How can I have a multicolumn label with blockarray before the block portion?

Question

I'm trying to have a multicolumn label but that doesn't seem to work. When I run the following, it fails:

\begin{equation*}
    \mathbf{C} =
    \begin{blockarray}{ccccccc}
        & \multicolumn{5}{c}{\text{Predicted}} & \\
        & 1 & 2 & 3 & \dots & n & \\
        \begin{block}{c(ccccc)c}
            1 & c_{11}   & c_{12} & c_{13} & \dots & c_{1n} & \multirow{4}{*}{\rotatebox{-90}{\text{Actual}}} \\
            2 & c_{21}   & c_{22} & c_{23} & \dots & c_{2n} & \\
            \vdots & \vdots   & \ddots &        &       & \vdots & \\
            n & c_{n1}   & c_{n2} & c_{n3} & \dots & c_{nn} & \\
        \end{block}
    \end{blockarray}
\end{equation*}

This is the closest I've gotten to a non-failing version but am unable to get the multicolumn label to work.

\begin{equation*}
    \mathbf{C} =
    \begin{blockarray}{ccccccc}
        & \text{Predicted} & & & & \\
        & 1 & 2 & 3 & \dots & n & \\
        \begin{block}{c(ccccc)c}
            1 & c_{11}   & c_{12} & c_{13} & \dots & c_{1n} & \multirow{4}{*}{\rotatebox{-90}{\text{Actual}}} \\
            2 & c_{21}   & c_{22} & c_{23} & \dots & c_{2n} & \\
            \vdots & \vdots   & \ddots &        &       & \vdots & \\
            n & c_{n1}   & c_{n2} & c_{n3} & \dots & c_{nn} & \\
        \end{block}
    \end{blockarray}
\end{equation*}

How can I get the "Predicted" label to span the remaining columns?

I'm happy to switch to another environment from blockarray but it seemed like this was the recommended package after looking at multiple alternatives for labeled matrixes.

Full document:

\documentclass{beamer}
    % PACKAGES
    \usepackage{blkarray}
    \usepackage{multirow}

\begin{document}

\begin{frame}

    \begin{equation*}
    \mathbf{C} =
        \begin{blockarray}{ccccccc}
            & \text{Predicted} & & & & & \\
            & 1 & 2 & 3 & \dots & n & \\
            \begin{block}{c(ccccc)c}
                1 & c_{11}   & c_{12} & c_{13} & \dots & c_{1n} & \multirow{4}{*}{\rotatebox{-90}{\text{Actual}}} \\
                2 & c_{21}   & c_{22} & c_{23} & \dots & c_{2n} & \\
                \vdots & \vdots   & \ddots &        &       & \vdots & \\
                n & c_{n1}   & c_{n2} & c_{n3} & \dots & c_{nn} & \\
            \end{block}
        \end{blockarray}
    \end{equation*}

\end{frame}

\end{document}

please always post a complete small document, not disconnected fragments, so people can run and debug the code. I'm surprised multirow works with blockarray at all, I'm certain that combination has never been tested by the authors of either package:-) — David Carlisle, Aug 25 '18 at 15:04
Will update with full document. Apologies for that.
I thought they'd work based on this question: https://tex.stackexchange.com/questions/10122/bordermatrix-with-blocks — sbeleidy, Aug 25 '18 at 15:08
@sbeleidy, as you found yourself, in the first example replace \multicolumn{5}{c}{\text{Predicted}} with \BAmulticolumn{5}{c}{\text{Predicted}} and it will works. — Zarko, Aug 25 '18 at 15:26
Well, that's embarassing. I completely missed that and saw it as multicolumn. Thank you @Zarko — sbeleidy, Aug 25 '18 at 15:45
@Zarko I'd completely forgotten about \BAmulticolumn (stupid command name, who wrote that package.....) — David Carlisle, Aug 25 '18 at 19:21
@DavidCarlisle, i know who is author :-). however after written 50+ packages (if i remember correctly) is difficult to remember all commands/macros names :-) — Zarko, Aug 25 '18 at 19:45

sbeleidy · Accepted Answer · 2018-08-26T18:03:54.883

Thanks to @Zarko pointing out that the way to do multicolumns when using blockarray is to use \BAmulticolumn instead of \multicolumn. The final result is from the following code:

\documentclass{beamer}

\usepackage{blkarray}
\usepackage{multirow}

\begin{document}

\begin{frame}

\begin{blockarray}{ccccccc}
    & \BAmulticolumn{5}{c}{\text{Predicted}} & \\
    & 1 & 2 & 3 & \dots & n & \\
    \begin{block}{c[ccccc]c}
        1 & c_{11}   & c_{12} & c_{13} & \dots & c_{1n} & \multirow{4}{*}{\rotatebox{-90}{\text{Actual}}} \\
        2 & c_{21}   & c_{22} & c_{23} & \dots & c_{2n} & \\
        \vdots & \vdots   & \ddots &        &       & \vdots & \\
        n & c_{n1}   & c_{n2} & c_{n3} & \dots & c_{nn} & \\
    \end{block}
\end{blockarray}

\end{frame}

\end{document}

nice answer (+1), however, please extend your code snippet to complete small document beginning with \documentclass... and end with \end{document} that members can simple copy and test it. — Zarko, Aug 25 '18 at 16:18

score 0 · Answer 2 · answered Apr 29 '21 at 16:42

With {NiceMatrix} of the eponymous package nicematrix.

\documentclass{beamer}
\usepackage{nicematrix}
\begin{document}
\begin{frame}
$\begin{NiceMatrix}
         & \Block{1-5}{\text{Predicted}} \
         & 1      & 2      & 3      & \dots & n      & \
  1      & c_{11} & c_{12} & c_{13} & \dots & c_{1n} & \Block{4-1}{\rotate\text{Actual}} \
  2      & c_{21} & c_{22} & c_{23} & \dots & c_{2n} & \
  \vdots & \vdots & \ddots &        &       & \vdots & \
  n      & c_{n1} & c_{n2} & c_{n3} & \dots & c_{nn} & \
\CodeAfter
  \SubMatrix[{3-2}{6-6}]
\end{NiceMatrix}$
\end{frame}
\end{document}

You need several compilations (because nicematrix uses PGF/Tikz nodes under the hood).

How can I have a multicolumn label with blockarray before the block portion?

2 Answers2