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\author{Laurent}
\begin{document}
\title{affy: Import Methods (HowTo)}

\maketitle
\tableofcontents
\section{Introduction}
This document describes briefly how to write import methods for
the \verb+affy+ package. As one might know, the Affymetrix data are
originally stored in files of type \verb+.CEL+ and \verb+.CDF+. The
package extracts and store the information contained in \verb+R+ data
structures using file parsers. This document outlines how to get the
data from other sources than the current\footnote{today's date is
early 2003} file formats.

As usual, loading the package in your \verb+R+ session is required. 
\begin{Sinput}
R> library(affy) ##load the affy package
\end{Sinput}
<<echo=F,results=hide>>=
library(affy)
@

{\bf note: this document only describes the process for .CEL files}

Knowing the slots of \verb+Cel+ and \verb+AffyBatch+ objects will
probably help you to achieve your goals. You may want to refer to
the respective help pages. Try \verb+help(Cel)+, \verb+help(AffyBatch)+.

\section{How-to}
 \subsection{Cel objects}
The functions \verb+getNrowForCEL+ and \verb+getNcolForCEL+ are assumed to return
the number of rows and the number of columns in the \verb+.CEL+ file respectively

You will also need to have access to the $X$ and $Y$ position for the probes in the
{\verb .CEL} file. The functions \verb+getPosXForCel+ and \verb+getPosYForCEL+
are assumed to return the $X$ and $Y$ positions respectively. The corresponding
probe intensities are assumed to be returned by the function \verb+getIntensitiesForCEL+.

If you stored {\bf all} the $X$ and $Y$ values that were in the \verb+.CEL+, the
functions verb+getNrowForCEL+ and \verb+getNcolForCEL+ can written:
<<<>>=
getNrowForCEL <- function() max(getPosXForCEL())
getNcolForCEL <- function() max(getPosYForCEL())
@ 

You will also need the name for the corresponding \verb+.CDF+ (although you will
probably no need the \verb+.CDF+ file itself, the cdf packages available for download
will probably be enough).

\begin{Sinput}
import.celfile <- function(celfile, ...) {

  cel.nrow <- getNrowForCEL(celfile)
  cel.ncol <- getNcolForCEL(celfile)
  x <- matrix(NA, nr=cel.nrow, nc=cel.ncol)

  cel.intensities <- getIntensitiesForCEL(celfile)

  cel.posx <- getPosXForCEL(celfile) # +1 if indexing starts at 0 (like in .CEL)
  cel.posy <- getPosYForCEL(celfile) # idem

  x[cbind(cel.posx, cel.posy)] <- cel.intensities

  mycdfName <- whatcdf("aCELfile.CEL")

  myCel <- new("Cel", exprs=x, cdfName=mycdfName)

  return(myCel)
}
\end{Sinput}

The function \verb+import.celfile+ can now replace the function \verb+read.celfile+
in the \verb+affy+ package


 \subsection{AffyBatch objects}

(scratch)
the use of \verb+...+ should make you able to override the function
read.celfile by a hack like:
\begin{Sinput}
read.celfile <- import.celfile
\end{Sinput}
The function \verb+read.affybatch+ should now function using your \verb+import.celfile+


\end{document}
