postscript - Updating a variable in a procedure called multiple tmies -

i'm trying generate postscript "bitmap" making repeated calls block-drawing procedure.

rather define position of block on each call, i'd procedure update starting position automatically, based on known block size of (say) 100 pixels, this:

% starting position /px { 72 } def /py { 720 } def  % block-drawing procedure, input= r-g-b /block {     setrgbcolor     px py moveto     0 100 rlineto     100 0 rlineto     0 100 neg rlineto     fill     % adjust px , py 100 , -100, ahead of next call  } def  % draw 3 increasingly lighter boxes, automatically shifted  0 0 0 box  .25 .25 .25 box .5 .5 .5 box % etc... 

is there simple way of achieving this?

yes, alter values of px , py.

note code defines px , py executable arrays, which, when executed, leave value on operand stack. there no need that, this:

/px 72 def /py 720 def 

works well.

i think missing fact 'def' associates key , value in current dictionary. since haven't started new dictionary, default one, userdict. if use 'def' again, associates new value same key in dictionary.

so if do:

% starting position /px 72 def /py 720 def  % block-drawing procedure, input= r-g-b /block {     setrgbcolor     px py moveto     0 100 rlineto     100 0 rlineto     0 100 neg rlineto     fill     % adjust px , py 100 , -100, ahead of next call      px 100 add /px exch def     py 100 sub /py exch def } def  % draw 3 increasingly lighter boxes, automatically shifted  0 0 0 box  .25 .25 .25 box .5 .5 .5 box % etc... 

that seem want.