I'm getting the error: ! Paragraph ended before \@@label was complete

Question

I've scoured this forum and it looks to me like I'm setting up the tree correctly, but I'm still getting this error, I have

\usepackage{tikz}
\usepackage{tikz-qtree}

in the header.

Here is the code that is causing the problem:

\begin{tikzpicture}

\Tree 
[.{Boolean Groups}
    [.{Abelian Groups}
        [.Groups]
    ]
    [.{Class 3} 
        [.{Class 5}]
        [.{Class 1}]
        [.{Class 2}]
        [.{Class 4}]
    ]
]

\end{tikzpicture}

Answer 1

Get used to this error, it's a common one with tikz-qtree: tikz-qtree requires a space before a node's contents and its closing ]. So you need to insert some spaces before each ] in your tree.

\Tree 
[.{Boolean Groups}
    [.{Abelian Groups}
        [.Groups ] % Here
    ]
    [.{Class 3} 
        [.{Class 5} ] % Here etc.
        [.{Class 1} ]
        [.{Class 2} ]
        [.{Class 4} ]
    ]
]

If you need to connect particular nodes in the tree with each other, you can add explicit \node commands and then use regular TikZ drawing commands to connect them. To connect 'Abelian Groups' to 'Class 5' make them each nodes:

\documentclass{article}
\usepackage{tikz-qtree}
\begin{document}

\begin{tikzpicture}

\Tree 
[.{Boolean Groups}
    [.\node (A) {Abelian Groups};
        [.Groups ]
    ]
    [.{Class 3} 
        [.\node (5) {Class 5}; ]
        [.{Class 1} ]
        [.{Class 2} ]
        [.{Class 4} ]
    ]
]
\draw (A) -- (5);
\end{tikzpicture}
\end{document}