dd.dart

 String addOperandToExpression(String operand, String mathsExpression) {
    if (operands.contains(mathsExpression[mathsExpression.length - 1])) {
      var toArray = mathsExpression.split("");
      toArray[toArray.length - 1] = operand;
      mathsExpression = toArray.join("");
    } else {
      operandsIndex.add(mathsExpression.length);
      mathsExpression += operand;
    }
    dotTaken = false;
    print(mathsExpression);
    return mathsExpression;
  }