% This file is embedded in datatool-user.pdf version 3.4.1 2025-04-25
% Example 92 Editing a Database with \DTLforeach
% Label: "ex:foreachedit"
% arara: pdflatex
% arara: pdfcrop
\documentclass[12pt]{article}
\pagestyle{empty}
 \begin{filecontents}[noheader,overwrite]{studentmarks.csv}
Surname,Forename,StudentNo,Assign1,Assign2,Assign3
"Smith, Jr",John,102689,68,57,72
"Brown",Jane,102647,75,84,80
"Brown",Jane,102646,64,92,79
"Brown",Andy,103569,42,52,54
"Adams",Zoë,105987,52,48,57
"Brady",Roger,106872,68,60,62
"Verdon",Clare,104356,45,50,48
\end{filecontents}

\usepackage{datatool}
\DTLsetup{store-datum,default-name=marks}
\DTLread{studentmarks.csv} 
\begin{document}
\DTLforeach{marks}{}{% 
 \DTLaction[
  keys={Assign1-},
  options={mean},
  datum={round=1},% round the result
  return={\Mean=mean}
 ]
 {current row aggregate}
 \DTLifnull{\Mean}% test the return value
 {}% row aggregate failed!
 {% average calculated successfully
 \DTLappendtorow{Average}{\Mean}% 
 }
} 

\DTLsortdata{marks}{Average=desc}
\DTLdisplaydb{marks} 
\end{document}
