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option). • If you happen to forget a “,” in an argument list—especially to print—you can produce some very odd error messages. • A parse error at the last line of the source often indicates a missing end keyword, sometimes quite a bit earlier. • An attribute setter is not being called. Within a class definition, Ruby will parse setter= as an assignment to a local variable, not as a method call. Use the form self.setter= to indicate the method call. class Incorrect attr_accessor :one, :two def initialize one = 1 # incorrect - sets local variable self.two = 2 end end obj = Incorrect.new obj.one → nil obj.two →2 • Objects that don’t appear to be properly set up may have been victims of an incor- rectly spelled initialize method. class Incorrect attr_reader :answer def initialise # < < < spelling error @answer = 42 end end ultimate = Incorrect.new ultimate.answer → nil The same kind of thing can happen if you misspell the instance variable name. class Incorrect attr_reader :answer def initialize @anwser = 42 #<« spelling error end end ultimate = Incorrect.new ultimate.answer → nil • Block parameters are in the same scope as local variables. If an existing local variable with the same name as a block parameter 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. c = "carbon" i = "iodine" elements = [ c, i ] elements.each_with_index do |element, i| # do some chemistry end c → "carbon" i →1 • Watch out for precedence issues, especially when using {} instead of do/end. def one(arg) if block_given? "block given to 'one' returns #{yield}" else arg end end def two if block_given? "block given to 'two' returns #{yield}" end end result1 = one two { "three" } result2 = one two do "three" end puts "With braces, result = #{result1}" puts "With do/end, result = #{result2}" produces: With braces, result = block given to 'two' returns three With do/end, result = block given to 'one' returns three • Output written to a terminal may be buffered. This means you may not see a mes- sage 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 Integer will work wonders (and will throw an exception if the input isn’t a well-formed integer). A common mistake Perl programmers make is while line = gets num1, num2 = line.split(/,/) # ... end You can rewrite this as while line = gets num1, num2 = line.split(/,/) num1 = Integer(num1) num2 = Integer(num2) # ... end Or, you could convert all the strings using map. while line = gets num1, num2 = line.split(/,/).map {|val| Integer(val) } # ... 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). arr = [1, 2] hash = { arr => "value" } hash[arr] → "value" arr[0] = 99 hash[arr] nil → hash.rehash {[99, 2]=>"value"} → hash[arr] "value" → • Make sure the class of the object you are using is what you think it is. If in doubt, use puts my_obj.class. • Make sure your method names start with a lowercase letter and class and constant names start with an uppercase letter. • If method calls aren’t doing what you’d expect, make sure you’ve put parentheses around the arguments. • Make sure the open parenthesis of a method’s parameter list butts up against the end of the method name with no intervening spaces. • Use irb and 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. |
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