Ruby transliteration using hash

Question

I try to make Cyrillic => Latin transliteration using hash, I use # encoding: utf-8 and ruby 1.9.3. I want this code to change the value of `file_name`. Why does this code leave `file_name` unchanged? ``` abc = Hash.new abc = {"a" => "a", "b" => "б", "v" => "в", 'g' => "г", 'd'=> "д", 'jo' => "ё", 'zh' => "ж", 'th' => "з", 'i' => "и", 'l' => "л", 'm' => "м", 'n' => "н",'p' => "п", 'r' => "р", 's' => "с", 't' => "т", 'u' => "у", 'f' => "ф", 'h' => "х", 'c' => "ц", 'ch' => "ч", 'sh' => "ш", 'sch' => "щ", 'y' => "ы",'u' => "ю", 'ja' => "я"} file_name.each_char do |c| abc.each {|key, value| if c == value then c = key end } end ```

Accepted Answer

The problem with `.each_char` is that the block variable - `c` in your question - does not point back to the character in the string allowing to alter the string in situ. There are ways you could make that per-character mapping work from there (using a `.map` followed by a `.join` for instance) - but they are inefficient compared to `.tr!` or `.gsub!` for your purpose, because breaking the string out into an array of characters and reconstructing it involves creating many Ruby objects.

I think you need to do something like

```
file_name.tr!( 'aбвгдилмнпрстуфхцыю', 'abvgdilmnprstufhcyu' )
```

which covers the single letter conversions very efficiently. You then have some multi-letter conversions. I would use `gsub!` for that, and an inverted copy of your hash

```
latin_of = {"ё"=>"jo", "ж"=>"zh", "з"=>"th", "ч"=>"ch", 
            "ш"=>"sh", "щ"=>"sch", "я"=>"ja"}
file_name.gsub!( /[ёжзчшщя]/ ) { |cyrillic| latin_of[ cyrillic ] }
```

Note, unlike `each_char`, the return value of the block in `.gsub!` is used to replace whatever you matched in the original string. The above code uses an inversion of your original hash to quickly find the correct Latin replacement for the matched Cyrillic character.

You don't need `tr!` . . . instead, if you prefer, just use an inversion of your original hash in one pass using this second syntax. The cost of using two methods probably means you don't really gain that much from using `.tr!`. But you should know about `String#tr!` method, it can be very handy.

Edit: As suggested in comments, `.gsub!` can do a lot more for you here. Assuming `latin_of` was the complete hash with Cyrillic keys and the Latin values, you could do this:

```
file_name.gsub!( Regexp.union(latin_of.keys), latin_of )
```

Two things to note:

`Regexp.union(latin_of.keys)` is taking an array of the keys you want to convert and ensuring `gsub` will find them ready for replacement in the `String`

`gsub!` accepts a hash as the second parameter, and converts each match by looking it up as a key and replacing it with the associated value - exactly the behaviour you are looking for.

Ruby transliteration using hash

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