r3v1z3 / eval-expression

Simple expression evaluator example project for Lazarus IDE

EvalExpression

Simple expression evaluator example project for Lazarus IDE. The project includes a form with a couple text fields and a label. Users can enter math expressions in the first field and get evaluated results as they type.

Evaluations are handled through TFPExpressionParser which is bundled with Lazarus.

The bulk of the code is as simple as this:

procedure Teval.ExpressionChange(Sender: TObject);
var
  FParser: TFPExpressionParser;
  resultValue: Double;
  exp: String;
begin
  exp := Expression.Text;
  if exp = '' then exp := '0';
  FParser := TFPExpressionParser.Create(nil);
  try
    FParser.BuiltIns := [bcMath];
    FParser.Expression := exp;
    resultValue := ArgToFloat(FParser.Evaluate);
    Result.caption := resultValue.ToString();
    Result.Color := clCream;
    FParser.Free;
  except
    Result.caption := 'nil';
    Result.Color := clYellow;
    FParser.Free;
  end;
end;

Usage

Simply clone the repo and open the project in Lazarus.