Thank you Danny. I used to program in Visual Basic and then Java with related stuff, such as HTML, Javascripts. I learned C and Pascal in school. Racket is quite different. It seems I need to use recursion much more. <div class="gmail_quote">On Thu, Jun 2, 2011 at 11:56 AM, Danny Yoo <<a href="mailto:dyoo@cs.wpi.edu">dyoo@cs.wpi.edu</a>> wrote: <blockquote style="BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8ex; PADDING-LEFT: 1ex" class="gmail_quote"> <div class="im">On Thu, Jun 2, 2011 at 1:00 PM, Yingjian Ma <<a href="mailto:yingjian.ma1955@gmail.com">yingjian.ma1955@gmail.com</a>> wrote: > Thank you Stephen. The code does not work in general. I know how to do it > now. </div>It sounds like you're beginning to learn Racket, which is cool. You may find the the Quick Start documentation at: <a href="http://docs.racket-lang.org/quick/index.html" target="_blank">http://docs.racket-lang.org/quick/index.html</a> a useful way to get started with Racket. At the very least, the quick start will show off some of Racket's capabilities. The Guide at: <a href="http://docs.racket-lang.org/guide/index.html" target="_blank">http://docs.racket-lang.org/guide/index.html</a> can be helpful, but it assumes that you already have some knowledge in general programming. What kind of background do you already have? Are you trying to learn Racket specifically, or general programming? If you're just trying to learn general programming, a systematic textbook, like How to Design Programs book, may help you: <a href="http://htdp.org/" target="_blank">http://htdp.org/</a> </blockquote></div>