# classq.f90 - Man Page

SRC/classq.f90

## Synopsis

### Functions/Subroutines

subroutine **classq** (n, x, incx, scale, sumsq)**CLASSQ** updates a sum of squares represented in scaled form.

## Function/Subroutine Documentation

### subroutine classq (integer n, complex(wp), dimension(*) x, integer incx, real(wp) scale, real(wp) sumsq)

**CLASSQ** updates a sum of squares represented in scaled form.

**Purpose:**

CLASSQ returns the values scale_out and sumsq_out such that (scale_out**2)*sumsq_out = 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. scale and sumsq must be supplied in SCALE and SUMSQ and scale_out and sumsq_out are overwritten on SCALE and SUMSQ respectively.

**Parameters***N*N is INTEGER The number of elements to be used from the vector x.

*X*X is COMPLEX array, dimension (1+(N-1)*abs(INCX)) The vector for which a scaled sum of squares is computed. x( i ) = X( 1 + ( i - 1 )*INCX ), 1 <= i <= n.

*INCX*INCX is INTEGER The increment between successive values of the vector x. If INCX > 0, X(1+(i-1)*INCX) = x(i) for 1 <= i <= n If INCX < 0, X(1-(n-i)*INCX) = x(i) for 1 <= i <= n If INCX = 0, x isn't a vector so there is no need to call this subroutine. If you call it anyway, it will count x(1) in the vector norm N times.

*SCALE*SCALE is REAL On entry, the value scale in the equation above. On exit, SCALE is overwritten by scale_out, the scaling factor for the sum of squares.

*SUMSQ*SUMSQ is REAL On entry, the value sumsq in the equation above. On exit, SUMSQ is overwritten by sumsq_out, the basic sum of squares from which scale_out has been factored out.

**Author**Edward Anderson, Lockheed Martin

**Contributors:**Weslley Pereira, University of Colorado Denver, USA Nick Papior, Technical University of Denmark, DK

**Further Details:**

Anderson E. (2017) Algorithm 978: Safe Scaling in the Level 1 BLAS ACM Trans Math Softw 44:1--28 https://doi.org/10.1145/3061665 Blue, James L. (1978) A Portable Fortran Program to Find the Euclidean Norm of a Vector ACM Trans Math Softw 4:15--23 https://doi.org/10.1145/355769.355771

Definition at line **123** of file **classq.f90**.

## Author

Generated automatically by Doxygen for LAPACK from the source code.

## Referenced By

The man page classq(3) is an alias of classq.f90(3).

Tue Nov 28 2023 12:08:41 Version 3.12.0 LAPACK