I checked the testcode source code, the expected return value for -788f and 959440.94f are both false. why?
While the use of an f at the end of a number can mean that it is a float, it also means the f isn't part of the number. In that case, the f is metadata about the number's representation. With the numbers -788f and 959440.94f, however, the f is being treated as part of the number string, which means that the string isn't numeric.