Example 8.1: Simple Loop to Draw an Octagon
4 setlinewidth
300 200 moveto
7 { %repeat
0 100 rlineto
45rotate
}bind repeat closepath stroke
Figure 8.1: Output of Example 8.1
output page
starting point 300,200
USING THE LOOP INDEX
Looping operators are useful because they let a task be performed more than once. They can be even more useful when you keep track of the loop index and use it effectively.
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Example 8.2: Ignoring Loop Index
gsave
250 250 translate
0 10 360 { %for
pop % do not use loop index this time 0 -50 moveto 200 0 lineto 0 50 lineto
10 rotate
} for stroke
grestore
Example 8.2 sets up the loop index to give 10-degree increments around a circle, but actually does not use the loop index explicitly within the loop. The results are shown in Figure 8.2.
Figure 8.2:Output of Example 8.2
output page
In Example 8.3 is a short program that uses the loop index to set the line weight of a series of stroked lines and to control their spacing; its results are shown in Figure 8.3.
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Example 8.3: Using Loop Index for Line Weight
00 100 translate
2 3 div setgray 0 0 612 250 rectfill
0 setgray
2 2 20 { %for
dup setlinewidth % use loop index as line weight 0 exch 12 mul moveto% and use for line spacing 612 0 rlineto
stroke
} for
Figure 8.3: Output of Example 8.3
output page
When using a loop index, make sure you either pop the unwanted data off the stack or use it within the loop body, or you will leave unwanted data on the operand stack which may affect other portions of the program.
Several looping operators push data onto the stack each time around the loop. In particular, the forall operator pushes each element of an array or string onto the stack; it pushes both the key and value for each dictionary entry onto the stack. Table 8.2 provides a summary of each looping operator and the data that are pushed onto the stack for each iteration of the loop.
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TIP |
Don’t forget that the for, forall, kshow, and filenameforall looping oper- |
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ators push values onto the operand stack that must either be used or |
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popped from the stack. |
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Table 8.2: Arguments Supplied by Looping Operators |
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Operator |
Data Pushed onto Operand Stack for Each Iteration |
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for |
loop index (integer) |
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forall |
for strings: integer ASCII code for each byte of string |
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for arrays: each element of the array, of arbitrary type |
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for dictionaries: both the key and the value for each entry |
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kshow |
character code for character just printed and for the |
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character about to be printed |
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loop |
none |
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repeat |
none |
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filenameforall |
a string containing each file name that matches the pattern |
Example 8.4 shows both a forall loop and a filenameforall loop being used to find out all the font names defined in FontDirectory and on the disk of a printer. Notice the way that the data are used inside the loop bodies.
Example 8.4: Finding Font Names with Looping Operators
/scratch 128 string def FontDirectory { %forall
pop % get rid of “value” part of dictionary entry scratch cvs print (\n) print
} bind forall
(fonts/*) { %filenameforall
dup length 6 sub 6 exch getinterval print (\n) print } scratch filenameforall
As you can see, there are a lot of ways to use the looping operators effectively, especially if you take advantage of the operators that push data onto the operand stack during execution of the loop body.
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