RawColView

[Clayton]

If I need a row of a Dense as a []float64, I can use someDense.RawRowView (https://godoc.org/gonum.org/v1/gonum/mat#Dense.RawRowView), is there any way to do the same for some column?  Something like RawColumnView?  I don't see it in the docs nor do I see it in the source code, though I do see ColView.  Is there a reason this does not exist?  Or has it just not been implemented yet?

[Dan Kortschak]

The data for a matrix is stored in row-major format, so it is easy to
give a slice representing a row. This cannot be done for column in
general (an n by 1 matrix can if the stride is also 1). So we don't
provide this. If you just want a []float64 that is a copy of column,
it's straightforward to iterate over the rows for the column you want
and copy the elements across, or you could CopyVec the column view into
a new VecDense with Inc=1 and then have the vector Data field in the
blas64.Vector returned by RawVector.

[Clayton]

Okay that makes sense.  Below is my solution, I believe it does what you suggest.

func RawColView(d *mat.Dense, col int) []float64 {

rows, _ := d.Dims()

results := mat.NewVecDense(rows, nil)

results.CopyVec(d.ColView(col))

return results.RawVector().Data

}

func TestRawColView(t *testing.T) {

// 4 5

// 9 8

d := mat.NewDense(2, 2, []float64{4.0, 5.0, 9.0, 8.0})

colView := RawColView(d, 1)

if !reflect.DeepEqual(colView, []float64{5.0, 8.0}) {

t.Error("incorrect raw column view")

}

}

[Brendan Tracey]

That solution gets a copy of the column, but does not get a _view_. If you do, say, `colView[0] = 15`, the elements of `d` are unchanged (unlike with RawRowView). If you just want a copy of the column, you can just do

col := mat.Col(nil, d, 1)

[Dan Kortschak]

I forget the things we have too often. This is what the OP wanted.

[Clayton]

yup this is what I wanted thanks!

[Sebastien Binet]

We may want to create an example for this operation as it comes back quite regularly on the mailing list...

Do you want to give this a try?

Cheers,

-s

sent from my droid

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[Clayton]

sure, would this example best live as an "example" in the documentation? I can make a pull request for it if you agree

[Clayton]

added a pull request with examples for mat.Col and mat.Row here: https://github.com/gonum/gonum/pull/777