LAPACK  3.10.0
LAPACK: Linear Algebra PACKage

◆ dgeqr2p()

subroutine dgeqr2p ( integer  M,
integer  N,
double precision, dimension( lda, * )  A,
integer  LDA,
double precision, dimension( * )  TAU,
double precision, dimension( * )  WORK,
integer  INFO 
)

DGEQR2P computes the QR factorization of a general rectangular matrix with non-negative diagonal elements using an unblocked algorithm.

Download DGEQR2P + dependencies [TGZ] [ZIP] [TXT]

Purpose:
 DGEQR2P computes a QR factorization of a real m-by-n matrix A:

    A = Q * ( R ),
            ( 0 )

 where:

    Q is a m-by-m orthogonal matrix;
    R is an upper-triangular n-by-n matrix with nonnegative diagonal
    entries;
    0 is a (m-n)-by-n zero matrix, if m > n.
Parameters
[in]M
          M is INTEGER
          The number of rows of the matrix A.  M >= 0.
[in]N
          N is INTEGER
          The number of columns of the matrix A.  N >= 0.
[in,out]A
          A is DOUBLE PRECISION array, dimension (LDA,N)
          On entry, the m by n matrix A.
          On exit, the elements on and above the diagonal of the array
          contain the min(m,n) by n upper trapezoidal matrix R (R is
          upper triangular if m >= n). The diagonal entries of R are
          nonnegative; the elements below the diagonal,
          with the array TAU, represent the orthogonal matrix Q as a
          product of elementary reflectors (see Further Details).
[in]LDA
          LDA is INTEGER
          The leading dimension of the array A.  LDA >= max(1,M).
[out]TAU
          TAU is DOUBLE PRECISION array, dimension (min(M,N))
          The scalar factors of the elementary reflectors (see Further
          Details).
[out]WORK
          WORK is DOUBLE PRECISION array, dimension (N)
[out]INFO
          INFO is INTEGER
          = 0: successful exit
          < 0: if INFO = -i, the i-th argument had an illegal value
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Further Details:
  The matrix Q is represented as a product of elementary reflectors

     Q = H(1) H(2) . . . H(k), where k = min(m,n).

  Each H(i) has the form

     H(i) = I - tau * v * v**T

  where tau is a real scalar, and v is a real vector with
  v(1:i-1) = 0 and v(i) = 1; v(i+1:m) is stored on exit in A(i+1:m,i),
  and tau in TAU(i).

 See Lapack Working Note 203 for details

Definition at line 133 of file dgeqr2p.f.

134 *
135 * -- LAPACK computational routine --
136 * -- LAPACK is a software package provided by Univ. of Tennessee, --
137 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
138 *
139 * .. Scalar Arguments ..
140  INTEGER INFO, LDA, M, N
141 * ..
142 * .. Array Arguments ..
143  DOUBLE PRECISION A( LDA, * ), TAU( * ), WORK( * )
144 * ..
145 *
146 * =====================================================================
147 *
148 * .. Parameters ..
149  DOUBLE PRECISION ONE
150  parameter( one = 1.0d+0 )
151 * ..
152 * .. Local Scalars ..
153  INTEGER I, K
154  DOUBLE PRECISION AII
155 * ..
156 * .. External Subroutines ..
157  EXTERNAL dlarf, dlarfgp, xerbla
158 * ..
159 * .. Intrinsic Functions ..
160  INTRINSIC max, min
161 * ..
162 * .. Executable Statements ..
163 *
164 * Test the input arguments
165 *
166  info = 0
167  IF( m.LT.0 ) THEN
168  info = -1
169  ELSE IF( n.LT.0 ) THEN
170  info = -2
171  ELSE IF( lda.LT.max( 1, m ) ) THEN
172  info = -4
173  END IF
174  IF( info.NE.0 ) THEN
175  CALL xerbla( 'DGEQR2P', -info )
176  RETURN
177  END IF
178 *
179  k = min( m, n )
180 *
181  DO 10 i = 1, k
182 *
183 * Generate elementary reflector H(i) to annihilate A(i+1:m,i)
184 *
185  CALL dlarfgp( m-i+1, a( i, i ), a( min( i+1, m ), i ), 1,
186  $ tau( i ) )
187  IF( i.LT.n ) THEN
188 *
189 * Apply H(i) to A(i:m,i+1:n) from the left
190 *
191  aii = a( i, i )
192  a( i, i ) = one
193  CALL dlarf( 'Left', m-i+1, n-i, a( i, i ), 1, tau( i ),
194  $ a( i, i+1 ), lda, work )
195  a( i, i ) = aii
196  END IF
197  10 CONTINUE
198  RETURN
199 *
200 * End of DGEQR2P
201 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
subroutine dlarf(SIDE, M, N, V, INCV, TAU, C, LDC, WORK)
DLARF applies an elementary reflector to a general rectangular matrix.
Definition: dlarf.f:124
subroutine dlarfgp(N, ALPHA, X, INCX, TAU)
DLARFGP generates an elementary reflector (Householder matrix) with non-negative beta.
Definition: dlarfgp.f:104
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