github.com/gonum/lapack@v0.0.0-20181123203213-e4cdc5a0bff9/native/dlanst.go (about)

     1  // Copyright ©2016 The gonum Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package native
     6  
     7  import (
     8  	"math"
     9  
    10  	"github.com/gonum/lapack"
    11  )
    12  
    13  // Dlanst computes the specified norm of a symmetric tridiagonal matrix A.
    14  // The diagonal elements of A are stored in d and the off-diagonal elements
    15  // are stored in e.
    16  func (impl Implementation) Dlanst(norm lapack.MatrixNorm, n int, d, e []float64) float64 {
    17  	if len(d) < n {
    18  		panic(badD)
    19  	}
    20  	if len(e) < n-1 {
    21  		panic(badE)
    22  	}
    23  	if n <= 0 {
    24  		return 0
    25  	}
    26  	switch norm {
    27  	default:
    28  		panic(badNorm)
    29  	case lapack.MaxAbs:
    30  		anorm := math.Abs(d[n-1])
    31  		for i := 0; i < n-1; i++ {
    32  			sum := math.Abs(d[i])
    33  			if anorm < sum || math.IsNaN(sum) {
    34  				anorm = sum
    35  			}
    36  			sum = math.Abs(e[i])
    37  			if anorm < sum || math.IsNaN(sum) {
    38  				anorm = sum
    39  			}
    40  		}
    41  		return anorm
    42  	case lapack.MaxColumnSum, lapack.MaxRowSum:
    43  		if n == 1 {
    44  			return math.Abs(d[0])
    45  		}
    46  		anorm := math.Abs(d[0]) + math.Abs(e[0])
    47  		sum := math.Abs(e[n-2]) + math.Abs(d[n-1])
    48  		if anorm < sum || math.IsNaN(sum) {
    49  			anorm = sum
    50  		}
    51  		for i := 1; i < n-1; i++ {
    52  			sum := math.Abs(d[i]) + math.Abs(e[i]) + math.Abs(e[i-1])
    53  			if anorm < sum || math.IsNaN(sum) {
    54  				anorm = sum
    55  			}
    56  		}
    57  		return anorm
    58  	case lapack.NormFrob:
    59  		var scale float64
    60  		sum := 1.0
    61  		if n > 1 {
    62  			scale, sum = impl.Dlassq(n-1, e, 1, scale, sum)
    63  			sum = 2 * sum
    64  		}
    65  		scale, sum = impl.Dlassq(n, d, 1, scale, sum)
    66  		return scale * math.Sqrt(sum)
    67  	}
    68  }