forked from lijiext/lammps
90 lines
2.6 KiB
Fortran
90 lines
2.6 KiB
Fortran
SUBROUTINE DLASSQ( N, X, INCX, SCALE, SUMSQ )
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*
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* -- LAPACK auxiliary routine (version 3.2) --
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* -- LAPACK is a software package provided by Univ. of Tennessee, --
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* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
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* November 2006
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*
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* .. Scalar Arguments ..
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INTEGER INCX, N
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DOUBLE PRECISION SCALE, SUMSQ
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* ..
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* .. Array Arguments ..
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DOUBLE PRECISION X( * )
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* ..
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*
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* Purpose
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* =======
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*
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* DLASSQ returns the values scl and smsq such that
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*
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* ( scl**2 )*smsq = x( 1 )**2 +...+ x( n )**2 + ( scale**2 )*sumsq,
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*
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* where x( i ) = X( 1 + ( i - 1 )*INCX ). The value of sumsq is
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* assumed to be non-negative and scl returns the value
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*
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* scl = max( scale, abs( x( i ) ) ).
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*
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* scale and sumsq must be supplied in SCALE and SUMSQ and
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* scl and smsq are overwritten on SCALE and SUMSQ respectively.
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*
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* The routine makes only one pass through the vector x.
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*
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* Arguments
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* =========
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*
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* N (input) INTEGER
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* The number of elements to be used from the vector X.
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*
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* X (input) DOUBLE PRECISION array, dimension (N)
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* The vector for which a scaled sum of squares is computed.
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* x( i ) = X( 1 + ( i - 1 )*INCX ), 1 <= i <= n.
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*
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* INCX (input) INTEGER
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* The increment between successive values of the vector X.
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* INCX > 0.
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*
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* SCALE (input/output) DOUBLE PRECISION
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* On entry, the value scale in the equation above.
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* On exit, SCALE is overwritten with scl , the scaling factor
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* for the sum of squares.
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*
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* SUMSQ (input/output) DOUBLE PRECISION
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* On entry, the value sumsq in the equation above.
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* On exit, SUMSQ is overwritten with smsq , the basic sum of
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* squares from which scl has been factored out.
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*
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* =====================================================================
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*
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* .. Parameters ..
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DOUBLE PRECISION ZERO
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PARAMETER ( ZERO = 0.0D+0 )
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* ..
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* .. Local Scalars ..
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INTEGER IX
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DOUBLE PRECISION ABSXI
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* ..
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* .. Intrinsic Functions ..
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INTRINSIC ABS
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* ..
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* .. Executable Statements ..
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*
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IF( N.GT.0 ) THEN
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DO 10 IX = 1, 1 + ( N-1 )*INCX, INCX
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IF( X( IX ).NE.ZERO ) THEN
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ABSXI = ABS( X( IX ) )
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IF( SCALE.LT.ABSXI ) THEN
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SUMSQ = 1 + SUMSQ*( SCALE / ABSXI )**2
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SCALE = ABSXI
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ELSE
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SUMSQ = SUMSQ + ( ABSXI / SCALE )**2
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END IF
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END IF
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10 CONTINUE
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END IF
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RETURN
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*
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* End of DLASSQ
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*
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END
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