Difference between revisions of "System Macros/Loops and Recursion"

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< '''Prev:''' [[System Macros/Branches and Decisions|Branches & Decisions]] | '''Top:''' [[System Macros]] | '''Next:''' [[System Macros/Action Processing|Action Processing]] >
  
TeX does not offer us powerfull for-loop mechanisms. On
+
TeX does not offer us powerful for-loop mechanisms. On
 
the other hand its recursion engine is quite unique. We
 
the other hand its recursion engine is quite unique. We
 
therefore identify the for-looping macros by this method.
 
therefore identify the for-looping macros by this method.
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number.
 
number.
  
=== <cmd>dorecurse</cmd> ===
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=== {{cmd|dorecurse}} ===
 
<texcode>
 
<texcode>
 
\dorecurse {n} {whatever we want}
 
\dorecurse {n} {whatever we want}
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=== <cmd>dostepwiserecurse</cmd> ===
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=== {{cmd|dostepwiserecurse}} ===
 
The simple command <code>\dorecurse</code> is
 
The simple command <code>\dorecurse</code> is
 
a special case of the more general:
 
a special case of the more general:
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</texcode>
 
</texcode>
  
=== <cmd>doloop</cmd> <cmd>exitloop</cmd> ===
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=== {{cmd|doloop}} {{cmd|exitloop}} ===
 
Sometimes loops are not determined by counters, but by
 
Sometimes loops are not determined by counters, but by
 
(a combinations of) conditions. We therefore implement a
 
(a combinations of) conditions. We therefore implement a
 
straightforward loop, which can only be left when we
 
straightforward loop, which can only be left when we
explictly exit it. Nesting is supported. First we present
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explicitly exit it. Nesting is supported. First we present
 
a more extensive alternative.
 
a more extensive alternative.
  
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</texcode>
 
</texcode>
  
When needed, one can call for <code>\looplevel</code> and
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When needed, one can call for <code>\recurselevel</code> and
<code>\loopdepth</code>.
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<code>\recursedepth</code>.
  
 
The loop is executed at least once, so beware of situations
 
The loop is executed at least once, so beware of situations
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for conditional errors.
 
for conditional errors.
  
[[Category:ConTeXt programming]]
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=== Recursion and expansion ===
[[Category:Inside ConTeXt]]
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 +
Using recursion to build tables requires some particular attention:
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 +
<context source="yes">
 +
\bTABLE
 +
\bTR
 +
\dorecurse{8}{\expanded{\bTD\recurselevel\eTD}}
 +
\eTR
 +
\eTABLE
 +
</context>
 +
 
 +
Multi-dimensional tables offer another illustration:
 +
 
 +
<context source="yes">
 +
\bTABLE
 +
\dorecurse{8}{
 +
\bTR
 +
\dorecurse{5}{\bTD #1,##1 \eTD}
 +
\eTR
 +
}
 +
\eTABLE
 +
</context>
 +
 
 +
Alternatively, (mkiv only?)
 +
 
 +
<context source="yes">
 +
\bTABLE
 +
    \dorecurse{8}{\bTR
 +
        \dorecurse{5}{\bTD \currentTABLErow,\currentTABLEcolumn \eTD}
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    \eTR}
 +
\eTABLE
 +
</context>
 +
 
 +
For further discussion on loops and expansion, see [[System_Macros/Expansion_Control|Expansion control]] as well as this [http://randomdeterminism.wordpress.com/2009/03/05/tex-programming-the-past-the-present-and-the-future/ blog post].
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< '''Prev:''' [[System Macros/Branches and Decisions|Branches & Decisions]] | '''Top:''' [[System Macros]] | '''Next:''' [[System Macros/Action Processing|Action Processing]] >
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 +
[[Category:Programming and Databases]]
 +
[[Category:Tools]]

Latest revision as of 13:27, 9 August 2020

< Prev: Branches & Decisions | Top: System Macros | Next: Action Processing >

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

\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
  {10}
  {\recurselevel:          % outer value
     \dorecurse          % inner loop
       {\recurselevel}     % outer value
       {\recurselevel}     % inner value
     \dorecurse          % inner loop
       {\recurselevel}     % outer value
       {\recurselevel}     % inner value
   \endgraf}

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.


\dostepwiserecurse

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} {...}

\doloop \exitloop

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.

\doloop
  {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:

\bTABLE
\bTR
\dorecurse{8}{\expanded{\bTD\recurselevel\eTD}}
\eTR
\eTABLE

Multi-dimensional tables offer another illustration:

\bTABLE
\dorecurse{8}{
	\bTR
	\dorecurse{5}{\bTD #1,##1 \eTD}
	\eTR
}
\eTABLE

Alternatively, (mkiv only?)

\bTABLE
    \dorecurse{8}{\bTR
        \dorecurse{5}{\bTD \currentTABLErow,\currentTABLEcolumn \eTD}
    \eTR}
\eTABLE

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

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