Multiple columns in table, problem with alignment

Question

The following code generates a table:

\begin{table}[!t]
% increase table row spacing, adjust to taste
\renewcommand{\arraystretch}{1.3}
\caption{An Example of a Table}
\label{table_example}
\centering
\begin{tabular}{|c|c|c|c|c|}
%\toprule
\hline
a & b & c & \multicolumn{2}{c|}{abcdefg fewfewfe}\\
%\midrule
& & & st1 & st2\\

\hline
a         & 1  & 2 &  3 & 4  \\
b         & 1  & 2 &  3 & 4 \\
%\bottomrule
\hline
\end{tabular}
\end{table}

I don't know why columns of st1 and st2 are not properly aligned. When I replace 'abcdefg fewfewfe' with a single word say 'abcde', the alignment will be good.

Answer 1

If you need the final two columns to have exactly equal widths and if they happen to be spanned by the string abcdefg fewfewfe, you could proceed as in the example below.

\documentclass{article}
\usepackage{array}
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}
\newlength{\mylen}
\settowidth\mylen{abcdefg fewfewfe}
\addtolength\mylen{-2\tabcolsep}
\addtolength\mylen{-\arrayrulewidth}
\setlength\mylen{\dimexpr\mylen/2\relax}
\begin{document}
\begin{table}[!t]
% increase table row spacing, adjust to taste
\renewcommand{\arraystretch}{1.3}
\caption{An Example of a Table}
\label{table_example}
\centering
\begin{tabular}{|c|c|c|C{\mylen}|C{\mylen}|}
%\toprule
\hline
a & b & c & \multicolumn{2}{c|}{abcdefg fewfewfe}\
%\midrule
& & & st1 & st2\
\hline
a         & 1  & 2 &  3 & 4  \
b         & 1  & 2 &  3 & 4 \
%\bottomrule
\hline
\end{tabular}
\end{table}
\end{document}

---

April 2023 Addendum to address the follow-up question by @MorganRogers: To achieve your objective, I suggest you replace the code block

\newlength\mylen
\settowidth\mylen{abcdefg fewfewfe}
\addtolength\mylen{-2\tabcolsep}
\addtolength\mylen{-\arrayrulewidth}
\setlength\mylen{\dimexpr\mylen/2\relax}

with

\newlength\mylen % or some other suitable name
\usepackage{calc}
\newcommand\CalcHalfWidth[2]{\setlength{#1}%
   {(\widthof{#2}-2\tabcolsep-\arrayrulewidth)/2}}

and then issue the directive

\CalcHalfWidth{\mylen}{abcdefg fewfewfe}

at the point where you wish to set the (usable) column width of the pair of columns in question. Of course, you're free to choose another length name than \mylen, and you'll have to replace abcdefg fewfewfe with the string that spans the pair columns in question.

Answer 2

Tabular adjusts column widths automatically to cell contents. It doesn't have enough information to equally space the st1 and st2 columns relative to the multicolumn header. It fits st1, then adjusts st2 to fit the rest of the multicolumn header. Your example does not work for single word headings. Remove the space between abcdefg and fewfewfe to see the same behavior.

If you specify the width of the two columns to be equal, then you get the behavior you seek.

\documentclass{article}

% The four lines came from 
% http://tex.stackexchange.com/questions/12703/how-to-create-fixed-width-table-columns-with-text-raggedright-centered-raggedlef
% I changed the m{#1} to p{#1}
\usepackage{array}
\newcolumntype{L}[1]{>{\raggedright\let\newline\\\arraybackslash\hspace{0pt}}p{#1}}
\newcolumntype{C}[1]{>{\centering\let\newline\\\arraybackslash\hspace{0pt}}p{#1}}
\newcolumntype{R}[1]{>{\raggedleft\let\newline\\\arraybackslash\hspace{0pt}}p{#1}}

\begin{document}
\begin{table}
% increase table row spacing, adjust to taste
\renewcommand{\arraystretch}{1.3}
\caption{An Example of a Table}
\label{table_example}
\centering
\begin{tabular}{|c|c|c|C{1cm}|C{1cm}|}
%\toprule
\hline
a & b & c & \multicolumn{2}{c|}{abcdefg fewfewfe}\\
%\midrule
& & & st1 & st2\\

\hline
a         & 1  & 2 &  3 & 4  \\
b         & 1  & 2 &  3 & 4 \\
%\bottomrule
\hline
\end{tabular}
\end{table}\end{document}

Answer 3

Because the caption (abcdefg fewfewfe) is wider than the columns the last column is widened. Below is one solution, which you may or may not like.

\documentclass{article}

\usepackage{minibox}
\begin{document}
\begin{table}[!t]
% increase table row spacing, adjust to taste
\renewcommand{\arraystretch}{1.3}
\caption{An Example of a Table}
\label{table_example}
\centering
\begin{tabular}{|c|c|c|c|c|}
%\toprule
\hline
a & b & c & \multicolumn{2}{c|}{\minibox{abcdefg\\fewfewfe}}\\
%\midrule
& & & st1 & st2\\

\hline
a         & 1  & 2 &  3 & 4  \\
b         & 1  & 2 &  3 & 4 \\
%\bottomrule
\hline
\end{tabular}
\end{table}


\end{document}