How to obtain the table in exact spacing

Question

Consider the following two tables:

The first table is the table that I wish to have and the second table is the table I obtained for which the code is as follow:

\begin{array}{|c|c|c|c|}
\cline { 2 - 4 } \multicolumn{1}{c|} {} & \|V-QR\|_{2} & \left\|I-Q^{\intercal}Q\right\|_{2} & \left\|I-QQ^{\intercal}\right\|_{2} \\
\hline \text {$\mathsf{CGS3\_QR}$} & & & \\
\text {$\mathsf{MGS\_QR}$} & & & \\
\hline
\end{array}

My only concern is the spacing because in the second table which I got the entries are too narrowed. I hope someone can help and thank you.

Answer 1

You can use the \makegapedcells command from makecell:

    \documentclass{article}

    \usepackage{amsmath, amssymb}
    \usepackage{makecell}

    \begin{document}

     \[ \setcellgapes{3pt}\makegapedcells
      \begin{array}{|*{4}{c|}}
        \cline { 2 - 4 } \multicolumn{1}{c|} {} & \|V-QR\|_{2} & \left\|I-Q^{\intercal}Q\right\|_{2} & \left\|I-QQ^{\intercal}\right\|_{2} \\
        \hline \text {$\mathsf{CGS3\_QR}$} & & & \\
        \text {$\mathsf{MGS\_QR}$} & & & \\
        \hline
      \end{array} \]

    \end{document} 

Answer 2

A small variation of nice @Bernard answer (+1) ...

• by use of mathtools package is defined \norm
• contents in the first column is cleanup of superfluous code

\documentclass{article}
\usepackage{mathtools, amssymb}
\DeclarePairedDelimiterX\norm[1]{\|}{\|}{#1}
\usepackage{makecell}

\begin{document}
    \[  \setcellgapes{3pt}
        \makegapedcells
\begin{array}{|*{4}{c|}}
    \cline {2-4} 
\multicolumn{1}{c|} {} 
    &   \norm*{V-QR}_{2}    
        &   \norm*{I-Q^{\intercal} Q}_{2} 
            &   \norm*{I-QQ^{\intercal}}_{2} \\
    \hline
\mathsf{CGS3\_QR}   &   &   &               \\
\mathsf{MGS\_QR}    &   &   &               \\
    \hline
\end{array} 
    \]
\end{document}