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}

Answer 2

forest allows you to draw trees using a slightly more intuitive syntax, I think:

• no need to leave spaces before the closing bracket, although it isn't a problem if you do so, either;

• no need to preface a new sub-tree with a dot - just use brackets with nesting to represent the tree structure;

• no need to explicitly make things TikZ nodes in order to name them - all nodes are TikZ nodes and can be given a name using name= and, if necessary, extra names using alias=.

In the following code, I've also used a phantom node in order to move the bottom left node outwards, so that the connection between abelian groups and class 5 is displayed more naturally.

\documentclass[tikz, border=5pt, varwidth, multi]{standalone}
\usepackage{forest}

\begin{document}
  \begin{forest}
    for tree={
      parent anchor=south,
      child anchor=north,
    }
    [Boolean Groups
        [Abelian Groups, name=abelian groups
            [Groups]
            [, phantom]
        ]
        [Class 3
            [Class 5, name=class 5]
            [Class 1]
            [Class 2]
            [Class 4]
        ]
    ]
    \draw (abelian groups.south) -- (class 5);
  \end{forest}
\end{document}