Databases
StackOverflow2013
Postsdbo
POWhy is a string key for a hash frozen?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POWhy is a string key for a hash frozen?
    primarykey
    data
    text
    <p>According to the <a href="http://www.ruby-doc.org/core-1.9.3/Hash.html#method-i-store">specification</a>, strings that are used as a key to a hash are duplicated and frozen. Other mutable objects do not seem to have such special consideration. For example, with an array key, the following is possible.</p> <pre><code>a = [0] h = {a =&gt; :a} h.keys.first[0] = 1 h # =&gt; {[1] =&gt; :a} h[[1]] # =&gt; nil h.rehash h[[1]] # =&gt; :a </code></pre> <p>On the other hand, a similar thing cannot be done with a string key.</p> <pre><code>s = "a" h = {s =&gt; :s} h.keys.first.upcase! # =&gt; RuntimeError: can't modify frozen String </code></pre> <p>Why is string designed to be different from other mutable objects when it comes to a hash key? Is there any use case where this specification becomes useful? What other consequences does this specification have?</p> <p><hr> I actually have a use case where absence of such special specification about strings may be useful. That is, I read with the <code>yaml</code> gem a manually written YAML file that describes a hash. the keys may be strings, and I would like to allow case insensitivity in the original YAML file. When I read a file, I might get a hash like this:</p> <pre><code>h = {"foo" =&gt; :foo, "Bar" =&gt; :bar, "BAZ" =&gt; :baz} </code></pre> <p>And I want to normalize the keys to lower case to get this:</p> <pre><code>h = {"foo" =&gt; :foo, "bar" =&gt; :bar, "baz" =&gt; :baz} </code></pre> <p>by doing something like this:</p> <pre><code>h.keys.each(&amp;:downcase!) </code></pre> <p>but that returns an error for the reason explained above.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USsawa
    1. USsawa
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POWhy is a string key for a hash frozen?
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POWhy is a string key for a hash frozen?
      1. USJon Taylor
      1. VTFavorite
    3. VO
      singulars
      1. POWhy is a string key for a hash frozen?
      1. This table or related slice is empty.
      1. VTUpMod
    1. COLooks like, for my purpose, the best I can do is `h.keys.each{|s| h.store(s.downcase, h.delete(s))}`.
      singulars
      1. POWhy is a string key for a hash frozen?
      1. USsawa
    2. COI can only guess at "why". As well as strings being a more common use case than arrays, I suspect freezing a string would be easier to implement. If I knew Perl, I'd look at whether Ruby is trying to be consistent with Perl in its hash behavior. If I was proficient in Japanese, I'd look at when the freezing of keys were implemented, and see if that was the result of a bug report or discussion on a mailing list (presumably in Japanese for something so early in Ruby's history).
      singulars
      1. POWhy is a string key for a hash frozen?
      1. USAndrew Grimm
    3. CO@AndrewGrimm [Here](http://doc.ruby-lang.org/ja/1.9.2/class/Hash.html) it says arrays and hashes do not make good keys for a hash because they can be modified, and strings are frozen so that you don't have to call rehash. Consistent with steenslag's answer.
      singulars
      1. POWhy is a string key for a hash frozen?
      1. USsawa
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload