README.md

Exercise: StringsAndBinaries-4

(Hard) Write a function that takes a single-quoted string of the form number [+-*/] number and returns the result of the calculation. The individual numbers do not have leading plus or minus signs.

Solution

See the calculator.exs file for the full module.

The first order of business is to split up the three tokens. We can do this pretty easily by converted the char list to a string and using the String.split function on a space. Once we have the three tokens, we should use the Float module to parse the first and third token into a float.

Finally, once we have the float representation of the numbers we are calculating, we can simply check which operation was passed in an apply it to the two numbers.

def calculate(equation) when is_list(equation) do
  [num1_string, op_string, num2_string] =
    List.to_string(equation)
    |> String.split(" ")

  {num1, _} = Float.parse(num1_string)
  {num2, _} = Float.parse(num2_string)
  case op_string do
    "+" -> num1 + num2
    "-" -> num1 - num2
    "*" -> num1 * num2
    "/" -> num1 / num2
  end
end

That wasn't so bad! Let's test that it works:

iex> Calculator.calculate '12 + 3'
15.0
iex> Calculator.calculate '14.5 - 12'
2.5
iex> Calculator.calculate '1623 * 0.5'
811.5
iex> Calculator.calculate '36 / 12'   
3.0