kata-expression-eval

Expression tree

The idea of this kata is to implement a simple expression tree able to represent the following expressions :

• Value is a literal integer
• Add is an integer addition
• Mul is an integer multiplication
• Var is a variable (it's value will be pass when evaluating the expression).

Each expression is implemented by a Java class, here is an example of usage :

  Value value = new Value(13);
  Add add = new Add(new Value(4), new Add(new Value(3), new Value(7)));
  Mul mul = new Mul(new Value(4), new Add(new Value(3), new Value(7)));
  Var variable = new Var("a");

An expression can also be constructed from a string in prefix form, operator first followed by the operands, (for example, "+ 2 * 3 7" is equivalent to (2 + (3 * 7))) using the method parse(text).

Once an expression is constructed, we can ask for a string representation using toString() or an evaluation using eval(variableMap). The variableMap associate a name of a variable to its value.

  Expr expr = Expr.parse("+ 4 * x 7");
  System.out.println(expr);  // (4 + (x * 7))
        
  int result = expr.eval(Map.of("x", 3));
  System.out.println(result);  // 25

So you can use the last Java LTS version 17.