%!PS
% Draw a series of circles along the diagonal
% Changing colors.  Tricky!
% Illustrates booleans, for loop
% Joseph O'Rourke

10 1 0 setrgbcolor % Green
2 setlinewidth

/inch {72 mul} def

% Place true on the stack if the stack top is an even integer
/even {
	2 mod 0 eq
} def

% Change orgin to center of page
4.25 inch 5.5 inch translate

% Draw circles on diagonal.
% Each is a separate path, and stroked separately.

% start incr end
0 10 200 {
        % Bind counter to i for later use. loop counter is always on top of stack.
	/i exch def
	newpath
	i 10 idiv % Integer divided the counter by 10, for even/odd check
	even
		{1 0 0 setrgbcolor} % Red
		{0 0 1 setrgbcolor} % Blue
		ifelse
	i i 20 0 360 arc % (x,y) = (i,i), i.e., on diagonal
	stroke

} for


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