LAPACK 3.3.1
Linear Algebra PACKage

slarot.f

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00001       SUBROUTINE SLAROT( LROWS, LLEFT, LRIGHT, NL, C, S, A, LDA, XLEFT,
00002      $                   XRIGHT )
00003 *
00004 *  -- LAPACK auxiliary test routine (version 3.1) --
00005 *     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
00006 *     November 2006
00007 *
00008 *     .. Scalar Arguments ..
00009       LOGICAL            LLEFT, LRIGHT, LROWS
00010       INTEGER            LDA, NL
00011       REAL               C, S, XLEFT, XRIGHT
00012 *     ..
00013 *     .. Array Arguments ..
00014       REAL               A( * )
00015 *     ..
00016 *
00017 *  Purpose
00018 *  =======
00019 *
00020 *     SLAROT applies a (Givens) rotation to two adjacent rows or
00021 *     columns, where one element of the first and/or last column/row
00022 *     for use on matrices stored in some format other than GE, so
00023 *     that elements of the matrix may be used or modified for which
00024 *     no array element is provided.
00025 *
00026 *     One example is a symmetric matrix in SB format (bandwidth=4), for
00027 *     which UPLO='L':  Two adjacent rows will have the format:
00028 *
00029 *     row j:     *  *  *  *  *  .  .  .  .
00030 *     row j+1:      *  *  *  *  *  .  .  .  .
00031 *
00032 *     '*' indicates elements for which storage is provided,
00033 *     '.' indicates elements for which no storage is provided, but
00034 *     are not necessarily zero; their values are determined by
00035 *     symmetry.  ' ' indicates elements which are necessarily zero,
00036 *      and have no storage provided.
00037 *
00038 *     Those columns which have two '*'s can be handled by SROT.
00039 *     Those columns which have no '*'s can be ignored, since as long
00040 *     as the Givens rotations are carefully applied to preserve
00041 *     symmetry, their values are determined.
00042 *     Those columns which have one '*' have to be handled separately,
00043 *     by using separate variables "p" and "q":
00044 *
00045 *     row j:     *  *  *  *  *  p  .  .  .
00046 *     row j+1:   q  *  *  *  *  *  .  .  .  .
00047 *
00048 *     The element p would have to be set correctly, then that column
00049 *     is rotated, setting p to its new value.  The next call to
00050 *     SLAROT would rotate columns j and j+1, using p, and restore
00051 *     symmetry.  The element q would start out being zero, and be
00052 *     made non-zero by the rotation.  Later, rotations would presumably
00053 *     be chosen to zero q out.
00054 *
00055 *     Typical Calling Sequences: rotating the i-th and (i+1)-st rows.
00056 *     ------- ------- ---------
00057 *
00058 *       General dense matrix:
00059 *
00060 *               CALL SLAROT(.TRUE.,.FALSE.,.FALSE., N, C,S,
00061 *                       A(i,1),LDA, DUMMY, DUMMY)
00062 *
00063 *       General banded matrix in GB format:
00064 *
00065 *               j = MAX(1, i-KL )
00066 *               NL = MIN( N, i+KU+1 ) + 1-j
00067 *               CALL SLAROT( .TRUE., i-KL.GE.1, i+KU.LT.N, NL, C,S,
00068 *                       A(KU+i+1-j,j),LDA-1, XLEFT, XRIGHT )
00069 *
00070 *               [ note that i+1-j is just MIN(i,KL+1) ]
00071 *
00072 *       Symmetric banded matrix in SY format, bandwidth K,
00073 *       lower triangle only:
00074 *
00075 *               j = MAX(1, i-K )
00076 *               NL = MIN( K+1, i ) + 1
00077 *               CALL SLAROT( .TRUE., i-K.GE.1, .TRUE., NL, C,S,
00078 *                       A(i,j), LDA, XLEFT, XRIGHT )
00079 *
00080 *       Same, but upper triangle only:
00081 *
00082 *               NL = MIN( K+1, N-i ) + 1
00083 *               CALL SLAROT( .TRUE., .TRUE., i+K.LT.N, NL, C,S,
00084 *                       A(i,i), LDA, XLEFT, XRIGHT )
00085 *
00086 *       Symmetric banded matrix in SB format, bandwidth K,
00087 *       lower triangle only:
00088 *
00089 *               [ same as for SY, except:]
00090 *                   . . . .
