FIll empty cells automatically with dummy values

Question

I am working on layoutin a huge table for my thesis. For this it would be great that the tabl is not just blank, it should contain all empty cells with dummy values like x It simply could do so by entering &x&x&x&x&x but everytime I changed my amount of columns I have to fill every row by hand with the exact amount of x&x&x&x which can be very fuzzy having 60+ rows.

Is there some easy step I can say at the beginning of my table like it is possible in tikz matrixwhere I can define empty cells = Valuexyz, so that all cells are filled with dummy values?

Just empty cells should be replaced. All with content should not.

MWE:

\documentclass[a4paper]{article} 
\usepackage{booktabs} 
\begin{document} 
\begin{tabular}{llllcccccccccccccccccccccccccccccccccccccccccccccccccccccccc}
\toprule
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&\\
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&\\
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&\\
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&\\
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&\\
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&\\
\bottomrule
\end{tabular}
\end{document}

My table look like this:

Answer 1

The following approach uses the cellcoll package to collect the cell content and then \IfEq of package xstring to test wether the cell is empty (then print -) or not (then print the cell content as is #1).

\documentclass{article}

\usepackage{xstring}
\usepackage{array,collcell}
\newcolumntype{C}{>{\collectcell\checkempty}c<{\endcollectcell}}
\def\checkempty#1{%
   \IfEq{#1}{}{-}{#1}%
}

\begin{document}
\begin{tabular}{CCC}
   &&X\\
   &&\\
   &X&\\
   O&X&\\
   X&&T\\
\end{tabular}
\end{document}

Answer 2

If you're after a method for easily changing the number of columns that your table has, this method should help. It works by using a loop to enter each &x that you have.

To change the number of columns that are in your table, simply adjust the \numcols parameter.

\documentclass[a4paper]{article} 
\usepackage{booktabs}

\newcounter{countA}%
\newcommand{\numcols}{20}

\newcommand{\repeatentry}[2]{%
    \def\myline{#1}%
    \setcounter{countA}{1}%
    \loop\ifnum\thecountA<#2%
        \stepcounter{countA}%
        \edef\myline{\myline & #1}%
    \repeat%
    \myline{}%
}

\begin{document} 
\begin{tabular}{|*{\numcols}{c|}}
    \toprule
    \repeatentry{x}{\numcols}\\
    \repeatentry{x}{\numcols}\\
    \repeatentry{x}{\numcols}\\
    \bottomrule
\end{tabular}
\end{document}

EDIT:

In order to allow formatting to enter the \repeatentry command, the following change must be made. This is done to protect the expansions of the formatting when \edef adds another column. This is preferred over using \noexpand, as the nesting of \edef requires a similar nesting of \noexpand.

\makeatletter
\newcommand{\repeatentry}[2]{%
    \def\myline{#1}%
    \setcounter{countA}{1}%
    \loop\ifnum\thecountA<#2%
        \stepcounter{countA}%
        \protected@edef\myline{\myline & #1}%
    \repeat%
    \myline{}%
}
\makeatother