(eplain.info)Iteration

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7.3 Iteration
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You can iterate through a comma-separated list of items with '\for'.
Here is an example:

     \for\name:=karl,kathy\do{%
        \message{\name}%
     }%

This writes 'karl' and 'kathy' to the terminal. Spaces before or after
the commas in the list, or after the ':=', are _not_ ignored. To strip
leading spaces off the items, use '\For':

     \For\name:=
        karl,
        kathy\do{%
           \message{\name}%
        }%

Note that trailing spaces are still _not_ ignored.

   Both '\for' and '\For' expand the first token of the item list fully,
so this is equivalent to the above:

     \def\namelist{karl,kathy}%
     \for\name:=\namelist\do ...

However, this won't work, either with '\for' or with '\For':

     \def\namelist{karl,kathy}%
     \For\name:= \namelist\do ...

because '\for' and '\For' expand the first token after ':=' which is
space, not '\namelist'.

   Eplain provides another kind of loops, which is an extension of plain
TeX's '\loop'. If you say:

     \loop
       LOOP-TEXT
     \if CONDITION
       IF-TEXT
     \repeat

then LOOP-TEXT will be repeated as long as CONDITION is satisfied ('\if'
can be any of the TeX's conditional commands, without the matching
'\fi'). Eplain extends this with the optional else clause:

     \loop
       LOOP-TEXT
     \if CONDITION
       IF-TEXT
     \else
       ELSE-TEXT
     \repeat

Here, LOOP-TEXT will be repeated as long as CONDITION is _not_
satisfied. This extension is from Victor Eijkhout's TeX by Topic
(page 104).