@nameuse with value \relax in conditionals

Question

This is a follow-up to this question: @nameuse as conditional where i asked about how to use \expandafter if i combine \@nameuse with \ifx. However, in my real-life example, i use \relax as a default value for dynamically assigned csnames. I want to know if these control sequences have a specific value, or if they are default. My naive approach would be something like:

\documentclass{standalone}

\makeatletter
\parindent\z@
\@namedef{blafasel}{\relax}


\begin{document}
\expandafter\expandafter\expandafter\ifx\@nameuse{blafasel}\relax
   true
\else
   false
\fi

\end{document}

but this doesn't work. In its simplest form it gives "false", but in more complex form (with xmltex, a macro definition, and another ifx-structure around) i get Extra \else errors. I'd like to understand why that is and what i need to do to make it work.

Bonus: I'd prefer a “plain” solution, i.e. no expl3-syntax or complex packages since i want to be as independent as possible from my code's user's configurations.

EDIT:

Here is the problem i'm facing: I want to store some information of the cells of a table. I'm using David Carlisle's xmltex mechanism to parse html tables. Elements are grabbed and transmitted directly into corresponding tex macros. The necessary counters are incremented at the beginning of every such macro.

In the cell definiton iterate over all cells and use a global macro

\gdef\SetCellProp#1#2{\expandafter\def\csname cell-\the\curtab-\the\currow-\the\curcel-#2\endcsname{#1}}

to store properties like the cells calculated width (content in \mybox; \@tempdima=\wd\mybox), its content, and its background-color. E.g., i am in the second table, third row, second column, then the width of that cell (e.g. "12mm") is stored with \SetCellProp{12mm}{width} which "creates" a control sequence named \cell-3-2-2-width that yields the value "12mm".

Likewise, i have want to store the biggest value for width so far using

\gdef\SetColProp#1#2{\expandafter\def\csname col-\the\curtab-\the\curcel-#2\endcsname{#1}}

The macro that returns that value is defined as

\gdef\GetCurColProp#1{\csname cell-\the\curtab-\the\curcel-#1\endcsname}

Note, that this definition doesn't care about the row i am in. Since i re-count cells each row, i need to decide whether to overwrite the maximum value, or not if it is present at all.

To do so, in each cell, we need to check if

• 1. a maxwidth for the column the current cell is in, is already given
• 2a. if no, make the current cell's width maxwidth for the whole column
• 2b. if yes, look if the value for the current width is larger than maxwidth for the column
• 3a if yes, make that current value the new maxwidth for the while column,
• 3b if no, leave everything as it is.

The part of my cell definition so far looks like:

  %% check if maxwidth for the col is already given:
  \ifx\csname cell-\the\curtab-\the\curcel-maxidth\endcsname\relax
    %% if no, take current width (\@tempdima) as maxwidth
    \expandafter\SetColProp\expandafter{\the\@tempdima}{maxwidth}%
  \else
    %% else check if current width is larger than  maxwidth:
    \expandafter\@tempdimb\GetCurColProp{maxwidth}% 
    \ifdim\@tempdimb<\@tempdima
      %% if so, reassign maxwidth with curwidth:
      \expandafter\SetColProp\expandafter{\the\@tempdima}{maxwidth}%
    \fi
  \fi

where \curtab counts the tables, \currow counts the current row, and \curcel the number of the cell in the row (gets reset to 1 at the beginning of every row). The aim is to get the maximum width of any column my table has.

I hope that explains my problem a little bit.

Answer 1

The problem you will confront will be that, unless you know in advance the types of arguments that may be passed to this test, they each may require differing numbers of expansions to finally reach the \relax or not-\relax condition.

Perhaps you need to fully expand your argument for the test:

\documentclass{standalone}

\makeatletter
\parindent\z@
\@namedef{blafasel}{\relax}


\begin{document}
\edef\tmp{\@nameuse{blafasel}}% MUST, HOWEVER, BE FULLY EXPANDABLE
\expandafter\ifx\tmp\relax
   true
\else
   false
\fi

\end{document}

Of course, this approach only works if the test argument is fully expandable.

---

Alternately, if it will always be a \@nameuse that is part of the test, then the use of 3x\expandafter only produces 2 expansions. You need 7x\expandafter to get 3 expansions, resulting in a true test.

Rule: to get n expansions, you need 2^n - 1 instances of \expandafter.

\documentclass{standalone}

\makeatletter
\parindent\z@
\@namedef{blafasel}{\relax}


\begin{document}
\expandafter\expandafter\expandafter\expandafter\expandafter%
  \expandafter\expandafter\ifx\@nameuse{blafasel}\relax
   true
\else
   false
\fi

\end{document}

Answer 2

You just need to give no initial definition to \blafasel. If you want to test whether it has received a definition, just do

\expandafter\ifx\csname blafasel\endcsname\relax
   <still not defined>%
\else
   <is defined>%
\fi

This exploits the fact that a token constructed with \csname is made equivalent to \relax if it has no previous definition.

Answer 3

\@nameuse{blafasel} expands to \csname blafasel\endcsname, which you expand once again yielding \blafasel. You then compare this to \relax, which is different. You have three possibilities of solving your problem:

1. You can expand \blafasel once more:

   \@namedef{blafasel}{\relax}%
   \expandafter\expandafter\expandafter\expandafter
   \expandafter\expandafter\expandafter\ifx\@nameuse{blafasel}\relax
       true
   \else
       false
   \fi
   

   Note that this \ifx will always follow the true branch when the expansion \blafasel starts with two equal tokens and is thus not a good test. Try \@namedef{blafasel}{OOH, LOOK, AN ERROR!}, for example.

2. You can define a different macro to compare it to:

   \@namedef{blafasel}{\relax}%
   \def\@test@macro{\relax}%
   \expandafter\expandafter\expandafter\ifx\@nameuse{blafasel}\@test@macro
       true
   \else
       false
   \fi
   

3. You can initialize your macros to be the same as \relax instead of a macro expanding to \relax:

   \def\@namelet#1#2{\expandafter\let\csname #1\endcsname#2}%
   \@namelet{blafasel}\relax
   \expandafter\expandafter\expandafter\ifx\@nameuse{blafasel}\relax
       true
   \else
       false
   \fi
   

   As egreg points out, you could just as well leave \blafasel undefined (i.e. omit the first two lines of this code block), since \csname will default to \relax, when the macro is not defined.