 LAPACK  3.8.0 LAPACK: Linear Algebra PACKage

## ◆ sgeequ()

 subroutine sgeequ ( integer M, integer N, real, dimension( lda, * ) A, integer LDA, real, dimension( * ) R, real, dimension( * ) C, real ROWCND, real COLCND, real AMAX, integer INFO )

SGEEQU

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

Purpose:
``` SGEEQU computes row and column scalings intended to equilibrate an
M-by-N matrix A and reduce its condition number.  R returns the row
scale factors and C the column scale factors, chosen to try to make
the largest element in each row and column of the matrix B with
elements B(i,j)=R(i)*A(i,j)*C(j) have absolute value 1.

R(i) and C(j) are restricted to be between SMLNUM = smallest safe
number and BIGNUM = largest safe number.  Use of these scaling
factors is not guaranteed to reduce the condition number of A but
works well in practice.```
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] A ``` A is REAL array, dimension (LDA,N) The M-by-N matrix whose equilibration factors are to be computed.``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,M).``` [out] R ``` R is REAL array, dimension (M) If INFO = 0 or INFO > M, R contains the row scale factors for A.``` [out] C ``` C is REAL array, dimension (N) If INFO = 0, C contains the column scale factors for A.``` [out] ROWCND ``` ROWCND is REAL If INFO = 0 or INFO > M, ROWCND contains the ratio of the smallest R(i) to the largest R(i). If ROWCND >= 0.1 and AMAX is neither too large nor too small, it is not worth scaling by R.``` [out] COLCND ``` COLCND is REAL If INFO = 0, COLCND contains the ratio of the smallest C(i) to the largest C(i). If COLCND >= 0.1, it is not worth scaling by C.``` [out] AMAX ``` AMAX is REAL Absolute value of largest matrix element. If AMAX is very close to overflow or very close to underflow, the matrix should be scaled.``` [out] INFO ``` INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value > 0: if INFO = i, and i is <= M: the i-th row of A is exactly zero > M: the (i-M)-th column of A is exactly zero```
Date
December 2016

Definition at line 141 of file sgeequ.f.

141 *
142 * -- LAPACK computational routine (version 3.7.0) --
143 * -- LAPACK is a software package provided by Univ. of Tennessee, --
144 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
145 * December 2016
146 *
147 * .. Scalar Arguments ..
148  INTEGER info, lda, m, n
149  REAL amax, colcnd, rowcnd
150 * ..
151 * .. Array Arguments ..
152  REAL a( lda, * ), c( * ), r( * )
153 * ..
154 *
155 * =====================================================================
156 *
157 * .. Parameters ..
158  REAL one, zero
159  parameter( one = 1.0e+0, zero = 0.0e+0 )
160 * ..
161 * .. Local Scalars ..
162  INTEGER i, j
163  REAL bignum, rcmax, rcmin, smlnum
164 * ..
165 * .. External Functions ..
166  REAL slamch
167  EXTERNAL slamch
168 * ..
169 * .. External Subroutines ..
170  EXTERNAL xerbla
171 * ..
172 * .. Intrinsic Functions ..
173  INTRINSIC abs, max, min
174 * ..
175 * .. Executable Statements ..
176 *
177 * Test the input parameters.
178 *
179  info = 0
180  IF( m.LT.0 ) THEN
181  info = -1
182  ELSE IF( n.LT.0 ) THEN
183  info = -2
184  ELSE IF( lda.LT.max( 1, m ) ) THEN
185  info = -4
186  END IF
187  IF( info.NE.0 ) THEN
188  CALL xerbla( 'SGEEQU', -info )
189  RETURN
190  END IF
191 *
192 * Quick return if possible
193 *
194  IF( m.EQ.0 .OR. n.EQ.0 ) THEN
195  rowcnd = one
196  colcnd = one
197  amax = zero
198  RETURN
199  END IF
200 *
201 * Get machine constants.
202 *
203  smlnum = slamch( 'S' )
204  bignum = one / smlnum
205 *
206 * Compute row scale factors.
207 *
208  DO 10 i = 1, m
209  r( i ) = zero
210  10 CONTINUE
211 *
212 * Find the maximum element in each row.
213 *
214  DO 30 j = 1, n
215  DO 20 i = 1, m
216  r( i ) = max( r( i ), abs( a( i, j ) ) )
217  20 CONTINUE
218  30 CONTINUE
219 *
220 * Find the maximum and minimum scale factors.
221 *
222  rcmin = bignum
223  rcmax = zero
224  DO 40 i = 1, m
225  rcmax = max( rcmax, r( i ) )
226  rcmin = min( rcmin, r( i ) )
227  40 CONTINUE
228  amax = rcmax
229 *
230  IF( rcmin.EQ.zero ) THEN
231 *
232 * Find the first zero scale factor and return an error code.
233 *
234  DO 50 i = 1, m
235  IF( r( i ).EQ.zero ) THEN
236  info = i
237  RETURN
238  END IF
239  50 CONTINUE
240  ELSE
241 *
242 * Invert the scale factors.
243 *
244  DO 60 i = 1, m
245  r( i ) = one / min( max( r( i ), smlnum ), bignum )
246  60 CONTINUE
247 *
248 * Compute ROWCND = min(R(I)) / max(R(I))
249 *
250  rowcnd = max( rcmin, smlnum ) / min( rcmax, bignum )
251  END IF
252 *
253 * Compute column scale factors
254 *
255  DO 70 j = 1, n
256  c( j ) = zero
257  70 CONTINUE
258 *
259 * Find the maximum element in each column,
260 * assuming the row scaling computed above.
261 *
262  DO 90 j = 1, n
263  DO 80 i = 1, m
264  c( j ) = max( c( j ), abs( a( i, j ) )*r( i ) )
265  80 CONTINUE
266  90 CONTINUE
267 *
268 * Find the maximum and minimum scale factors.
269 *
270  rcmin = bignum
271  rcmax = zero
272  DO 100 j = 1, n
273  rcmin = min( rcmin, c( j ) )
274  rcmax = max( rcmax, c( j ) )
275  100 CONTINUE
276 *
277  IF( rcmin.EQ.zero ) THEN
278 *
279 * Find the first zero scale factor and return an error code.
280 *
281  DO 110 j = 1, n
282  IF( c( j ).EQ.zero ) THEN
283  info = m + j
284  RETURN
285  END IF
286  110 CONTINUE
287  ELSE
288 *
289 * Invert the scale factors.
290 *
291  DO 120 j = 1, n
292  c( j ) = one / min( max( c( j ), smlnum ), bignum )
293  120 CONTINUE
294 *
295 * Compute COLCND = min(C(J)) / max(C(J))
296 *
297  colcnd = max( rcmin, smlnum ) / min( rcmax, bignum )
298  END IF
299 *
300  RETURN
301 *
302 * End of SGEEQU
303 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:62
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:69
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