Array — dynamic !{decl.} parameter

Question

I use !{=} in array to make compact equation, but on the last line I'd like to have \approx instead of =.

Is it possible ?

\documentclass{standalone}

\usepackage{array,tabularx}

\begin{document}

$\begin{array}{r!{=}l}
    TS  &TU \times\cos\hat{T} \\
        &7.5\times\cos\,41.5\\
        &5.6\\ % \approx here instead of =
\end{array}$

\end{document}

Answer 1

I'm not sure this is really time-saving.

\documentclass[border=2]{standalone}

\usepackage{array}

\newcommand{\noeq}[1]{\multicolumn{1}{r!{#1}}{}}

\begin{document}

$\begin{array}{r!{=}l}
TS             &TU \times\cos\hat{T} \\
               &7.5\times\cos 41.5 \\
\noeq{\approx} &5.6
\end{array}$

\end{document}

I'd do it like this; note that the spacings are the normal ones around the relation symbols.

\documentclass[border=2]{standalone}

\usepackage{array}

\begin{document}

$\begin{array}{r@{}>{{}}l}
TS &= TU \times\cos\hat{T} \\
   &= 7.5\times\cos 41.5 \\
   &\approx 5.6
\end{array}$

\end{document}

This might be even more appealing, as it automatically takes care of the space at either side.

\documentclass[border=2]{standalone}

\usepackage{amsmath}

\begin{document}

$\begin{aligned}
TS &= TU \times\cos\hat{T} \\
   &= 7.5\times\cos 41.5 \\
   &\approx 5.6
\end{aligned}$

\end{document}

Answer 2

For extra flexibility, do consider creating a 3-column array, with the middle column containing the relational operator -- either = or \approx.

\documentclass{article}
\usepackage{array} % for "\newcolumntype" macro
\newcolumntype{C}{>{{}}c<{{}}}
\begin{document}

$\setlength\arraycolsep{0pt}
  \begin{array}{rCl}
     TS &=&TU \times\cos\hat{T} \\
        &=&7.5\times\cos\,41.5\\
        &\approx&5.6 
  \end{array}$
\end{document}

Answer 3

\documentclass{standalone}

\usepackage{array}

\newif\ifUseApprox
\protected\def\Equal{\ifUseApprox\approx\else=\fi}
\def\EqualUseApprox{\global\UseApproxtrue}
\def\EqualUseEqual {\global\UseApproxfalse}

\begin{document}

$\begin{array}{r!{\Equal}l}
    TS  &TU \times\cos\hat{T} \\
        &7.5\times\cos\,41.5\\
   \EqualUseApprox     &5.6\\ % \approx here instead of =
\end{array}$

\end{document}

\documentclass[varwidth, border=10pt]{standalone}

\usepackage{array, shortvrb}

\newif\ifUseApprox
\protected\def\Equal{\ifUseApprox\approx\else=\fi}
\def\EqualUseApprox{\global\UseApproxtrue}
\def\EqualUseEqual {\global\UseApproxfalse}

\begin{document}
\hsize 6cm

$\begin{array}{r!{\Equal}l}
    TS  &TU \times\cos\hat{T} \\
        &7.5\times\cos\,41.5\\
   \EqualUseApprox     &5.6\\ % \approx here instead of =
\end{array}$

I now want to have a line \verb|\approx 7.5\times 0.75| 
followed by a line with \verb|= 5.63| !

\EqualUseEqual % reset

$\begin{array}{r!{\Equal}l}
    TS  &TU \times\cos\hat{T} \\
        &7.5\times\cos\,41.5\\
   \EqualUseApprox     &5.6\\ % \approx here instead of =
        &7.5\times 0.75\\
   \EqualUseEqual      &5.63\\ % again using =
\end{array}$

\end{document}