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How to pass a logic expression as string and return the response in Flutter

How do i test a expression in a string and evaluate it:

String expression = "'Cat' != 'Dog'";

I want that something like that:

if(expression) // return it logic result

like this:

if('Dog' != 'Doc'){
   return true;
}else{
   return false;
}

That in flutter, thanks in advance.

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2

Answers


  1. Chosen as BEST ANSWER
    0

    I had to create my own function that resolve my problem:

    dynamic evaluateExpression(String expression) {
  // Remove white spaces from the expression
  expression = expression.replaceAll(' ', '');

  // Parse the expression
  dynamic evaluate(String expr) {
    if (expr == 'true') return true;
    if (expr == 'false') return false;
    if (RegExp(r'^d+$').hasMatch(expr)) return int.parse(expr);
    if (RegExp(r'^[+-]?([0-9]+([.][0-9]*)?|[.][0-9]+)$').hasMatch(expr)) {
      return double.parse(expr);
    }
    return expr; // Assume it's a string
  }

  // Evaluate the expression recursively
  dynamic evaluateHelper(String expr) {
    // Handle string comparisons separately
    if (expr.contains('==')) {
      final parts = expr.split('==');
      return evaluate(parts[0]) == evaluate(parts[1]);
    } else if (expr.contains('!=')) {
      final parts = expr.split('!=');
      return evaluate(parts[0]) != evaluate(parts[1]);
    } else if (expr.contains('<=')) {
      final parts = expr.split('<=');
      return evaluate(parts[0]).compareTo(evaluate(parts[1])) <= 0;
    } else if (expr.contains('>=')) {
      final parts = expr.split('>=');
      return evaluate(parts[0]).compareTo(evaluate(parts[1])) >= 0;
    } else if (expr.contains('<')) {
      final parts = expr.split('<');
      return evaluate(parts[0]).compareTo(evaluate(parts[1])) < 0;
    } else if (expr.contains('>')) {
      final parts = expr.split('>');
      return evaluate(parts[0]).compareTo(evaluate(parts[1])) > 0;
    }

    // No comparison operators found, return null
    return null;
  }

  // Evaluate the expression
  return evaluateHelper(expression);
}

    I hope it helps someone.


  2. 0

    as @Vilsad PP mentioned in the comment we can use dart_eval package for this kind of things

    import 'package:dart_eval/dart_eval.dart';

void main() {
  var data = eval("'dog' != 'cat'");
  print(data); //true
}
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