cungr2.f

          SUBROUTINE CUNGR2( M, N, K, A, LDA, TAU, WORK, INFO )
    *
    *  -- LAPACK routine (version 3.1) --
    *     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
    *     November 2006
    *
    *     .. Scalar Arguments ..
          INTEGER            INFO, K, LDA, M, N
    *     ..
    *     .. Array Arguments ..
          COMPLEX            A( LDA, * ), TAU( * ), WORK( * )
    *     ..
    *
    *  Purpose
    *  =======
    *
    *  CUNGR2 generates an m by n complex matrix Q with orthonormal rows,
    *  which is defined as the last m rows of a product of k elementary
    *  reflectors of order n
    *
    *        Q  =  H(1)' H(2)' . . . H(k)'
    *
    *  as returned by CGERQF.
    *
    *  Arguments
    *  =========
    *
    *  M       (input) INTEGER
    *          The number of rows of the matrix Q. M >= 0.
    *
    *  N       (input) INTEGER
    *          The number of columns of the matrix Q. N >= M.
    *
    *  K       (input) INTEGER
    *          The number of elementary reflectors whose product defines the
    *          matrix Q. M >= K >= 0.
    *
    *  A       (input/output) COMPLEX array, dimension (LDA,N)
    *          On entry, the (m-k+i)-th row must contain the vector which
    *          defines the elementary reflector H(i), for i = 1,2,...,k, as
    *          returned by CGERQF in the last k rows of its array argument
    *          A.
    *          On exit, the m-by-n matrix Q.
    *
    *  LDA     (input) INTEGER
    *          The first dimension of the array A. LDA >= max(1,M).
    *
    *  TAU     (input) COMPLEX array, dimension (K)
    *          TAU(i) must contain the scalar factor of the elementary
    *          reflector H(i), as returned by CGERQF.
    *
    *  WORK    (workspace) COMPLEX array, dimension (M)
    *
    *  INFO    (output) INTEGER
    *          = 0: successful exit
    *          < 0: if INFO = -i, the i-th argument has an illegal value
    *
    *  =====================================================================
    *
    *     .. Parameters ..
          COMPLEX            ONE, ZERO
          PARAMETER          ( ONE = ( 1.0E+0, 0.0E+0 ),
         $                   ZERO = ( 0.0E+0, 0.0E+0 ) )
    *     ..
    *     .. Local Scalars ..
          INTEGER            I, II, J, L
    *     ..
    *     .. External Subroutines ..
          EXTERNAL           CLACGV, CLARF, CSCAL, XERBLA
    *     ..
    *     .. Intrinsic Functions ..
          INTRINSIC          CONJG, MAX
    *     ..
    *     .. Executable Statements ..
    *
    *     Test the input arguments
    *
          INFO = 0
          IF( M.LT.0 ) THEN
             INFO = -1
          ELSE IF( N.LT.M ) THEN
             INFO = -2
          ELSE IF( K.LT.0 .OR. K.GT.M ) THEN
             INFO = -3
          ELSE IF( LDA.LT.MAX( 1, M ) ) THEN
             INFO = -5
          END IF
          IF( INFO.NE.0 ) THEN
             CALL XERBLA( 'CUNGR2', -INFO )
             RETURN
          END IF
    *
    *     Quick return if possible
    *
          IF( M.LE.0 )
         $   RETURN
    *
          IF( K.LT.M ) THEN
    *
    *        Initialise rows 1:m-k to rows of the unit matrix
    *
             DO 20 J = 1, N
                DO 10 L = 1, M - K
                   A( L, J ) = ZERO
       10       CONTINUE
                IF( J.GT.N-M .AND. J.LE.N-K )
         $         A( M-N+J, J ) = ONE
       20    CONTINUE
          END IF
    *
          DO 40 I = 1, K
             II = M - K + I
    *
    *        Apply H(i)' to A(1:m-k+i,1:n-k+i) from the right
    *
             CALL CLACGV( N-M+II-1, A( II, 1 ), LDA )
             A( II, N-M+II ) = ONE
             CALL CLARF( 'Right', II-1, N-M+II, A( II, 1 ), LDA,
         $               CONJG( TAU( I ) ), A, LDA, WORK )
             CALL CSCAL( N-M+II-1, -TAU( I ), A( II, 1 ), LDA )
             CALL CLACGV( N-M+II-1, A( II, 1 ), LDA )
             A( II, N-M+II ) = ONE - CONJG( TAU( I ) )
    *
    *        Set A(m-k+i,n-k+i+1:n) to zero
    *
             DO 30 L = N - M + II + 1, N
                A( II, L ) = ZERO
       30    CONTINUE
       40 CONTINUE
          RETURN
    *
    *     End of CUNGR2
    *
          END