Open File and Read

I mentioned I needed a project to work on and Zach Beane mentioned an SVG parser. I thought it sounded really cool, but I have never programmed a parser before in any language. So, I started writing code yesterday that hopefully will turn into an SVG parser. I started with simply opening an SVG and reading it line by line.

An SVG is actually created by using XML. I heard you can use javascript to do this, but I don’t know much about that yet. I have been reading the w3.org site for SVG information. They had an example of an upside down triangle, so I grabbed that to test my code. Here is the XML code that makes up the SVG. If you copied this to a text file and then saved it as a .svg, you could open the file with your web browser and see the red upside down triangle it creates.

<?xml version=”1.0″ standalone=”no”?>
<!DOCTYPE svg PUBLIC “-//W3C//DTD SVG 1.1//EN”
“http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd”>
<svg width=”4cm” height=”4cm” viewBox=”0 0 400 400″
xmlns=”http://www.w3.org/2000/svg” version=”1.1″>
<title>Example triangle01- simple example of a ‘path’</title>
<desc>A path that draws a triangle</desc>
<rect x=”1″ y=”1″ width=”398″ height=”398″
fill=”none” stroke=”blue” />
<path d=”M 100 100 L 300 100 L 200 300 z”
fill=”red” stroke=”blue” stroke-width=”3″ />
</svg>

I have been using CUSP lately to do all of my Common Lisp coding. Especially after talking with Jasko at the programming meetup we had last week. I like using it, plus I can use it on Windows and that is all I have at work, so it makes it more convenient.

What took me the longest with this code was the Windows pathname. I wasn’t sure how SBCL looks at that. SBCL is what CUSP uses as it’s implementation, by the way. Which is great, because I want to use SBCL. I had to play with it quite a bit but finally figured out the pathname. I placed my tri.svg file in my C:\lispstuff directory. So after playing in the REPL and trying different options, I figured out SBCL reads it as “C:\\lispstuff\\tri.svg”.

Once I had that, I knew the best thing to use to open a file and read it was the Common Lisp macro WITH-OPEN-FILE. Peter Seibel talks about this in his Practical Common Lisp book in chapter 14. Then I used a do loop to step through each line of the file and “format” to print them out. Here is the code:
Keep in mind this code is indented, but WordPress really sucks when it comes to handling code. I need to switch this to a different blog.
I uploaded the actual text file so you could see the indention, get it here.

(defun pjmain ()
(with-open-file (stream “C:\\lispstuff\\tri.svg”)
(do ((line (read-line stream nil)
(read-line stream nil)))
((null line))
(format t “~A~%” line))))

I will have to revisit this code for the parser, because I need a way for it to ask the person what SVG file they would like to parse. The next thing I need to do is try to parse this. I have a lot of reading to do. I know Zach mentioned he only needed the path data of the SVG, so I will concentrate on obtaining that first.

Any tips or suggestions on any of this is definitely welcome. I am a beginner after all.