[R] comparing two unequal matrices without for loop?

[William Dunlap]

Look into merge() or, if you want to work at
a lower level, match().

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com 

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of David Winsemius
> Sent: Sunday, August 28, 2011 11:07 AM
> To: Paul Hammer
> Cc: r-help at r-project.org
> Subject: Re: [R] comparing two unequal matrices without for loop?
> 
> 
> On Aug 28, 2011, at 1:20 PM, Paul Hammer wrote:
> 
> > Dear folks,
> >
> > I would like to compare two unequal matrices.
> 
> "Unequal " is hardly an adequate data description.
> 
> > At the moment I realize
> > this with two for loop and an if function but that's definitely too
> > slow
> > :(...
> >
> > Here is my code:
> >
> > for (i in 1:length(all_expressed_genes[,1])) {
> >   for (j in 1:length(kegg_gene_pVal[,1])) {
> >     if (all_expressed_genes[j,1] == kegg_gene_pVal[i,1]) {
> >       kegg_gene_pVal[j,4] =  all_expressed_genes[i,6]
> >     }
> >   }
> > }
> >
> > Are there any faster solutions for that easy code?
> 
> I'm trying to figure out what this is supposed to be doing. You are
> making N^2 comparisons and transferring results from one vector of
> length N to another vector of length N. Is this really what the (non-
> existent) problem definition would have told us to do if you had
> provided a problem definition? It appears there would be a great
> possibility of overwriting earlier operations. And it appears to be
> doing N^2-N unnecessary operations as well.
> 
> > And why is R so
> > dramatically slow when using for, while or other loop functions?
> 
> Blaming the tool is generally a risky idea. Axes can be used
> effectively or ineffectively. I'm guessing inadequate "operator"
> training  and/or confused problem specification.
> 
> *---------------------*
> > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> *---------------------*
> 
> 
> David Winsemius, MD
> West Hartford, CT
> 
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.