Alignment in `longtable'

Question

\documentclass[12pt] {article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{multirow}
\usepackage{multicol}
\usepackage{longtable}

\begin{document}
    \begin{longtable}{|c|c|c|}
    \hline
    \pmb{Operation} & \pmb{Codes}   & \pmb{Output}\\\hline
    \endhead
    \multirow{3}{*}{Transpose}  &{\fontfamily{qcr}\selectfont A=Matrix([[1,2,3],[3,2,1],[1,1,5]])}  & \multirow{3}{*}{$\left(
    \begin{matrix}
        1 &2 &3\\
        3 &2 &1\\
        1 &1 &5
    \end{matrix}
    \right)$}\\

    & {\fontfamily{qcr}\selectfont B=A.transpose()} &\\
    & {\fontfamily{qcr}\selectfont show(B)} &\\\hline
    \multirow{2}{*}{Determinant}    &{\fontfamily{qcr}\selectfont A=Matrix([[1,2,3],[3,2,1],[1,1,5]])}  & \multirow{2}{*}{-16}\\
    & {\fontfamily{qcr}\selectfont A.det()} &   \\\hline 
    
    \multirow{3}{*}{Adjoint}    &{\fontfamily{qcr}\selectfont A=Matrix([[1,2,3],[3,2,1],[1,1,5]])}  & \multirow{3}{*}{$\left(
        \begin{matrix}
            1 &2 &3\\
            3 &2 &1\\
            1 &1 &5
        \end{matrix}
        \right)$}\\
    & {\fontfamily{qcr}\selectfont B=A.adjugate()}  &\\
    &{\fontfamily{qcr}\selectfont show(B)}  &\\\hline
\end{longtable}
\end{document}

It is appearing through my coding that

1. in the 3rd column (Output column), then parenthesis touched the top and bottom horizontal lines
2. also in the mid column, the lines are not left aligned. How can I solve these problems?

Answer 1

Here's a solution that doesn't require all those \fontfamily{qcr}\selectfont directives.

\documentclass[12pt]{article}
\usepackage{geometry}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{multirow}
%%%\usepackage{multicol} % not needed
\usepackage{longtable}

% new:
\usepackage{array,courier}

\begin{document}
\begin{longtable}{| l | >{\ttfamily}l | c |}
    
\hline
\textbf{Operation} & \multicolumn{1}{c|}{\textbf{Codes}}  & \textbf{Output}\\
\hline
\endhead
    
% body of table:
    \multirow{3}{*}{Transpose}  & 
    A=Matrix([[1,2,3],[3,2,1],[1,1,5]])  & 
    \multirow{3}{*}{\small $
        \begin{pmatrix}
            1 &2 &3\\
            3 &2 &1\\
            1 &1 &5
        \end{pmatrix}
        $} \\

    & B=A.transpose() & \\
    & show(B) & \\
    \hline
    \multirow{2}{*}{Determinant}    &
    A=Matrix([[1,2,3],[3,2,1],[1,1,5]])  & 
    \multirow{2}{*}{$-16$}\\
    & A.det() & \\
    \hline  
    \multirow{3}{*}{Adjoint}    &
    A=Matrix([[1,2,3],[3,2,1],[1,1,5]])  & 
    \multirow{3}{*}{\small $
        \begin{pmatrix}
            1 &2 &3\\
            3 &2 &1\\
            1 &1 &5
        \end{pmatrix}
        $} \\
    & B=A.adjugate()  & \\
    & show(B)  & \\
\hline
\end{longtable}
\end{document}

---

Addendum to address the OP's follow-up query: The two main macros provided by LaTeX for drawing horizontal lines in table-like structures -- \hline and \cline -- are not particularly sophisticated. In particular, the spacing between lines drawn by \hline and \cline and the material above/below the lines is frequently woefully inadequate. You've encountered this yourself, haven't you?

Now, one could try a "bandaid approach" to fixing this issue, by inserting some vertical whitespace padding, by hand, above or below various lines. I would like to recommend that you consider doing something very different: Load the booktabs package and learn to use its user-level macros: \toprule, \midrule, \bottomrule, \cmidrule, and \addlinespace. Whitespace can be every bit as effective at forming a visual separator as a black line can. A happy side-effect of using the macros of the booktabs package is that it's hardly ever necessary to contemplate engaging in the kind of visual formatting you are mentioning in your follow-up comments.

Oh, getting rid of all vertical rules is also something that you should become very familiar with. Trust me, those vertical lines won't be missed.

\documentclass[12pt]{article}
\usepackage{geometry}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{multirow}
%%%\usepackage{multicol} % not needed
\usepackage{longtable}

% new:
\usepackage{array,courier,booktabs}

\begin{document}
\begin{longtable}{@{} l >{\ttfamily}l c @{}}
    
    \toprule
    \textbf{Operation} & 
    \multicolumn{1}{c}{\textbf{Codes}} & 
    \textbf{Output}\\
    \midrule
    \endhead

    \bottomrule 
    \endlastfoot
    
% body of table:
    \multirow{3}{*}{Transpose} & 
    A=Matrix([[1,2,3],[3,2,1],[1,1,5]]) & 
    \multirow{3}{*}{$
    \begin{pmatrix}
        1 &2 &3\\
        3 &2 &1\\
        1 &1 &5
    \end{pmatrix}$} \\
   
  & B=A.transpose() &\\
  & show(B) & \\
    \addlinespace
    
    \multirow{2}{*}{Determinant} &
    A=Matrix([[1,2,3],[3,2,1],[1,1,5]]) & 
    \multirow{2}{*}{$-16$}\\
  & A.det() & \\
    \addlinespace 
    
    \multirow{3}{*}{Adjoint} &
    A=Matrix([[1,2,3],[3,2,1],[1,1,5]]) & 
    \multirow{3}{*}{$
    \begin{pmatrix}
        1 &2 &3\\
        3 &2 &1\\
        1 &1 &5
    \end{pmatrix}$}\\
  & B=A.adjugate() & \\
  & show(B) & \\

\end{longtable}
\end{document}