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A "trivial" (easy to understand but not practical) conversion function

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• A trivial conversion method to convert string numbers to its 2's complement code
  • Before discussing the parseInt( ) method, I want to show you what this method need to do by using a trivial solution.

    There is a 1-to-1 correspondence between String representation and 2's complement representation of numbers:

    String representation 2's complement representation (32 bits) ----------------------- ---------------------------------- "0" 00000000000000000000000000000000 "1" 00000000000000000000000000000001 "2" 00000000000000000000000000000010 "3" 00000000000000000000000000000011 ... ... "-1" 11111111111111111111111111111111 "-2" 11111111111111111111111111111110 "-3" 11111111111111111111111111111101 ... ...

  • You can convert a number string into its 2's complement binary number by testing every possible string input and return the corresponding integer representation as follows:

    /* --------------------------------------------------------------- Trivial conversion: match the input string to every possible number string and return the corresponding 2's complement binary number --------------------------------------------------------------- */ public static int parseInt( String s ) { if ( s.equals("0") ) return 0; // The return statement will return the binary number // 00000000000000000000000000000000 !! else if ( s.equals("1") ) return 1; // returns: 00000000000000000000000000000001 else if ( s.equals("2") ) return 2; // returns: 00000000000000000000000000000010 ... else if ( s.equals("10") ) return 10; // returns: 00000000000000000000000000001010 else if ( s.equals("11") ) return 11; // returns: 00000000000000000000000000001011 ... else if ( s.equals("-1") ) return -1; // returns: 11111111111111111111111111111111 else if ( s.equals("-2") ) 1eturn -2; // returns: 11111111111111111111111111111110 ... }

  • This method is not practical because you will need to write 2³² (= 4294967296) different cases with the if-statements !!!

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• Dictatical comment
  • I want to pause a bit and make sure that you know that:

    When you write this statement in Java: return 5; // Or some other decimal number (it's used in the above program), the Java compiler will generate computer instructions that will: return 5 as a 2's complement binary number, i.e.: 00000000000000000000000000000101