mfpic-doc.pdf.

4.12 FOR ADVANCED USERS. 72
where 〈variable〉 is as before, while 〈start〉, 〈delta〉 and 〈stop〉 are numeric values. If 〈delta〉 is
positive the loop is skipped entirely if 〈stop〉 is less than 〈start〉. Otherwise the loop is executed
successively with the variable equal to 〈start〉, then 〈start〉 + 〈delta〉 then 〈start〉 + 2〈delta〉, etc., as
long as the variable is not greater than 〈stop〉. The behavior is similar if 〈delta〉 is negative, except
the loop is repeated only as long as the variable is not less than 〈stop〉. If 〈delta〉 is 0, then the
METAFONT run will generate an error.
Note that the index variable (center and radius in the above two examples) is a temporary
METAFONT variable. If mplabels is turned on, this variable will work as expected in the location
parameter of a \tlabel command, but if it is used in the label part, it will be interpreted as TEX
code and printed as is. The index variable reverts to its previous state outside the loop. That is, if it
existed before the loop, it regains its previous value after the loop, and if it was undefined before the
loop, it is again undefined after.
The single word ‘upto’ can be used as an abbreviation for “step 1 until” and ‘downto’ for
“step -1 until” in for-loop headers. Spaces are not significant in for-loop headers, except to
distinguish the keywords (e.g. step) from variable names that might be used (e.g., for 〈start〉).
\mfpwhile{〈condition〉}
〈MFPIC commands〉
\endmfpwhile
The 〈condition〉 should be an expression that can be either true or false about a METAFONT
variable that changes at some time during the loop body. The loop body is executed (by METAFONT)
as long as the condition is true. Example:
\setmfvariable{numeric}{R}{20}
\mfpwhile{R > 1}
\rect{(0,0), (R,3R)}
\mfcmd{R:=R/2}
\endmfpwhile
There are no MFPIC commands to systematically change a variable, so in this example we have
resorted to directly writing a METAFONT assignment command via \mfcmd (see subsection 4.12.3
above) that reduces R by half. The loop will be executed with R having the successive values 20,
10, 5, 2.5, and 1.25. The resulting picture could have been achieved with \mfpfor using this list of
values.
\mfploop
〈MFPIC commands〉
\mfpuntil{〈condition〉}
〈MFPIC commands〉
\endmfploop
The body of this loop will be repeated until the 〈condition〉 becomes true. The condition should
be some expression that can be either true or false about a variable that changes during the loop execution.
It should eventually become true. If an mfploop environment does not contain an \mfpuntil
command, then the \endmfploop command will generate a warning message. If the warning is ignored,
and the user has not otherwise arranged for loop termination, 19 the .mf file will contain an
infinite loop. The \mfpuntil command will break the loop at whatever point it occurs. Example:
19 Perhaps by means of \mfsrc commands. It is because of this possibility that only a warning is produced and not an error.
If the warning becomes annoying, adding \mfpuntil{false} to the loop will silence it. This command will never break the
loop because the condition false (of course) never becomes true.