 17, 1980, 403-409).```
Parameters
 [in] SIDE ``` SIDE is CHARACTER*1 Specifies whether A is multiplied on the left or right by U. = 'L': Multiply A on the left (premultiply) by U = 'R': Multiply A on the right (postmultiply) by U' = 'C' or 'T': Multiply A on the left by U and the right by U' (Here, U' means U-transpose.)``` [in] INIT ``` INIT is CHARACTER*1 Specifies whether or not A should be initialized to the identity matrix. = 'I': Initialize A to (a section of) the identity matrix before applying U. = 'N': No initialization. Apply U to the input matrix A. INIT = 'I' may be used to generate square or rectangular orthogonal matrices: For M = N and SIDE = 'L' or 'R', the rows will be orthogonal to each other, as will the columns. If M < N, SIDE = 'R' produces a dense matrix whose rows are orthogonal and whose columns are not, while SIDE = 'L' produces a matrix whose rows are orthogonal, and whose first M columns are orthogonal, and whose remaining columns are zero. If M > N, SIDE = 'L' produces a dense matrix whose columns are orthogonal and whose rows are not, while SIDE = 'R' produces a matrix whose columns are orthogonal, and whose first M rows are orthogonal, and whose remaining rows are zero.``` [in] M ``` M is INTEGER The number of rows of A.``` [in] N ``` N is INTEGER The number of columns of A.``` [in,out] A ``` A is REAL array, dimension (LDA, N) On entry, the array A. On exit, overwritten by U A ( if SIDE = 'L' ), or by A U ( if SIDE = 'R' ), or by U A U' ( if SIDE = 'C' or 'T').``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,M).``` [in,out] ISEED ``` ISEED is INTEGER array, dimension (4) On entry ISEED specifies the seed of the random number generator. The array elements should be between 0 and 4095; if not they will be reduced mod 4096. Also, ISEED(4) must be odd. The random number generator uses a linear congruential sequence limited to small integers, and so should produce machine independent random numbers. The values of ISEED are changed on exit, and can be used in the next call to SLAROR to continue the same random number sequence.``` [out] X ``` X is REAL array, dimension (3*MAX( M, N )) Workspace of length 2*M + N if SIDE = 'L', 2*N + M if SIDE = 'R', 3*N if SIDE = 'C' or 'T'.``` [out] INFO ``` INFO is INTEGER An error flag. It is set to: = 0: normal return < 0: if INFO = -k, the k-th argument had an illegal value = 1: if the random numbers generated by SLARND are bad.```

Definition at line 145 of file slaror.f.

146*
147* -- LAPACK auxiliary routine --
148* -- LAPACK is a software package provided by Univ. of Tennessee, --
149* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
150*
151* .. Scalar Arguments ..
152 CHARACTER INIT, SIDE
153 INTEGER INFO, LDA, M, N
154* ..
155* .. Array Arguments ..
156 INTEGER ISEED( 4 )
157 REAL A( LDA, * ), X( * )
158* ..
159*
160* =====================================================================
161*
162* .. Parameters ..
163 REAL ZERO, ONE, TOOSML
164 parameter( zero = 0.0e+0, one = 1.0e+0,
165 \$ toosml = 1.0e-20 )
166* ..
167* .. Local Scalars ..
168 INTEGER IROW, ITYPE, IXFRM, J, JCOL, KBEG, NXFRM
169 REAL FACTOR, XNORM, XNORMS
170* ..
171* .. External Functions ..
172 LOGICAL LSAME
173 REAL SLARND, SNRM2
174 EXTERNAL lsame, slarnd, snrm2
175* ..
176* .. External Subroutines ..
177 EXTERNAL sgemv, sger, slaset, sscal, xerbla
178* ..
179* .. Intrinsic Functions ..
180 INTRINSIC abs, sign
181* ..
182* .. Executable Statements ..
183*
184 info = 0
185 IF( n.EQ.0 .OR. m.EQ.0 )
186 \$ RETURN
187*
188 itype = 0
189 IF( lsame( side, 'L' ) ) THEN
190 itype = 1
191 ELSE IF( lsame( side, 'R' ) ) THEN
192 itype = 2
193 ELSE IF( lsame( side, 'C' ) .OR. lsame( side, 'T' ) ) THEN
194 itype = 3
195 END IF
196*
197* Check for argument errors.
