title: How can I convert a "String-Calculation-Row" WITH operand into double?
body: <p>Can anybody help me please with a String-Conversion?
  This row is for example a String I want to convert: </p>
  
  <p>(This is from user via a Textfield:)
  NSString *testString = @"3 + 4 * 5.2 - 6"; </p>
  
  <p>And my way at the moment ist that, but the operands changes ...:
  double temp = number1 + number2 * number3 - number4;</p>
  
  <p>The result should be a double and the operands have to recognized. The numbers are not the problem at all, but the operands. An operand like "*" or "/" cannot convert into double with doubleValue. Is there a possibility to do that so from a String? </p>
  
  <p>Thanks in advance! :-)</p>