Math & Science ⇒ Arrays and Expansion

[kazza]

Recently I have found myself using arrays with quite fancy format specifications (particually for case distinction of mixfix operators). For example

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\begin{array}{@{\llbracket}l@{,}l@{\models}l@{\rrbracket~=~}l}
M & s & true & true \\
...
\end{array}

Then the inevitable occoured, I wanted to tweak (bold) one of the lines of the array. So I defined the following

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\newcommand{\ifother}[2]{{{{\ifx\myother\undefined{#2}\else{#1}\fi}}}}
\newcommand{\startother}{\def\myother{}}
\makeatletter
\newcommand{\finishother}{\let\myother\@undefined}
\makeatother

with the intention to use it in the format specification for the array to beable to switch between one symbol and another. I.e.

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\begin{array}{@{\ifother{\llbracket}{\boldsymbol{\llbracket}}}l@{,}...}
... \\
\startother ... \\
... \\
\finishother ... \\
... \\
\end{array}

But I soon found that the array expands its arguments early, so the ifx test occours once at the begining of the array and its expanded value is reused.

As a concrete example

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\begin{array}{@{\ifother{b}{a}~}l}
1 \startother\\
2 \\
3 \endother \\
4 \\
\end{array}

outputs:

a 1
a 2
a 3
a 4

but i would like it to output:

a 1
b 2
b 3
a 4

Ive done some experiments with noexpand/expandafter/aftergroup but could not find the right combination. Could anyone help?

[kazza]

ah, I found my own solution. I had to use the latex protect command, and use global defs. i.e.

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    \newcommand{\ifother}[2]{{{{\ifx\myother\undefined{#2}\else{#1}\fi}}}}
    \newcommand{\startother}{\global\def\myother{}}
    \makeatletter
    \newcommand{\finishother}{\global\let\myother\@undefined}
    \makeatother

and

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\begin{array}{@{\protect\ifother{b}{a}~}l}