macro for empty nodes in tikz-qtree

You need to encourage \missing to expand at the right time:

\documentclass{article} \usepackage{tikz} \usepackage{tikz-qtree} \makeatletter \let\old@@children\@@children \def\@@children{\futurelet\my@next\my@@children} \def\my@@children{% \ifx\my@next\missing\else \expandafter\@gobble \fi \expandafter\old@@children} \makeatother \newcommand{\missing}{ \edge[draw=none]; {} } %intended usage inside tikzpicture enviornment: %\Tree [.3 1 % \missing ] %To producce the same result as code below but %it gave me an error: %! Undefined control sequence. %\missing -> \edge % [draw=none]; {} \begin{document} \begin{tikzpicture} \Tree [.3 1 \missing ] \end{tikzpicture} \end{document} 

If you are drawing trees, I urge you to look at forest which is extremely powerful and uses a similar syntax to tikz-qtree/qtree. It also has a built in style for missing nodes:

\documentclass[tikz]{standalone} \usepackage{forest} \begin{document} \begin{forest} [3 [1 ] [,phantom ] ] \end{forest} \end{document} 

If you want to just pass a missing to the relevant node and have the phantom constructed, you can do something like this, which will have the same output as the code above:

\documentclass[tikz,multi,varwidth]{standalone} \usepackage{forest} \begin{document} \forestset{ missing/.style={ before typesetting nodes={ append={[, phantom]}, }, }, } \begin{forest} [3, missing [1 ] ] \end{forest} \end{document} 

This can be used for non-binary trees where nodes may have more than 2 children. If you might sometimes want to add your missing node in the middle of a node's children in such a tree, you can adapt missing to take an argument equal to the number of the children occurring before the missing node.

For example:

\documentclass[tikz,multi,varwidth]{standalone} \usepackage{forest} \begin{document} \forestset{ missing/.style={ for children={ if n=#1{ before typesetting nodes={ insert after={[, phantom]}, }, }{}, }, }, } \begin{forest} [3, missing=3 [1 ] [2 ] [3 ] [4 ] ] \end{forest} \end{document} 

Something slightly more complex is needed if you might want the first child to be missing i.e. if missing=0 is a possibility:

\documentclass[tikz,multi,varwidth]{standalone} \usepackage{forest} \begin{document} \forestset{ missing/.style={ if={equal(#1,0)}{ before typesetting nodes={ prepend={[, phantom]}, }, }{ for children={ if n=#1{ before typesetting nodes={ insert after={[, phantom]}, }, }{}, }, }, }, } \begin{forest} [0 [1 ] [2, missing=0 [2 ] [3 ] ] [3, missing=1 [1 ] [3 ] ] [4 ] ] \end{forest} \end{document} 

This might also be useful in binary trees since you can use 0 for left child missing and 1 for right child missing:

\documentclass[tikz,multi,varwidth]{standalone} \usepackage{forest} \begin{document} \forestset{ missing/.style={ if={equal(#1,0)}{ before typesetting nodes={ prepend={[, phantom]}, }, }{ for children={ if n=#1{ before typesetting nodes={ insert after={[, phantom]}, }, }{}, }, }, }, } \begin{forest} [0 [1, missing=1 [1 ] ] [2, missing=0 [2 ] ] ] \end{forest} \end{document}