Re: [eigen] Returning an expression unifying two types of matrices |

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*To*: eigen@xxxxxxxxxxxxxxxxxxx*Subject*: Re: [eigen] Returning an expression unifying two types of matrices*From*: Rhys Ulerich <rhys.ulerich@xxxxxxxxx>*Date*: Thu, 24 Nov 2011 13:39:15 -0600*Dkim-signature*: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :content-type; bh=w0sX6thsa2snWhnDdS//s953HLQxsBUr/5O8ivjD+pg=; b=Go5/cf7pxDKhQpl3M9euqo7xZgMSD2006wdfbche+imkhFi5I1SuiM2AnrTcoK7uW3 GWCdb4cmRey5cAHh3bGwWAixNE6SjigkuXEHAbQq6G1Y4Z0uV1355ymklDJ+tmHTDBSL BVci4LdFtnD7wAhDxdhutL41ZND1McD47yhJA=

On Nov 24, 2011 8:19 AM, "Jens Mueller" <jens.k.mueller@xxxxxx> wrote:

>

> Hi,

>

> I'm facing a problem and don't know how to approach it.

> In my code I have many expressions of the form y.transpose() * getG() *

> y, where y is a column vector of length n and getG() returns a reference

> to a nxn matrix.

> Sometimes I know that G is diagonal (at run-time) and I'd like to use

> this information when evaluating the expressions (storing a diagonal is

> also much cheaper).

> I need to store G somewhere to return it. But G is either a full matrix

> or a diagonal matrix. So what do I store?

Store neither. Pass vector y into a method that, like getG, knows how to compute either inner product _expression_ (using G diagonal or full efficiently)__.__ Return the scalar result.

- Rhys

**Follow-Ups**:**Re: [eigen] Returning an expression unifying two types of matrices***From:*Jens Mueller

**References**:**[eigen] Returning an expression unifying two types of matrices***From:*Jens Mueller

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