-10
-10

What happened here is that -1 is as we discussed 65535 for unsigned integers. So on line 10, the computer multiplied 10 * 65535 which resulted to 655350 but as an integer number, this is larger than 16 bits. Then it gets truncated to 16 bits which results into 65526 which is obviously wrong as a result. Moving to line 20 we hit the first pitfall discussed in the opening statement: The result of SGN {-1} which is -1 gets converted into an unsigned integer itself so you end up with the exact same situation as with line 10; a multiplication of 10 with 65535. The pitfall therefore here is that SGN{...} must apply to the entirety of the integer expression, so if there are other non-signed expressions they must be taken into consideration when writing each statement! Line 30 produces finally what we were aiming for, but that also happens with line 40! So both are correct but which is the right way to do it?

The answer to that question lies with what we discussed above regarding the "pitfall" with integer expressions. The subexpression SGN {-1} will get evaluated to whatever is in the enclosing expression. So if the enclosing expression is an unsigned expression, the result of the subexpression will also become converted to unsigned; ergo since the entirety of the integer expression of line 30 is a signed expression, the signed subexpression is unnecessary and may even delay execution (especially in very complex calculations). The right way therefore to do it, is the way defined in line 40. Obviously this also applied to our initial example which is best written as:

PRINT % SGN {1 -32300- (-1)}

which is much neater to write AND read!

Exercises

1. Using the discussion about the unary ! operator and 16 bit binary numbers, calculate and print on screen the two's complement for the signed 32 bit integer: 650323