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55 lines
1.2 KiB
Go
55 lines
1.2 KiB
Go
// Copyright ©2014 The gonum Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package stat
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import (
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"github.com/gonum/matrix/mat64"
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)
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// CovarianceMatrix calculates a covariance matrix (also known as a
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// variance-covariance matrix) from a matrix of data.
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func CovarianceMatrix(x mat64.Matrix) *mat64.Dense {
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// matrix version of the two pass algorithm. This doesn't use
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// the correction found in the Covariance and Variance functions.
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r, _ := x.Dims()
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// determine the mean of each of the columns
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b := ones(1, r)
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b.Mul(b, x)
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b.Scale(1/float64(r), b)
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mu := b.RowView(0)
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// subtract the mean from the data
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xc := mat64.DenseCopyOf(x)
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for i := 0; i < r; i++ {
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rv := xc.RowView(i)
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for j, mean := range mu {
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rv[j] -= mean
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}
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}
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// todo: avoid matrix copy?
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var xt mat64.Dense
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xt.TCopy(xc)
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// It would be nice if we could indicate that this was a symmetric
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// matrix.
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var ss mat64.Dense
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ss.Mul(&xt, xc)
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ss.Scale(1/float64(r-1), &ss)
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return &ss
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}
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// ones is a matrix of all ones.
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func ones(r, c int) *mat64.Dense {
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x := make([]float64, r*c)
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for i := range x {
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x[i] = 1
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}
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return mat64.NewDense(r, c, x)
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}
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