saving computed number registers for access outside body of “let” operation

Question

Section 14.15 of the PGF manual (pp 150-) describes the let operation. On pg 152 (first example) it is applied to saving coordinate (tuple values) to point registers p1 and p2 using the coordinate function to enable accessing computed coordinates outside the body of the let operation. It seems to me that it should be possible to save and reuse number registers in a similar fashion, but that does not appear to be the case. See proposed code below. I want to use the computed length \n1 as the length of the subrectangle DATASET, but this does not seem possible within the constraints of this syntax. Ideas? Workarounds?

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes,arrows, positioning, calc}
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{tikzpicture}
\setlength\PreviewBorder{5em}%
\begin{document}
\begin{tikzpicture}[auto]
  \tikzstyle{file} = [ellipse, rounded corners, minimum height=1em, minimum width=1em, align=center, draw]
  \tikzstyle{block} = [rectangle, rounded corners, minimum height=2em, minimum width=15em, align=center, draw]
  \node [block, minimum height=10em] (DB){\textbf{Database}};
  % Set coordinate points for sub-rectangles
  % See pg 420 of 2.10 PGF manual for anchors
  % 'let' is defined on pg 150 in Section 14.15 (The Let Operation)
  \path
  let
  \p1 = ($(DB.east)$),
  \p2 = ($(DB.west)$),
  \n1 = {veclen(\x1-\x2,\y1-\y2)/2}
  in
  coordinate(p1) at (\p1)
  coordinate(p2) at (\p2)
  %number(n1) at (\n1) % illustrative syntax
  ;
  % Use n1 for 'minimum width' instead of 2em
  \node [block, draw, minimum width=2em] (DATASET) at ($(DB.north)!2/7!(DB.south)$) {Dataset};
\end{tikzpicture}
\end{document}

Answer 1

You can define a .code key like

\pgfkeys{/tikz/savenumber/.code 2 args={\global\edef#1{#2}}}

which allows you to use

[savenumber={<macro to save to>}{<macro to save>}]

(note the square brackets) somewhere in a TikZ path (including during a let operation).

Your example would thus look like

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes,arrows, positioning, calc}
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{tikzpicture}
\setlength\PreviewBorder{5em}%

\pgfkeys{/tikz/savenumber/.code 2 args={\global\edef#1{#2}}}

\begin{document}
\begin{tikzpicture}[auto]
  \tikzstyle{file} = [ellipse, rounded corners, text width=12em, text height=3em, minimum height=1em, minimum width=1em, align=center, draw]
  \tikzstyle{block} = [rectangle, rounded corners, text width=12em, text height=3em, minimum height=2em, minimum width=15em, align=center, draw]
  \node [block, minimum height=10em] (DB){\textbf{Database}};
  % Set coordinate points for sub-rectangles
  % See pg 420 of 2.10 PGF manual for anchors
  % 'let' is defined on pg 150 in Section 14.15 (The Let Operation)
  \path
  let
  \p1 = ($(DB.east)$),
  \p2 = ($(DB.west)$),
  \n1 = {veclen(\x1-\x2,\y1-\y2)/2}
  in
  coordinate(p1) at (\p1)
  coordinate(p2) at (\p2)
  [savenumber={\n}{\n1}]
  ;
  % Use n1 for 'minimum width' instead of 2em
  \node [block, draw, minimum width=\n] (DATASET) at ($(DB.north)!2/7!(DB.south)$) {Dataset};
\end{tikzpicture}
\end{document}