[Ruby] Why should we use symbol for key in Hash

Using symbols not only saves time when doing comparisons, but also saves memory, because they are only stored once

1. Symbol and String

– Symbols in Ruby are basically “immutable strings” .. that means that they can not be changed, and it implies that the same symbol when referenced many times throughout your source code, is always stored as the same entity, e.g. has the same object id.

– Strings on the other hand are mutable, they can be changed anytime. This implies that Ruby needs to store each string you mention throughout your source code in it’s separate entity, e.g. if you have a string “name” multiple times mentioned in your source code, Ruby needs to store these all in separate String objects, because they might change later on (that’s the nature of a Ruby string).

2. Use in Hash

– If you use a string as a Hash key, Ruby needs to evaluate the string and look at it’s contents (and compute a hash function on that) and compare the result against the (hashed) values of the keys which are already stored in the Hash.

– If you use a symbol as a Hash key, it’s implicit that it’s immutable, so Ruby can basically just do a comparison of the (hash function of the) object-id against the (hashed) object-ids of keys which are already stored in the Hash. (much faster)

Downside: Each symbol consumes a slot in the Ruby interpreter’s symbol-table, which is never released. Symbols are never garbage-collected. So a corner-case is when you have a large number of symbols (e.g. auto-generated ones). In that case you should evaluate how this affects the size of your Ruby interpreter.

Notes:

If you do string comparisons, Ruby can compare symbols just by their object ids, without having to evaluate them. That’s much faster than comparing strings, which need to be evaluated.

If you access a hash, Ruby always applies a hash-function to compute a “hash-key” from whatever key you use. You can imagine something like an MD5-hash. And then Ruby compares those “hashed keys” against each other.

Refer: http://stackoverflow.com/posts/8189435/revisions

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