# 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