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Ruby Programming
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But It Doesn't Work!

So you've read through enough of the book, you start to write your very own Ruby program, and it doesn't work. Here's a list of common gotchas and other tips.

  • Attribute setter not being called. Within an object, Ruby will parse setter= as an assignment to a local variable, not as a method call. Use self.setter= to indicate the method call.

  • A parse error at the last line of the source often indicates a missing end keyword.

  • Make sure that the type of the object you are using is what you think it is. If in doubt, use Object#type to check the type of an object.

  • Make sure that your methods start with a lowercase letter and that classes and constants start with an uppercase letter.

  • If you happen to forget a ``,'' in an argument list---especially to print---you can produce some very odd error messages.

  • Block parameters are actually local variables. If an existing local of the same name exists when the block executes, that variable will be modified by the call to the block. This may or may not be a good thing.

  • Watch out for precedence, especially when using {} instead of do/end.

  • Make sure that the open parenthesis of a method's parameter list butts up against the end of the method name with no intervening spaces.

  • Output written to a terminal may be buffered. This means that you may not see a message you write immediately. In addition, if you write messages to both $stdout and $stderr, the output may not appear in the order you were expecting. Always use nonbuffered I/O (set sync=true) for debug messages.

  • If numbers don't come out right, perhaps they're strings. Text read from a file will be a String, and will not be automatically converted to a number by Ruby. A call to to_i will work wonders. A common mistake Perl programmers make is:

    while gets
      num1, num2 = split /,/
      # ...
    end
    

  • Unintended aliasing---if you are using an object as the key of a hash, make sure it doesn't change its hash value (or arrange to call Hash#rehash if it does).

  • Use trace_var to watch when a variable changes value.

  • Use the debugger.

  • Use Object#freeze . If you suspect that some unknown portion of code is setting a variable to a bogus value, try freezing the variable. The culprit will then be caught during the attempt to modify the variable.

There's one major technique that makes writing Ruby code both easier and more fun. Develop your applications incrementally. Write a few lines of code, then run them. Write a few more, then run those. One of the major benefits of an untyped language is that things don't have to be complete before you use them.
Ruby Programming
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  Published under the terms of the Open Publication License Design by Interspire