Adding a Label and caption to a matrix

Question

Using LaTeX, I am trying to add a caption to my matrix, something like this;

I don't want to add it as a table and when I add a caption it gives an error, \caption outside float, probably because I am trying to add a caption inside equation tags. Is there an easy way to accomplish this. Thank you.

Edit: Apologies for not giving the code. This is the code for the matrix

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
    \begin{bmatrix}
    \cos\theta_i  &   -\sin\theta_i \cdot \cos\alpha_i   &   \sin\theta_i \cdot \cos\alpha_i    &   ln \cdot \cos\theta_i \\
    \sin\theta_i  &   \cos\theta_i \cdot \cos\alpha_i    &   -\cos\theta_i \cdot \sin\alpha_i   &   ln \cdot sin\theta_i \\
    0            &   \sin\theta_i \cdot \cos\alpha_i    &   \cos\alpha_i                &   d_i              \\
    0            &   0                          &   0                          &     1          
    \end{bmatrix}
\end{equation}
\captionof{Matrix 3.1: }{HTM of Manipulator}
\end{document}

It does give the error undefined control sequence

Answer 1

You could use the \captionof macro from the caption package to add captions outside of floats:

\documentclass{article}
\usepackage{amsmath}
\usepackage{caption}
\DeclareCaptionType[within=section]{mat}[Matrix]

\begin{document}
\section{Section}

\begin{equation}
    \begin{bmatrix}
    \cos\theta_i  &   -\sin\theta_i \cdot \cos\alpha_i   &   \sin\theta_i \cdot \cos\alpha_i    &   ln \cdot \cos\theta_i \\
    \sin\theta_i  &   \cos\theta_i \cdot \cos\alpha_i    &   -\cos\theta_i \cdot \sin\alpha_i   &   ln \cdot sin\theta_i \\
    0            &   \sin\theta_i \cdot \cos\alpha_i    &   \cos\alpha_i                &   d_i              \\
    0            &   0                          &   0                          &     1          
    \end{bmatrix}
\end{equation}
\captionof{mat}{HTM of Manipulator}
\end{document}