00091 *                       A(i+1-j,j), LDA-1, XLEFT, XRIGHT )
00092 *
00093 *               [ note that i+1-j is just MIN(i,K+1) ]
00094 *
00095 *       Same, but upper triangle only:
00096 *                    . . .
00097 *                       A(K+1,i), LDA-1, XLEFT, XRIGHT )
00098 *
00099 *       Rotating columns is just the transpose of rotating rows, except
00100 *       for GB and SB: (rotating columns i and i+1)
00101 *
00102 *       GB:
00103 *               j = MAX(1, i-KU )
00104 *               NL = MIN( N, i+KL+1 ) + 1-j
00105 *               CALL SLAROT( .TRUE., i-KU.GE.1, i+KL.LT.N, NL, C,S,
00106 *                       A(KU+j+1-i,i),LDA-1, XTOP, XBOTTM )
00107 *
00108 *               [note that KU+j+1-i is just MAX(1,KU+2-i)]
00109 *
00110 *       SB: (upper triangle)
00111 *
00112 *                    . . . . . .
00113 *                       A(K+j+1-i,i),LDA-1, XTOP, XBOTTM )
00114 *
00115 *       SB: (lower triangle)
00116 *
00117 *                    . . . . . .
00118 *                       A(1,i),LDA-1, XTOP, XBOTTM )
00119 *
00120 *  Arguments
00121 *  =========
00122 *
00123 *  LROWS  - LOGICAL
00124 *           If .TRUE., then SLAROT will rotate two rows.  If .FALSE.,
00125 *           then it will rotate two columns.
00126 *           Not modified.
00127 *
00128 *  LLEFT  - LOGICAL
00129 *           If .TRUE., then XLEFT will be used instead of the
00130 *           corresponding element of A for the first element in the
00131 *           second row (if LROWS=.FALSE.) or column (if LROWS=.TRUE.)
00132 *           If .FALSE., then the corresponding element of A will be
00133 *           used.
00134 *           Not modified.
00135 *
00136 *  LRIGHT - LOGICAL
00137 *           If .TRUE., then XRIGHT will be used instead of the
00138 *           corresponding element of A for the last element in the
00139 *           first row (if LROWS=.FALSE.) or column (if LROWS=.TRUE.) If
00140 *           .FALSE., then the corresponding element of A will be used.
00141 *           Not modified.
00142 *
00143 *  NL     - INTEGER
00144 *           The length of the rows (if LROWS=.TRUE.) or columns (if
00145 *           LROWS=.FALSE.) to be rotated.  If XLEFT and/or XRIGHT are
00146 *           used, the columns/rows they are in should be included in
00147 *           NL, e.g., if LLEFT = LRIGHT = .TRUE., then NL must be at
00148 *           least 2.  The number of rows/columns to be rotated
00149 *           exclusive of those involving XLEFT and/or XRIGHT may
00150 *           not be negative, i.e., NL minus how many of LLEFT and
00151 *           LRIGHT are .TRUE. must be at least zero; if not, XERBLA
00152 *           will be called.
00153 *           Not modified.
00154 *
00155 *  C, S   - REAL
00156 *           Specify the Givens rotation to be applied.  If LROWS is
00157 *           true, then the matrix ( c  s )
00158 *                                 (-s  c )  is applied from the left;
00159 *           if false, then the transpose thereof is applied from the
00160 *           right.  For a Givens rotation, C**2 + S**2 should be 1,
00161 *           but this is not checked.
00162 *           Not modified.
00163 *
00164 *  A      - REAL array.
00165 *           The array containing the rows/columns to be rotated.  The
00166 *           first element of A should be the upper left element to
00167 *           be rotated.
00168 *           Read and modified.
