What's the difference between a string and a symbol in Ruby?

The main difference is that multiple symbols representing a single value are identical whereas this is not true with strings. For example:

irb(main):007:0> :test.object_id
=> 83618
irb(main):008:0> :test.object_id
=> 83618
irb(main):009:0> :test.object_id
=> 83618

Those are three references to the symbol :test, which are all the same object.

irb(main):010:0> "test".object_id
=> -605770378
irb(main):011:0> "test".object_id
=> -605779298
irb(main):012:0> "test".object_id
=> -605784948

Those are three references to the string "test", but are all different objects.

This means that using symbols can potentially save a good bit of memory depending on the application. It is also faster to compare symbols for equality since they are the same object, comparing identical strings is much slower since the string values need to be compared instead of just the object ids.

As far as when to use which, I usually use strings for almost everything except things like hash keys where I really want a unique identifier, not a string.

What are the differences between Symbols and Strings?

1. Symbols are immutable: Their value remains constant.
2. Multiple uses of the same symbol have the same object ID and are the same object compared to string which will be a different object with unique object ID, everytime.
3. You can't call any of the String methods like split on Symbols.

From Understanding Differences Between Symbols & Strings in Ruby

If you know Chinese, you can also read 理解 Ruby Symbol.

The statement:

foo = "bar"

creates a new object in memory. If we repeat the statement:

foo = "bar"

We create another object.

To understand it more clearly please try this code in IRB:

foo = "bar"
puts "string #{foo} with object id = #{foo.object_id}"
foo = "bar"
puts "string #{foo} with object id = #{foo.object_id}"

You will get output like:

string bar with object id = 70358547221180
string bar with object id = 70358548927060

which clearly shows there are two different object for the same string. Now if you use a symbol it will create one object per symbol so:

foo = :bar
puts "symbol #{foo} with object id = #{foo.object_id}"
foo = :bar
puts "symbol #{foo} with object id = #{foo.object_id}"

shows:

symbol bar with object id = 7523228
symbol bar with object id = 7523228

which means there is only one object for :bar.

Further, Symbols are immutable and you can't call any of the String methods like upcase or split on Symbols.

Comparing Symbols are faster than comparing Strings. Symbols can be thought of as constant/immutable strings that form a unique set that are effectively converted to memory pointers on the heap. This means comparing two symbols is fast because you are just comparing two integers (memory pointers).

Strings are mutable so the memory pointer to their value on the heap can change after modification. This means comparison operations are slower because duplicates can exist that are semantically equivalent.

Use a symbol when you are sure that the value will remain constant, for example use symbols for hash keys. Use a string when you want to change the value or want to use a string method on it.