TESTING/MATGEN/slarot.f(3)

💖 NAME 💖

TESTING/MATGEN/slarot.f — that's the name, bestie. Short, cryptic, iconic. 🎀

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🦋 SYNOPSIS 🦋

✨ Functions/Subroutines ✨

subroutine slarot (lrows, lleft, lright, nl, c, s, a, lda, xleft,
    xright)

SLAROT

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✨ Function/Subroutine Documentation ✨

Here's the full signature in all its glory:

subroutine slarot (logical lrows, logical lleft, logical lright, integer nl,
    real c, real s, real, dimension( * ) a, integer lda, real xleft, real
    xright)

SLAROT

💅 Purpose 💅

Okay so SLAROT applies a Givens rotation to two adjacent rows or columns. It's built for matrices stored in some format other than GE, so it can use or modify matrix elements for which no array element is provided — one element of the first and/or last column/row gets special handling. 💅

One example is a symmetric matrix in SB format (bandwidth=4), for which UPLO='L'. Two adjacent rows will have this format:

row j:     C> C> C> C> C> .  .  .  .
row j+1:      C> C> C> C> C> .  .  .  .

Quick legend, because this diagram is doing a lot: '*' indicates elements for which storage is provided, '.' indicates elements for which no storage is provided but are not necessarily zero — their values are determined by symmetry. ' ' indicates elements which are necessarily zero, and have no storage provided. 🫶

Those columns which have two '*'s can be handled by SROT. Those columns which have no '*'s can be ignored, since as long as the Givens rotations are carefully applied to preserve symmetry, their values are determined. Those columns which have one '*' have to be handled separately, by using separate variables and :

row j:     C> C> C> C> C> p  .  .  .
row j+1:   q  C> C> C> C> C> .  .  .  .

The element p would have to be set correctly, then that column is rotated, setting p to its new value. The next call to SLAROT would rotate columns j and j+1, using p, and restore symmetry. The element q would start out being zero, and be made non-zero by the rotation. Later, rotations would presumably be chosen to zero q out. 😭

Typical Calling Sequences: rotating the i-th and (i+1)-st rows. The original's little underline situation is ------- ------- ---------, and honestly? Slay.

General dense matrix:

CALL SLAROT(.TRUE.,.FALSE.,.FALSE., N, C,S,
        A(i,1),LDA, DUMMY, DUMMY)

General banded matrix in GB format:

j = MAX(1, i-KL )
NL = MIN( N, i+KU+1 ) + 1-j
CALL SLAROT( .TRUE., i-KL.GE.1, i+KU.LT.N, NL, C,S,
        A(KU+i+1-j,j),LDA-1, XLEFT, XRIGHT )

[ note that i+1-j is just MIN(i,KL+1) ]

Symmetric banded matrix in SY format, bandwidth K, lower triangle only:

j = MAX(1, i-K )
NL = MIN( K+1, i ) + 1
CALL SLAROT( .TRUE., i-K.GE.1, .TRUE., NL, C,S,
        A(i,j), LDA, XLEFT, XRIGHT )

Same, but upper triangle only:

NL = MIN( K+1, N-i ) + 1
CALL SLAROT( .TRUE., .TRUE., i+K.LT.N, NL, C,S,
        A(i,i), LDA, XLEFT, XRIGHT )

Symmetric banded matrix in SB format, bandwidth K, lower triangle only:

[ same as for SY, except:]
    . . . .
        A(i+1-j,j), LDA-1, XLEFT, XRIGHT )

[ note that i+1-j is just MIN(i,K+1) ]

Same, but upper triangle only:

     . . .
        A(K+1,i), LDA-1, XLEFT, XRIGHT )

Rotating columns is just the transpose of rotating rows, except for GB and SB: (rotating columns i and i+1)

GB:

j = MAX(1, i-KU )
NL = MIN( N, i+KL+1 ) + 1-j
CALL SLAROT( .TRUE., i-KU.GE.1, i+KL.LT.N, NL, C,S,
        A(KU+j+1-i,i),LDA-1, XTOP, XBOTTM )

[note that KU+j+1-i is just MAX(1,KU+2-i)]

SB: (upper triangle)

     . . . . . .
        A(K+j+1-i,i),LDA-1, XTOP, XBOTTM )

SB: (lower triangle)

     . . . . . .
        A(1,i),LDA-1, XTOP, XBOTTM )

Now the args, bestie — every single one, no skipping:

• LROWS — LOGICAL. If .TRUE., then SLAROT will rotate two rows. If .FALSE., then it will rotate two columns. Not modified.
• LLEFT — LOGICAL. If .TRUE., then XLEFT will be used instead of the corresponding element of A for the first element in the second row (if LROWS=.FALSE.) or column (if LROWS=.TRUE.). If .FALSE., then the corresponding element of A will be used. Not modified.
• LRIGHT — LOGICAL. If .TRUE., then XRIGHT will be used instead of the corresponding element of A for the last element in the first row (if LROWS=.FALSE.) or column (if LROWS=.TRUE.). If .FALSE., then the corresponding element of A will be used. Not modified.
• NL — INTEGER. The length of the rows (if LROWS=.TRUE.) or columns (if LROWS=.FALSE.) to be rotated. If XLEFT and/or XRIGHT are used, the columns/rows they are in should be included in NL, e.g., if LLEFT = LRIGHT = .TRUE., then NL must be at least 2. The number of rows/columns to be rotated exclusive of those involving XLEFT and/or XRIGHT may not be negative, i.e., NL minus how many of LLEFT and LRIGHT are .TRUE. must be at least zero; if not, XERBLA will be called. Not modified.
• C, S — REAL. They specify the Givens rotation to be applied. If LROWS is true, then the matrix:

( c  s )
(-s  c )

is applied from the left; if false, then the transpose thereof is applied from the right. For a Givens rotation, C**2 + S**2 should be 1, but this is not checked. Not modified.

• A — REAL array. The array containing the rows/columns to be rotated. The first element of A should be the upper left element to be rotated. Read and modified.
• LDA — INTEGER. The leading dimension of A. If A contains a matrix stored in GE or SY format, then this is just the leading dimension of A as dimensioned in the calling routine. If A contains a matrix stored in band (GB or SB) format, then this should be one less than the leading dimension used in the calling routine. Thus, if A were dimensioned A(LDA,*) in SLAROT, then A(1,j) would be the j-th element in the first of the two rows to be rotated, and A(2,j) would be the j-th in the second, regardless of how the array may be stored in the calling routine.

bestie, important caveat: [A cannot, however, actually be dimensioned thus, since for band format, the row number may exceed LDA, which is not legal FORTRAN.]

If LROWS=.TRUE., then LDA must be at least 1, otherwise it must be at least NL minus the number of .TRUE. values in XLEFT and XRIGHT. Not modified.

• XLEFT — REAL. If LLEFT is .TRUE., then XLEFT will be used and modified instead of A(2,1) (if LROWS=.TRUE.) or A(1,2) (if LROWS=.FALSE.). Read and modified.
• XRIGHT — REAL. If LRIGHT is .TRUE., then XRIGHT will be used and modified instead of A(1,NL) (if LROWS=.TRUE.) or A(NL,1) (if LROWS=.FALSE.). Read and modified.

🫶 Author 🫶

• Univ. of Tennessee
• Univ. of California Berkeley
• Univ. of Colorado Denver
• NAG Ltd.

🎀 Definition 🎀

Definition at line 224 of file slarot.f.

🧚 Author 🧚

Generated automatically by Doxygen for LAPACK from the source code.

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