00169 *
00170 *  LDA    - INTEGER
00171 *           The "effective" leading dimension of A.  If A contains
00172 *           a matrix stored in GE or SY format, then this is just
00173 *           the leading dimension of A as dimensioned in the calling
00174 *           routine.  If A contains a matrix stored in band (GB or SB)
00175 *           format, then this should be *one less* than the leading
00176 *           dimension used in the calling routine.  Thus, if
00177 *           A were dimensioned A(LDA,*) in SLAROT, then A(1,j) would
00178 *           be the j-th element in the first of the two rows
00179 *           to be rotated, and A(2,j) would be the j-th in the second,
00180 *           regardless of how the array may be stored in the calling
00181 *           routine.  [A cannot, however, actually be dimensioned thus,
00182 *           since for band format, the row number may exceed LDA, which
00183 *           is not legal FORTRAN.]
00184 *           If LROWS=.TRUE., then LDA must be at least 1, otherwise
00185 *           it must be at least NL minus the number of .TRUE. values
00186 *           in XLEFT and XRIGHT.
00187 *           Not modified.
00188 *
00189 *  XLEFT  - REAL
00190 *           If LLEFT is .TRUE., then XLEFT will be used and modified
00191 *           instead of A(2,1) (if LROWS=.TRUE.) or A(1,2)
00192 *           (if LROWS=.FALSE.).
00193 *           Read and modified.
00194 *
00195 *  XRIGHT - REAL
00196 *           If LRIGHT is .TRUE., then XRIGHT will be used and modified
00197 *           instead of A(1,NL) (if LROWS=.TRUE.) or A(NL,1)
00198 *           (if LROWS=.FALSE.).
00199 *           Read and modified.
00200 *
00201 *  =====================================================================
00202 *
00203 *     .. Local Scalars ..
00204       INTEGER            IINC, INEXT, IX, IY, IYT, NT
00205 *     ..
00206 *     .. Local Arrays ..
00207       REAL               XT( 2 ), YT( 2 )
00208 *     ..
00209 *     .. External Subroutines ..
00210       EXTERNAL           SROT, XERBLA
00211 *     ..
00212 *     .. Executable Statements ..
00213 *
00214 *     Set up indices, arrays for ends
00215 *
00216       IF( LROWS ) THEN
00217          IINC = LDA
00218          INEXT = 1
00219       ELSE
00220          IINC = 1
00221          INEXT = LDA
00222       END IF
00223 *
00224       IF( LLEFT ) THEN
00225          NT = 1
00226          IX = 1 + IINC
00227          IY = 2 + LDA
00228          XT( 1 ) = A( 1 )
00229          YT( 1 ) = XLEFT
00230       ELSE
00231          NT = 0
00232          IX = 1
00233          IY = 1 + INEXT
00234       END IF
00235 *
00236       IF( LRIGHT ) THEN
00237          IYT = 1 + INEXT + ( NL-1 )*IINC
00238          NT = NT + 1
00239          XT( NT ) = XRIGHT
00240          YT( NT ) = A( IYT )
00241       END IF
00242 *
00243 *     Check for errors
00244 *
00245       IF( NL.LT.NT ) THEN
00246          CALL XERBLA( 'SLAROT', 4 )
00247          RETURN
00248       END IF
00249       IF( LDA.LE.0 .OR. ( .NOT.LROWS .AND. LDA.LT.NL-NT ) ) THEN
00250          CALL XERBLA( 'SLAROT', 8 )
00251          RETURN
00252       END IF
00253 *
00254 *     Rotate
00255 *
00256       CALL SROT( NL-NT, A( IX ), IINC, A( IY ), IINC, C, S )
00257       CALL SROT( NT, XT, 1, YT, 1, C, S )
00258 *
00259 *     Stuff values back into XLEFT, XRIGHT, etc.
00260 *
00261       IF( LLEFT ) THEN
00262          A( 1 ) = XT( 1 )
00263          XLEFT = YT( 1 )
00264       END IF
00265 *
00266       IF( LRIGHT ) THEN
00267          XRIGHT = XT( NT )
00268          A( IYT ) = YT( NT )
00269       END IF
00270 *
00271       RETURN
00272 *
00273 *     End of SLAROT
00274 *
00275       END
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