LaTex table column center align along upper column using tabular

Question

I want to center align val and test along upper columns which are something long and temp

\begin{table*}
\centering
\begin{tabular}{l c c c c}
\hline
\toprule
\multirow{2}{4em}{\textbf{temp}} & \multicolumn{2}{c} 
{\textbf{temp}$\downarrow$}  & \multicolumn{2}{c} 
{\textbf{something long}} \\
\cline{2-5}
 & val & test & val & test \\
\hline
\verb|temp| & & 0.7758 & & 62.02 \\
\verb|temp| & & 0.9679 & & 43.44 \\
\verb|temp| & & 0.7289 & & 70.34 \\
\verb|temp| \\
\verb|temp| & & \textbf{0.6877} & & \textbf{74.75} \\
\hline
\verb|Upperbound| & \multicolumn{2}{c}{0.7758} & \multicolumn{2} 
{c}{0.7758} \\
\bottomrule
\hline
\end{tabular}
\end{table*}

Answer 1

I'm not sure I understand what you mean by "table column center align along upper column using tabular" and/or "center align val and test along upper columns which are something long and temp". If you want the four data columns to have the same width, the following solution may be of interest to you.

\documentclass[twocolumn]{article} % or some other suitable document class
\usepackage{multirow} % for '\multirow' package
\usepackage{booktabs} % for well-spaced horizontal rules
\usepackage{array}    % for 'w' column type
\newlength\mylen
\begin{document}

\begin{table*}
%% Determine value of "\mylen" (usable width of data cols):
\settowidth{\mylen}{\textbf{something long}}
  \setlength{\mylen}{\dimexpr(\mylen-2\tabcolsep)/2\relax}
   
\centering
\begin{tabular}{@{} l *{4}{wc{\mylen}} @{}} % all data cols have same, fixed width
\toprule
\multirow{2.2}{*}{\textbf{temp}} 
  & \multicolumn{2}{c}{\textbf{temp}$\downarrow$}  
  & \multicolumn{2}{c@{}}{\textbf{something long}} \\
\cmidrule(lr){2-3} \cmidrule(l){4-5}
 & val & test & val & test \\
\midrule
\verb|temp| & & 0.7758 & & 62.02 \\
\verb|temp| & & 0.9679 & & 43.44 \\
\verb|temp| & & 0.7289 & & 70.34 \\
\verb|temp| & & & & \\
\verb|temp| & & \textbf{0.6877} & & \textbf{74.75} \\
\midrule
\verb|Upperbound| 
  & \multicolumn{2}{c}{0.7758} 
  & \multicolumn{2}{c@{}}{0.7758} \\
\bottomrule
\end{tabular}
\end{table*}
\end{document}