Last occurance of a character in a String

[noodle]

Hi,

 

I'm super uber new with Elixir. I read the document for string, but I still can't find a way to do it. I'm trying to find the last occurrence of a character in a String something like String.indexOf() but I can't seem to find a way to do it without splitting the string by the spaces and build them back together.

 

For example, if I have this string:

text = "My dog is cute."

 

Is there a way I can say text.last_index(' ') and it will return 9?

 

Thanks.

[Alexei Sholik]

I don't think there is a builtin way, but it's doable.

Enum.reduce String.codepoints(text), {0, -1}, fn(str, { i, last_occurrence })->
  if str == " " do
    last_occurrence = i
  end
  { i + 1, last_occurrence }
end

You have some choices here:

* String.codepoints and String.graphemes may return different results depending on the string content (if it's heavy with non-ASCII characters). You could also use binary_to_list to get just a list of bytes.

* you could reverse the list of codepoints/bytes first and then use Enum.find_index.

We also have a proposal for String.find function https://github.com/elixir-lang/elixir/issues/1119. Presumably, it could accept an option to make is search backwards. You would then be able to subtract the returned value from the string length.

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Best regards
Alexei Sholik

[José Valim]

There is String.last.

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Lead Developer

[Yvan Godin]

Hi 

only for the fun 

 x = "azerty qsdf wxcv"

 length(String.codepoints x) - (length(Enum.take_while Enum.reverse((String.codepoints x)) , fn(x) -> x != " " end))

 12

Yvan

[Alexei Sholik]

Yvan, taking a length of a list is better avoided in any practical usage, it's O(n). Treat lists like arrays in C: you don't know their size unless you keep it in a variable somewhere ;)

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[Yvan Godin]

OK I better understand your solution now

thank's 

[Yvan Godin]

if I not mistake Regex seem better there, no ? 

f = function do              

   x -> Regex.index(%r/[^ ]*$/u, x)-1

end

f1 =  function do

   x -> Enum.reduce String.codepoints(x), {0, -1}, fn(str, { i, last_occurrence })->

        if str == " " do

             last_occurrence = i

        end

        { i + 1, last_occurrence }

        end

end

 :timer.tc(f , [x]) 

 {79,11}

 

 :timer.tc(f1 , [x]) 

 {519,{16,11}}

[Alexei Sholik]

Yvan, this looks good. I didn't know of Regex.index function.

[noodle]

José, if I understood last correctly, it only returns the last value of the string. Is that right? 

What I really want is the index of a character in a string.

[noodle]

Agreed. It looks neat and clean. Thanks Yvan.

[noodle]

Yvan, thank you for the proposed solutions. I was thinking of regex too. Thank your for the solution.