parsing numbers from strings

[Elliot Finley]

I have some strings that represent numbers.  The only catch is, I don't know ahead of time whether they contain a decimal point or not.

iex(12)> String.to_integer("45")

45

iex(13)> String.to_integer("45.0")

** (ArgumentError) argument error

    :erlang.binary_to_integer("45.0")

iex(11)> String.to_float("45.0")

45.0

iex(11)> String.to_float("45")

** (ArgumentError) argument error

    :erlang.binary_to_float("45")

I could write a string_to_num function that checks the string to see if it contains a period and then use the appropriate String.to_X function, but that seems very clumsy.

Any pointers on how to handle this would be appreciated.

Thanks,

Elliot

[David Elkins]

A question for the experts, is it considered clumsy still if one does a try and rescue/catch instead of searching for period in string, eg

defmodule MyModule do
  def string_to_num(string) do
    try do
      String.to_integer(string)
    rescue
      ArgumentError -> String.to_float(string)
    end
  end
end

IO.puts inspect MyModule.string_to_num("45")
IO.puts inspect MyModule.string_to_num("45.0")

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[Jason M Barnes]

Partially stolen from http://stackoverflow.com/questions/4328719/erlang-binary-string-to-integer-or-float

defmodule MyString do

  def to_number(val) do

    num = String.to_char_list(val)

    case :string.to_float(num) do

      {:error, :no_float} -> :erlang.list_to_binary(num)

      {float, _} -> float

    end

  end

end

It does seem odd that String.to_float() won’t accept numbers without periods, though.

On Sun, Mar 1, 2015 at 8:19 PM, Elliot Finley <efinle...@gmail.com> wrote:

[José Valim]

String.to_integer and String.to_float do conversions. If you want parsing of input, you should look at Integer.parse and Float.parse. Is it also able to return a float from Float.parse("1").

José Valim

www.plataformatec.com.br

Skype: jv.ptec

Founder and Lead Developer

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[greggreg]

defmodule MyString do
   def to_number(val) do

 
     case val =~ "." do   # <- check if it has a `.`
       true  -> Float.parse val
       false -> Integer.parse val
     end
   end
end

If you wanna just return the number and not the rest of the binary:

defmodule MyString do
   def to_number(val) do

 
     num = case val =~ "." do
       true  -> Float.parse val
       false -> Integer.parse val
     end

     case num do
      :error   -> :error
      {num, _} -> num
     end
   end
end

Note: Integer.parse and Float.parse can't handle the "readable" way to write numbers: Integer.parse("1_234_567") == {1, "_234_567"}