How to execute a bitwise logical expression which is a string and the operator isn't known before hand

Question

I have to execute a logical expression which is a string.

For example :

string s = "2 << 1"

How could I execute above "s" with the bitwise operator unknown at the time of executing.

Split the string and use a `switch-case` or `if-else` for the operators? — jegtugado, Feb 12 '19 at 07:29
Possible duplicate of [How can I evaluate a C# expression dynamically?](https://stackoverflow.com/questions/53844/how-can-i-evaluate-a-c-sharp-expression-dynamically) — John Wu, Feb 12 '19 at 07:56
This question is a trap, unfortunately, because any answer that answers your direct question exactly is only going to lead to further questions in the comment, like "Yeah, ok, so `< — Lasse V. Karlsen, Feb 12 '19 at 08:27

score 1 · Answer 1 · answered Feb 12 '19 at 07:38

You may try the following:

string s = "2 << 1";
string operator_ = (new string(s.Where(c => !char.IsDigit(c)).ToArray())).Trim();
int operand1 = Convert.ToInt32(s.Substring(0, s.IndexOf(operator_)).Trim());
int operand2 = Convert.ToInt32(s.Substring(s.IndexOf(operator_) + operator_.Length).Trim());

int result = 0;
switch (operator_)
{
    case "<<":
         result = operand1 << operand2;
         break;
    case ">>":
         result = operand1 >> operand2;
         break;
}
Console.WriteLine(string.Format("{0} {1} {2} = {3}", operand1, operator_, operand2, result));

score 0 · Answer 2 · answered Feb 12 '19 at 08:07

You can try using regular expressions here in order to extract arguments and operation:

  using System.Text.RegularExpressions; 

  ...

  // Let's extract operations collection 
  // key: operation name, value: operation itself 
  Dictionary<string, Func<string, string, string>> operations =
    new Dictionary<string, Func<string, string, string>>() {
    { "<<", (x, y) => (long.Parse(x) << int.Parse(y)).ToString() },
    { ">>", (x, y) => (long.Parse(x) >> int.Parse(y)).ToString() }
  };

  string source = "2 << 1";

  var match = Regex.Match(source, @"(-?[0-9]+)\s*(\S+)\s(-?[0-9]+)");

  string result = match.Success
    ? operations.TryGetValue(match.Groups[2].Value, out var op) 
       ? op(match.Groups[1].Value, match.Groups[3].Value)
       : "Unknown Operation"  
    : "Syntax Error";

  // 4
  Console.Write(result);

How to execute a bitwise logical expression which is a string and the operator isn't known before hand

2 Answers2