dart-naive-eval

readme.md

dart-naive-eval

Created with <3 with dartpad.dev.

main.dart

num evaluate(String expression) {
  var tokens = expression.split(' ');
  print(tokens);
  // Stack for numbers: 'values'
  List<num> values = [];
  // Stack for Operators: 'ops'
  List<String> ops = [];

  for (var i = 0; i < tokens.length; i++) {
    if (num.tryParse(tokens[i]) != null) {
      // dart reafctor: removed the while here as it is not needed and bad
      values.add(num.parse(tokens[i]));
    }

    // Current token is an opening
    // brace, push it to 'ops'
    else if (tokens[i] == '(') {
      ops.add(tokens[i]);
    }

    // Closing brace encountered,
    // solve entire brace
    else if (tokens[i] == ')') {
      while (ops.last != '(') {
        values.add(applyOp(
            ops.removeLast(), values.removeLast(), values.removeLast()));
      }
      ops.removeLast();
    }

    // Current token is an operator.
    else if (tokens[i] == '+' ||
        tokens[i] == '-' ||
        tokens[i] == '*' ||
        tokens[i] == '/') {
      // While top of 'ops' has same
      // or greater precedence to current
      // token, which is an operator.
      // Apply operator on top of 'ops'
      // to top two elements in values stack
      print(ops);
      print(values);
      
      while (ops.isNotEmpty && hasPrecedence(tokens[i], ops.last)) {
        values.add(applyOp(
            ops.removeLast(), values.removeLast(), values.removeLast()));
      }

      // Push current token to 'ops'.
      ops.add(tokens[i]);
    }
  }

  // Entire expression has been
  // parsed at this point, apply remaining
  // ops to remaining values
  while (ops.isNotEmpty) {
    values.add(
        applyOp(ops.removeLast(), values.removeLast(), values.removeLast()));
  }

  // Top of 'values' contains
  // result, return it
  return values.removeLast();
}

bool hasPrecedence(String op1, String op2) {
  if (op2 == '(' || op2 == ')') {
    return false;
  }
  if ((op1 == '*' || op1 == '/') && (op2 == '+' || op2 == '-')) {
    return false;
  } else {
    return true;
  }
}

num applyOp(String op, num b, num a) {
  switch (op) {
    case '+':
      return a + b;
    case '-':
      return a - b;
    case '*':
      return a * b;
    case '/':
      if (b == 0) {
        throw "Cannot divide by zero";
      }
      return a / b;
  }
  return 0;
}

void main() {
  print(evaluate("100 * 2 + 12")); // 212
  print(evaluate("100 * ( 2 + 12 )")); // 1400
  print(evaluate( "100 * ( 2 + 12 ) / 14")); // 100
}