---
name: "classq.f90"
section: "3"
platform: "linux"
package: "lapack-doc"
version: "3.12.1-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/classq.f90"
---

# classq.f90(3)

## NAME

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

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