box the size of A containing B

Question

A thing I'd often like to do is put in my document a box the size of some text A and containing some other text B. For instance, I have two tikz nodes, one containing $n$ and one containing $n+1$, and I want them to be the same size (but not larger than necessary, as I would get by specifying a minimum width).

If I want the text B to be right- or left-aligned, I can do the trick with a combination of phantom and mathrlap/mathllap. However, if I want it to be centered, I don't see an easy way to do it.

Is there a simple command in some package to do this? It looks like a common task.

Minimal example: I want to alter the bounding box of $n$ so that the two circles are the same size (and $n$ is still centered in the circle).

\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
 \node[circle,draw=black] (A) {$n$};
 \node[circle,draw=black] (B) [right of=A] {$n+1$};
\end{tikzpicture}
\end{document}

Answer 1

As far as making the two circles the same size, this is what I propose. It basically overlays the $n$ on the phantom of $n+1$.

\documentclass{standalone}
\usepackage{stackengine}[2013-09-11]
\def\stacktype{L}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[node distance=1.2cm]
 \node[circle,draw=black] (A) {\stackon[0pt]{\phantom{$n+1$}}{$n$}};
 \node[circle,draw=black] (B) [right of=A] {$n+1$};
\end{tikzpicture}
\end{document}

Answer 2

Having the n+1 node first might help things to get the size of the node directly but you can use the limited width("<text>") function of pgfmath to get the text width. Then you can center it. By limited I mean if your text is a complex TeX macro it usually fails to understand or expand properly so extra protection is required. However for straightforward text it is quite handy.

\documentclass[tikz]{standalone}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
 \node[circle,draw=black,align=center,text width={width("$n+1$")}] (A) {$n$};
 \node[circle,draw=black] (B) [right=0 of A] {$n+1$};
\end{tikzpicture}
\end{document}

Finally a minor nuisance is explained here if you wish to have an idea why of= syntax might not be a good idea

Difference between "right of=" and "right=of" in PGF/TikZ

Answer 3

Here's a dirtier solution which:

1. easily extends to more than one nodes (without manually keeping score which one is the largest)
2. differentiates between width, height and depth (the widest node is not necessarily the tallest!), and
3. aligns the baselines of the text of all nodes.

You'll have to add this to your preamble:

\makeatletter
\newbox\eq@box
\newdimen\eq@w
\newdimen\eq@h
\newdimen\eq@d
\def\rb{\eq@w=0pt\eq@h=0pt\eq@d=0pt}
\def\sb#1{%
  \setbox\eq@box\vbox{\hbox{#1}}%
  \ifnum\eq@w<\wd\eq@box \eq@w=\wd\eq@box \fi%
  \ifnum\eq@h<\ht\eq@box \eq@h=\ht\eq@box \fi%
  \ifnum\eq@d<\dp\eq@box \eq@d=\dp\eq@box \fi%
}
\def\ub#1{\raisebox{0pt}[\eq@h][\eq@d]{\makebox[\eq@w][c]{#1}}}
\makeatother

Then you just use the three commands:

1. \rb for resetting boxes
2. \sb{t} for declaring that you'll later have text t in a node
3. \ub{t} for actually using text t in a node

Here's your example:

\begin{tikzpicture}[node distance=1.5cm]
 \rb\sb{$n$}\sb{$n+1$}
 \node[circle,draw=black] (A) {\ub{$n$}};
 \node[circle,draw=black] (B) [right of=A] {\ub{$n+1$}};
\end{tikzpicture}

And here's one more which may show you the difference from other solutions that you'll come up with.

\begin{tikzpicture}[node distance=3cm]
 \rb
 \sb{$a_1+a$}
 \sb{$a_{a_2}+a^b$}
 \sb{$a_{a_{a_3}}+a^{b^{c^d}}$}
 \sb{$a_{a_{a_4}}+a^{b^{c^{d^e}}}$}
 \node[circle,draw=black] (P1) [below of=A] {\ub{$a_1+a$}};
 \node[circle,draw=black] (P2) [right of=P1] {\ub{$a_{a_2}+a^b$}};
 \node[circle,draw=black] (P3) [right of=P2] {\ub{$a_{a_{a_3}}+a^{b^{c^d}}$}};
 \node[circle,draw=black] (P4) [right of=P3] {\ub{$a_{a_{a_4}}+a^{b^{c^{d^e}}}$}};
\end{tikzpicture}