[Tex/LaTex] Redefine command when inside a specific other command possible

conditionalsmacrosnesting

How, in general, can I (re)define commands based on in which other command they are nested?

A more specific example: I have a custom latex command (say, \code{}) that makes text appear bold. However, when used inside another custom command (such as in \question{Will you use \code{command 1} or \code{command 2}?}), I want the \code{} text to be NOT bold. So I want to redefine \code{} when inside another command.

BTW: When using CSS to format HTML pages, it would go something like this:

.code {font-weight: bold;}
.question .code {font-weight: normal;}

Best Answer

Yes, you can.

\newcommand\question[1]{{% extra brace
  \renewcommand\code[1]{\textit{##1}}% double #
  whatever you want for #1}}

Explanation:

Unless we are totally replacing the existing definition, command redefinitions are usually tweaks of existing commands. So, we still need access to the original functionality. Hence, command redefinitions are usually done in two stages: The first is to save the existing definition under a new name, and the second is to use this older definition when required.

First save the original definition of \multicolumn as \OldMulticolumn. You can do this with:
```
\let\OldMulticolumn\multicolumn
```
In this case \let is fine, but if you wish to redefine a macro that has optional parameters, you have to use \LetLtxMacro from the letltxmacro package . A detailed description of \LetLtxMacro can be found at this question at closed square root symbol. So in this case it is not necessary to use \LetLtxMacro, but is fine to do so.
Then, we redefine \multicolumn as desired and use \OldMulticolumn when we need to invoke the original multicolumn functionality:
```
\DeclareExpandableDocumentCommand{\multicolumn}{O{2} O{c |} m}{%
    \OldMulticolumn{#1}{#2}{#3}%
}%
```
From the link in the References section the command has to be expandable since it is used within a tabular environment, so \DeclareExpandableDocumentCommand works.
The next part {O{2} O{c |} m} provides the specification of the three parameters.

The first O{2} says that the first parameter, #1, is an optional parameter (specified in square brackets) and if it not provided it defaults to 2. In this particular case this is the number of columns that the \multicolumn is to span.

The next O{c |} says that the second parameter, #2, is also optional and if not provided it defaults to c |. This is the alignment specification that would be the second parameter of the original \multicolumn.

The last parameter is of type m which means that this is a mandatory parameter that needs to be provided with the usual {}.

So, based in the arg spec of ...{\MultiColumn}{O{2} O{c |} m} (using a different name here to hopefully avoid confustion with the existing \multicolumn) the following three would equivalent:
```
\\MultiColumn{xyz}%         
\\MultiColumn[2]{xyz}%
\\MultiColumn[2][c |]{xyz}%
```
all three have the parameters #1=2, #2=c |, #3=xyz.

For more details please refer to the xparse package documentation.

References:

The tabular environment requires expandable content. See Misplaced \omit. \multispan with \newcommand for more details.

Code:

\documentclass{article}
\usepackage{xparse}
\usepackage{letltxmacro}

\LetLtxMacro{\OldMulticolumn}{\multicolumn}

\DeclareExpandableDocumentCommand{\multicolumn}{O{2} O{c |} m}{%
    \OldMulticolumn{#1}{#2}{#3}%
}%


\begin{document}
\begin{tabular}{|l | l |l |}\hline
    a & \multicolumn{some long centered content}\\\hline
    b & c & d\\\hline
    a & \multicolumn[2][l|]{left content}\\ \hline
\end{tabular}
\end{document}

The above is the normal usage of \multicolumn. Note that in order to specify the alignment, the number of columns needs to be specified. One could certainly enhance the macro to test for an empty parameter and replace it with the default value -- that is using it as \multicolumn}[][r |]{x} would be equivalent to \multicolumn}[2][r |]{x}.

However, in my cases the number of columns to span in a table is usually fixed so I prefer to swap the order of the options and so that I can just provide the alignment. Now using \multicolumn[r |]{right content} I get right aligned content spanning the 2 columns:

enter image description here

Code:

\documentclass{article}
\usepackage{xparse}
\usepackage{letltxmacro}

\LetLtxMacro{\OldMulticolumn}{\multicolumn}

\DeclareExpandableDocumentCommand{\multicolumn}{O{c |} O{2} m}{%
    \OldMulticolumn{#2}{#1}{#3}%
}%


\begin{document}
\begin{tabular}{|l | l |l |}\hline
    a & \multicolumn{some long centered content}\\\hline
    b & c & d\\\hline
    a & \multicolumn[r |]{right content}\\ \hline
\end{tabular}
\end{document}

Define a macro that holds the value that \f@series assumes for bold

If you only want to cover b versus m then

\def\extracted@font@series@aux#1#2\stop{#1}
\edef\extracted@font@series{\expandafter\extracted@font@series@aux\f@series?\stop}

will do the job: you'll have a single letter which you can then test using for example

\if b\extracted@font@series TRUE\else FALSE\fi

Things can be a lot more complex: you might have a variety of both text and math mode weights in play. For siunitx I need to cover all of this, allowing for the full range of text weights, etc., as listed in the linked question. That's a question of checking for the special cases (where the weight is m but it's not in \f@series), then taking the weight you find and matching it to a math version. That can involve a two-letter weight, of course. You just have to work systematically.

Best Answer

Related Solutions

[Tex/LaTex] Redefine existing command

Explanation:

References:

Code:

Code:

Define a macro that holds the value that \f@series assumes for bold

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