Hashes and Symbols

Hashes are essentially a generalization of arrays: You can think of hashes basically like arrays, but not limited to integer indices. (In fact, some languages, especially Perl, sometimes call hashes associative arrays for this reason.) Instead, hash indices, or keys, can be almost any object. For example, we can use strings as keys:

Hashes are indicated with curly braces containing key-value pairs; a pair of braces with no key-value pairs—i.e., {}—is an empty hash. It’s important to note that the curly braces for hashes have nothing to do with the curly braces for blocks. (Yes, this can be confusing.) Although hashes resemble arrays, one important difference is that hashes don’t generally guarantee keeping their elements in a particular order.8 If order matters, use an array.  

Instead of defining hashes one item at a time using square brackets, it’s easy to use a literal representation with keys and values separated by =>, called a ‘‘hashrocket’’:

   Here I’ve used the usual Ruby convention of putting an extra space at the two ends of the hash—a convention ignored by the console output. (Don’t ask me why the spaces are conventional; probably some early influential Ruby programmer liked the look of the extra spaces, and the convention stuck.)

        So far we’ve used strings as hash keys, but in Rails it is much more common to use symbols instead. Symbols look kind of like strings, but prefixed with a colon instead of surrounded by quotes. For example, :name is a symbol. You can think of symbols as basically strings without all the extra baggage:9

 Symbols are a special Ruby data type shared with very few other languages, so they may seem weird at first, but Rails uses them a lot, so you’ll get used to them fast.
               In terms of symbols as hash keys, we can define a user hash as follows:

We see here from the last example that the hash value for an undefined key is simply nil. 

          Since it’s so common for hashes to use symbols as keys, Ruby 1.9 supports a new syntax just for this special case: