System Macros/Loops and Recursion

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TeX does not offer us powerful for-loop mechanisms. On the other hand its recursion engine is quite unique. We therefore identify the for-looping macros by this method. The most simple alternative is the one that only needs a number.


\dorecurse {n} {whatever we want}

This macro can be nested without problems and therefore be used in situations where Plain TeX's \loop macro ungracefully fails. The current value of the counter is available in \recurselevel, before as well as after the whatever we want stuff.

\dorecurse               % inner loop
  {\recurselevel:          % outer value
     \dorecurse          % inner loop
       {\recurselevel}     % outer value
       {\recurselevel}     % inner value
     \dorecurse          % inner loop
       {\recurselevel}     % outer value
       {\recurselevel}     % inner value

In this example the first, second and fourth \recurselevel concern the outer loop, while the third and fifth one concern the inner loop. The depth of the nesting is available for inspection in \recursedepth.

Both \recurselevel and \recursedepth are macros. The real conters are hidden from the user because we don't want any interference.


The simple command \dorecurse is a special case of the more general:

\dostepwiserecurse {from} {to} {step} {action}

This commands accepts positive and negative steps. Illegal values are handled as good as possible and the macro accepts numbers and counters.

\dostepwiserecurse  {1} {10}  {2} {...}
\dostepwiserecurse {10}  {1} {-2} {...}

{{cmd|doloop</cmd> {{cmd|exitloop</cmd>

Sometimes loops are not determined by counters, but by (a combinations of) conditions. We therefore implement a straightforward loop, which can only be left when we explicitly exit it. Nesting is supported. First we present a more extensive alternative.

  {Some kind of typesetting punishment \par
   \ifnum\pageno>100 \exitloop \fi}

When needed, one can call for \recurselevel and \recursedepth.

The loop is executed at least once, so beware of situations like:

\doloop {\exitloop some commands}

It's just a matter of putting the text into the \if statement that should be there anyway, like in:

\doloop {\ifwhatever \exitloop \else some commands\fi}

You can also quit a loop immediately, by using \exitloopnow instead. Beware, this is more sensitive for conditional errors.

Recursion and expansion

Using recursion to build tables requires some particular attention:


Multi-dimensional tables offer another illustration:

	\dorecurse{5}{\bTD #1,##1 \eTD}

Alternatively, (mkiv only?)

        \dorecurse{5}{\bTD \currentTABLErow,\currentTABLEcolumn \eTD}

For further discussion on loops and expansion, see Expansion control as well as this blog post.

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