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    <p>There are two ways to create initial values with for a <code>Hash</code>.</p> <p>One is to pass a single object in to <code>Hash.new</code>. This works well in many situations, especially if the object is a frozen value, but if the object has internal state, this may have unexpected side-effects. Since the same object is shared between <em>all</em> keys without an assigned value, modifying the internal state for one will show up in all.</p> <pre><code>a_hash = Hash.new "initial value" a_hash['a'] #=&gt; "initial value" # op= methods don't modify internal state (usually), since they assign a new # value for the key. a_hash['b'] += ' owned by b' #=&gt; "initial value owned by b" # other methods, like #&lt;&lt; and #gsub modify the state of the string a_hash['c'].gsub!(/initial/, "c's") a_hash['d'] &lt;&lt; " modified by d" a_hash['e'] #=&gt; "c's value modified by d" </code></pre> <p>Another initialization method is to pass <code>Hash.new</code> a block, which is invoked each time a value is requested for a key that has no value. This allows you to use a distinct value for each key. </p> <pre><code>another_hash = Hash.new { "new initial value" } another_hash['a'] #=&gt; "new initial value" # op= methods still work as expected another_hash['b'] += ' owned by b' # however, if you don't assign the modified value, it's lost, # since the hash rechecks the block every time an unassigned key's value is asked for another_hash['c'] &lt;&lt; " owned by c" #=&gt; "new initial value owned by c" another_hash['c'] #=&gt; "new initial value" </code></pre> <p>The block is passed two arguments: the hash being asked for a value, and the key used. This gives you the option of assigning a value for that key, so that the same object will be presented each time a particular key is given.</p> <pre><code>yet_another_hash = Hash.new { |hash, key| hash[key] = "#{key}'s initial value" } yet_another_hash['a'] #=&gt; "a's initial value" yet_another_hash['b'] #=&gt; "b's initial value" yet_another_hash['c'].gsub!('initial', 'awesome') yet_another_hash['c'] #=&gt; "c's awesome value" yet_another_hash #=&gt; { "a" =&gt; "a's initial value", "b" =&gt; "b's initial value", "c" =&gt; "c's awesome value" } </code></pre> <p>This last method is the one I most often use. It's also useful for caching the result of an expensive calculation.</p>
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    1. COinitialize with a block is one I will start using. Good one!
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