 LAPACK  3.8.0 LAPACK: Linear Algebra PACKage

## ◆ slassq()

 subroutine slassq ( integer N, real, dimension( * ) X, integer INCX, real SCALE, real SUMSQ )

SLASSQ updates a sum of squares represented in scaled form.

Purpose:
``` SLASSQ  returns the values  scl  and  smsq  such that

( scl**2 )*smsq = x( 1 )**2 +...+ x( n )**2 + ( scale**2 )*sumsq,

where  x( i ) = X( 1 + ( i - 1 )*INCX ). The value of  sumsq  is
assumed to be non-negative and  scl  returns the value

scl = max( scale, abs( x( i ) ) ).

scale and sumsq must be supplied in SCALE and SUMSQ and
scl and smsq are overwritten on SCALE and SUMSQ respectively.

The routine makes only one pass through the vector x.```
Parameters
 [in] N ``` N is INTEGER The number of elements to be used from the vector X.``` [in] X ``` X is REAL array, dimension (N) The vector for which a scaled sum of squares is computed. x( i ) = X( 1 + ( i - 1 )*INCX ), 1 <= i <= n.``` [in] INCX ``` INCX is INTEGER The increment between successive values of the vector X. INCX > 0.``` [in,out] SCALE ``` SCALE is REAL On entry, the value scale in the equation above. On exit, SCALE is overwritten with scl , the scaling factor for the sum of squares.``` [in,out] SUMSQ ``` SUMSQ is REAL On entry, the value sumsq in the equation above. On exit, SUMSQ is overwritten with smsq , the basic sum of squares from which scl has been factored out.```
Date
December 2016

Definition at line 105 of file slassq.f.

105 *
106 * -- LAPACK auxiliary routine (version 3.7.0) --
107 * -- LAPACK is a software package provided by Univ. of Tennessee, --
108 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
109 * December 2016
110 *
111 * .. Scalar Arguments ..
112  INTEGER incx, n
113  REAL scale, sumsq
114 * ..
115 * .. Array Arguments ..
116  REAL x( * )
117 * ..
118 *
119 * =====================================================================
120 *
121 * .. Parameters ..
122  REAL zero
123  parameter( zero = 0.0e+0 )
124 * ..
125 * .. Local Scalars ..
126  INTEGER ix
127  REAL absxi
128 * ..
129 * .. External Functions ..
130  LOGICAL sisnan
131  EXTERNAL sisnan
132 * ..
133 * .. Intrinsic Functions ..
134  INTRINSIC abs
135 * ..
136 * .. Executable Statements ..
137 *
138  IF( n.GT.0 ) THEN
139  DO 10 ix = 1, 1 + ( n-1 )*incx, incx
140  absxi = abs( x( ix ) )
141  IF( absxi.GT.zero.OR.sisnan( absxi ) ) THEN
142  IF( scale.LT.absxi ) THEN
143  sumsq = 1 + sumsq*( scale / absxi )**2
144  scale = absxi
145  ELSE
146  sumsq = sumsq + ( absxi / scale )**2
147  END IF
148  END IF
149  10 CONTINUE
150  END IF
151  RETURN
152 *
153 * End of SLASSQ
154 *
logical function sisnan(SIN)
SISNAN tests input for NaN.
Definition: sisnan.f:61
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