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    <p>Here is how to write a reversing program in general. I'm not giving you Java, I'm giving you "pseudocode".</p> <pre><code>function print_reverse(file) local_variable line line = read_from(file) if (we are not at end of file) print_reverse(file) print line </code></pre> <p>Because <code>line</code> is a local variable, you get a new instance of it on each call to <code>print_reverse()</code>. When you read in the whole file, and you want to print the lines in reverse, the lines need to be stored somewhere. In this recursive function, the lines are stored one at a time, one in each call to <code>print_reverse()</code>.</p> <p>I like to think of recursive functions as "winding" further and further until they hit a limit, then "unwinding" as they come back out. The limit is called the "basis case". With any recursive function you need to have a clear idea of what your basis case is. For <code>print_reverse()</code>, the basis case is hitting the end of file on the input file.</p> <p>After <code>print_reverse()</code> hits its basis case, it stops calling itself recursively; it prints a line and then unwinds. As each recursive call ends, it returns to the previous recursive call, which then in turn prints its line and unwinds again. This continues until the first call prints its line and terminates, at which point the recursion is finished and all lines have been printed.</p> <p>So, to summarize: when "winding" we read a line and save it, the basis case is end of input, and when "unwinding" we print a saved line. Since unwinding occurs in the exact opposite order of winding, the lines print in reversed order.</p> <p>If the input file is very large, this recursive solution may use up all the available stack space, in which case the program will crash. If you wanted to write a file-reversing program that could handle input files of any size, recursion is not going to work. However, look at how clean and simple this program is. Some problems are easier to code and understand if you use a recursive solution.</p> <p>Reversing a file is pretty easy to do iteratively; just use a loop to read each line from the file and keep appending lines to some sort of list, then loop over the list in reverse printing lines. But other programs are elegantly simple when you write them recursively, and much harder if you don't. For example, the "Towers of Hanoi" puzzle has a very clean recursive solution.</p> <p><a href="http://www.mathcs.emory.edu/~cheung/Courses/170/Syllabus/13/hanoi.html" rel="nofollow">http://www.mathcs.emory.edu/~cheung/Courses/170/Syllabus/13/hanoi.html</a></p>
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