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    <p><a href="http://en.wikipedia.org/wiki/Ruby_(programming_language)#.22Gotchas.22" rel="noreferrer">Wikipedia Ruby gotchas</a></p> <p>From the article:</p> <ul> <li>Names which begin with a capital letter are treated as constants, so local variables should begin with a lowercase letter.</li> <li>The characters <code>$</code> and <code>@</code> do not indicate variable data type as in Perl, but rather function as scope resolution operators.</li> <li>To denote floating point numbers, one must follow with a zero digit (<code>99.0</code>) or an explicit conversion (<code>99.to_f</code>). It is insufficient to append a dot (<code>99.</code>), because numbers are susceptible to method syntax.</li> <li>Boolean evaluation of non-boolean data is strict: <code>0</code>, <code>""</code> and <code>[]</code> are all evaluated to <code>true</code>. In C, the expression <code>0 ? 1 : 0</code> evaluates to <code>0</code> (i.e. false). In Ruby, however, it yields <code>1</code>, as all numbers evaluate to <code>true</code>; only <code>nil</code> and <code>false</code> evaluate to <code>false</code>. A corollary to this rule is that Ruby methods by convention — for example, regular-expression searches — return numbers, strings, lists, or other non-false values on success, but <code>nil</code> on failure (e.g., mismatch). This convention is also used in Smalltalk, where only the special objects <code>true</code> and <code>false</code> can be used in a boolean expression.</li> <li>Versions prior to 1.9 lack a character data type (compare to C, which provides type <code>char</code> for characters). This may cause surprises when slicing strings: <code>"abc"[0]</code> yields <code>97</code> (an integer, representing the ASCII code of the first character in the string); to obtain <code>"a"</code> use <code>"abc"[0,1]</code> (a substring of length 1) or <code>"abc"[0].chr</code>.</li> <li><p>The notation <code>statement until expression</code>, unlike other languages' equivalent statements (e.g. <code>do { statement } while (not(expression));</code> in C/C++/...), actually never runs the statement if the expression is already <code>true</code>. This is because <code>statement until expression</code> is actually syntactic sugar over</p> <pre><code>until expression statement end </code></pre> <p>, the equivalent of which in C/C++ is <code>while (not(expression)) statement;</code> just like <code>statement if expression</code> is an equivalent to</p> <pre><code>if expression statement end </code></pre> <p>However, the notation</p> <pre><code>begin statement end until expression </code></pre> <p>in Ruby will in fact run the statement once even if the expression is already true.</p></li> <li>Because constants are references to objects, changing what a constant refers to generates a warning, but modifying the object itself does not. For example, <code>Greeting &lt;&lt; " world!" if Greeting == "Hello"</code> does not generate an error or warning. This is similar to <code>final</code> variables in Java, but Ruby does also have the functionality to "freeze" an object, unlike Java.</li> </ul> <p>Some features which differ notably from other languages:</p> <ul> <li><p>The usual operators for conditional expressions, <code>and</code> and <code>or</code>, do not follow the normal rules of precedence: <code>and</code> does not bind tighter than <code>or</code>. Ruby also has expression operators <code>||</code> and <code>&amp;&amp;</code> which work as expected.</p></li> <li><p><code>def</code> inside <code>def</code> doesn't do what a Python programmer might expect:</p> <pre><code>def a_method x = 7 def print_x; puts x end print_x end </code></pre> <p>This gives an error about <code>x</code> not being defined. You need to use a <code>Proc</code>.</p></li> </ul> <p>Language features</p> <ul> <li>Omission of parentheses around method arguments may lead to unexpected results if the methods take multiple parameters. The Ruby developers have stated that omission of parentheses on multi-parameter methods may be disallowed in future Ruby versions; the current (November 2007) Ruby interpreter throws a warning which encourages the writer not to omit <code>()</code>, to avoid ambiguous meaning of code. Not using <code>()</code> is still common practice, and can be especially nice to use Ruby as a human readable domain-specific programming language itself, along with the method called <code>method_missing()</code>.</li> </ul>
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    1. CORuby 1.9 lacks character data type, too. In 1.8, index operator returned a Fixnum; in 1.9, it is equivalent to slicing an one-character string.
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