MetaFun - Pragma ADE

1.8 Loops
Yet another programming concept present in METAPOST is the loop statement, the familiar ‘for loop'
of all programming languages.
for i=0 step 2 until 20 :
draw (0,i) ;
endfor ;
As explained convincingly in Niklaus Wirth's book on algorithms and datastructures, the for loop
is the natural companion to an array. Given an array of length n, you can construct a path out of
the points that make up the array.
draw for i=0 step 1 until n-1 : p[i] .. endfor p[n] ;
If the step increment is not explicitly stated, it has an assumed value of 1. We can shorten the
previous loop construct as follows:
draw for i=0 upto n-1 : p[i] .. endfor p[n] ;
After seeing if in action, the following for loop will be no surprise:
draw origin for i=0 step 10 until 100 : ..{down}(i,0) endfor ;
This gives the zig--zag curve:
You can use a loop to iterate over a list of objects. A simple 3--step iteration is:
for i=p,q,r :
fill i withcolor .8white ;
draw i withcolor red ;
endfor ;
Using for in this manner can sometimes save a bit of typing. The list can contain any expression,
and may be of different types.
In the previous example the i is an independent variable, local to the for loop. If you want to
change the loop variable itself, you need to use forsuffixes. In the next loop the paths p, q and r
are all shifted.
forsuffixes i = p, q, r :
i := i shifted (3cm,2cm) ;
endfor ;
Sometimes you may want to loop forever until a specific condition occurs. For this, METAPOST
provides a special looping mechanism:
Loops Welcome to MetaPost
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