198*
199 IF( itype.EQ.0 ) THEN
200 info = -1
201 ELSE IF( m.LT.0 ) THEN
202 info = -3
203 ELSE IF( n.LT.0 .OR. ( itype.EQ.3 .AND. n.NE.m ) ) THEN
204 info = -4
205 ELSE IF( lda.LT.m ) THEN
206 info = -6
207 END IF
208 IF( info.NE.0 ) THEN
209 CALL xerbla( 'SLAROR', -info )
210 RETURN
211 END IF
212*
213 IF( itype.EQ.1 ) THEN
214 nxfrm = m
215 ELSE
216 nxfrm = n
217 END IF
218*
219* Initialize A to the identity matrix if desired
220*
221 IF( lsame( init, 'I' ) )
222 \$ CALL slaset( 'Full', m, n, zero, one, a, lda )
223*
224* If no rotation possible, multiply by random +/-1
225*
226* Compute rotation by computing Householder transformations
227* H(2), H(3), ..., H(nhouse)
228*
229 DO 10 j = 1, nxfrm
230 x( j ) = zero
231 10 CONTINUE
232*
233 DO 30 ixfrm = 2, nxfrm
234 kbeg = nxfrm - ixfrm + 1
235*
236* Generate independent normal( 0, 1 ) random numbers
237*
238 DO 20 j = kbeg, nxfrm
239 x( j ) = slarnd( 3, iseed )
240 20 CONTINUE
241*
242* Generate a Householder transformation from the random vector X
243*
244 xnorm = snrm2( ixfrm, x( kbeg ), 1 )
245 xnorms = sign( xnorm, x( kbeg ) )
246 x( kbeg+nxfrm ) = sign( one, -x( kbeg ) )
247 factor = xnorms*( xnorms+x( kbeg ) )
248 IF( abs( factor ).LT.toosml ) THEN
249 info = 1
250 CALL xerbla( 'SLAROR', info )
251 RETURN
252 ELSE
253 factor = one / factor
254 END IF
255 x( kbeg ) = x( kbeg ) + xnorms
256*
257* Apply Householder transformation to A
258*
259 IF( itype.EQ.1 .OR. itype.EQ.3 ) THEN
260*
261* Apply H(k) from the left.
262*
263 CALL sgemv( 'T', ixfrm, n, one, a( kbeg, 1 ), lda,
264 \$ x( kbeg ), 1, zero, x( 2*nxfrm+1 ), 1 )
265 CALL sger( ixfrm, n, -factor, x( kbeg ), 1, x( 2*nxfrm+1 ),
266 \$ 1, a( kbeg, 1 ), lda )
267*
268 END IF
269*
270 IF( itype.EQ.2 .OR. itype.EQ.3 ) THEN
271*
272* Apply H(k) from the right.
273*
274 CALL sgemv( 'N', m, ixfrm, one, a( 1, kbeg ), lda,
275 \$ x( kbeg ), 1, zero, x( 2*nxfrm+1 ), 1 )
276 CALL sger( m, ixfrm, -factor, x( 2*nxfrm+1 ), 1, x( kbeg ),
277 \$ 1, a( 1, kbeg ), lda )
278*
279 END IF
280 30 CONTINUE
281*
282 x( 2*nxfrm ) = sign( one, slarnd( 3, iseed ) )
283*
284* Scale the matrix A by D.
285*
286 IF( itype.EQ.1 .OR. itype.EQ.3 ) THEN
287 DO 40 irow = 1, m
288 CALL sscal( n, x( nxfrm+irow ), a( irow, 1 ), lda )
289 40 CONTINUE
290 END IF
291*
292 IF( itype.EQ.2 .OR. itype.EQ.3 ) THEN
293 DO 50 jcol = 1, n
294 CALL sscal( m, x( nxfrm+jcol ), a( 1, jcol ), 1 )
295 50 CONTINUE
296 END IF
297 RETURN
298*
299* End of SLAROR
300*
subroutine slaset(UPLO, M, N, ALPHA, BETA, A, LDA)
SLASET initializes the off-diagonal elements and the diagonal elements of a matrix to given values.
Definition: slaset.f:110
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
real function slarnd(IDIST, ISEED)
SLARND
Definition: slarnd.f:73
subroutine sscal(N, SA, SX, INCX)
SSCAL
Definition: sscal.f:79
real(wp) function snrm2(n, x, incx)
SNRM2
Definition: snrm2.f90:89
subroutine sger(M, N, ALPHA, X, INCX, Y, INCY, A, LDA)
SGER
Definition: sger.f:130
subroutine sgemv(TRANS, M, N, ALPHA, A, LDA, X, INCX, BETA, Y, INCY)
SGEMV
Definition: sgemv.f:156
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