CODEWITHSUNDEEP
rubyarraysenumerator
pguardiario
pguardiario

Reputation: 54984

Why return an enumerator?

I''m curious about why ruby returns an Enumerator instead of an Array for something that seems like Array is an obvious choice. For example:

'foo'.class
# => String

Most people think of a String as an array of chars.

'foo'.chars.class
# => Enumerator

So why does String#chars return an Enumerable instead of an Array? I'm assuming somebody put a lot of thought into this and decided that Enumerator is more appropriate but I don't understand why.

Upvotes: 12

Views: 3876

Answers (6)

Mark Reed
Mark Reed

Reputation: 95252

  1. Abstraction - the fact that something may be an Array is an implementation detail you don't care about for many use cases. For those where you do, you can always call .to_a on the Enumerable to get one.

  2. Efficiency - Enumerators are lazy, in that Ruby doesn't have to build the entire list of elements all at once, but can do so one at a time as needed. So only the number you need is actually computed. Of course, this leads to more overhead per item, so it's a trade-off.

  3. Extensibility - the reason chars returns an Enumerable is because it is itself implemented as an enumerator; if you pass a block to it, that block will be executed once per character. That means there's no need for e.g. .chars.each do ... end; you can just do .chars do ... end. This makes it easy to construct operation chains on the characters of the string.

Upvotes: 5

steenslag
steenslag

Reputation: 80065

This completely in accordance with the spirit of 1.9: to return enumerators whenever possible. String#bytes, String#lines, String#codepoints, but also methods like Array#permutation all return an enumerator.

In ruby 1.8 String#to_a resulted in an array of lines, but the method is gone in 1.9.

Upvotes: 2

DGM
DGM

Reputation: 26979

'Most people think of a String as an array of chars' ... only if you think like C or other languages. IMHO, Ruby's object orientation is much more advanced than that. Most Array operations tend to be more Enumerable like, so it probably makes more sense that way.

An array is great for random access to different indexes, but strings are rarely accessed by a particular index. (and if you are trying to to access a particular index, I suspect you are probably doing school work)

If you are trying to inspect each character, Enumerable works. With Enumberable, you have access to map, each, inject, among others. Also for substitution, there are string functions and regular expressions.

Frankly, I can't think of a real world need for an array of chars.

Upvotes: 1

Reactormonk
Reactormonk

Reputation: 21700

If you want an Array, call #to_a. The difference between Enumerable and Array is that one is lazy and the other eager. It's the good old memory (lazy) vs. cpu (eager) optimization. Apparently they chose lazy, also because

str = "foobar"
chrs = str.chars
chrs.to_a # => ["f", "o", "o", "b", "a", "r"]
str.sub!('r', 'z')
chrs.to_a # => ["f", "o", "o", "b", "a", "z"]

Upvotes: 7

swati
swati

Reputation: 1757

Actually 'foo'.chars passes each character in str to the given block, or returns an enumerator if no block is given.

Check it :

irb(main):017:0> 'foo'.chars
=> #<Enumerable::Enumerator:0xc8ab35 @__args__=[], @__object__="foo", @__method__=:chars>
irb(main):018:0> 'foo'.chars.each {|p| puts p}
f
o
o
=> "foo"

Upvotes: 0

Daren Thomas
Daren Thomas

Reputation: 70314

Maybe a string in ruby is mutable? Then having an Array isn't really an obvious choice - the length could change, for instance. But you will still want to enumerate the characters...

Also, you don't really want to be passing around the actual storage for the characters of a string, right? I mean, I don't remember much ruby (it's been a while), but if I were designing the interface, I'd only hand out "copies" for the .chars method/attribute/whatever. Now... Do you want to allocate a new array each time? Or just return a little object that knows how to enumerate the characters in the string? Thus, keeping the implementation hidden.

So, no. Most people don't think of a string as an array of chars. Most people think of a string as a string. With a behavior defined by the library/language/runtime. With an implementation you only need to know when you want to get nasty and all private with stuff below the abstraction belt.

Upvotes: 0

Related Questions