Two problems on programming

[rosicky2005]

First, I found some coding like this:

 
int current01;
current01 = !current01

what does !current01 mean? What will the value of !current01 be?


Here is another weird code:

#if MKL_PIVOT
		_rowpivot[k] = _rowpivot[maxrow];
#else
		std::swap(_rowpivot[k], _rowpivot[maxrow]);
#endif
                                SwapCols(rXmin + halfDeltaX,float(maxrow+.5f));

Why '#' is added befor if,else, endif? What is the aim doing that?
(MKL_PIVOT is enum)

Is that just the same as:

if (MKL_PIVOT)
		_rowpivot[k] = _rowpivot[maxrow];
else
		std::swap(_rowpivot[k], _rowpivot[maxrow]);

                                SwapCols(rXmin + halfDeltaX,float(maxrow+.5f));

which SwapCols(rXmin + halfDeltaX,float(maxrow+.5f)); will still run anyway?

[Desolation]

1. current01 will hold an undetermined value since it is not given any value in the first place.
2. Those are pre-processor conditions. It means that the conditions are evaluated before compilation and that the code matching the condition is "pasted" and the rest is "cut". Therefore, the first #if asks if MKL_PIVOT is a defined macro, if it is, proceed, if it is not, jump to the line after. #endif closes that #if statement and then SwapCols() gets executed no matter what just happened.

[rosicky2005]

But the code

 current01 = !current01

run for several times. And current01 = 1 initially.

If !current01 is undetermined value, then is that a swap between 1 and the undetermined will happen?

[Desolation]

You didn't specify that bit that current01 was set to 1 in the first place. ! is the boolean negation operator. So if the value of current01 is non-zero (or true), it will become false (0) and if it is 0, it will become true (1).

0 == !1 would evaluate to true because 0 == 0.

[rosicky2005]

You didn't specify that bit that current01 was set to 1 in the first place. ! is the boolean negation operator. So if the value of current01 is non-zero (or true), it will become false (0) and if it is 0, it will become true (1).

0 == !1 would evaluate to true because 0 == 0.

So, it is swap of 1 and 0 if I run several times!!
Right??

[Salem]

Yes, it will alternate 0 1 0 1 0 1

[jafet]

Salem: Isn't it more like [0] [non-0] [0] [non-0] [0] to be pedantic? Or does the standard specify operator! to return 0/1 only?

[Mario F.]

No. Well not for built-in arithmetic types. The operator is called unary-negation. It basically inverts the bits on its operand. Effectively switching between 0 and 1.

As for non built-in types... the sky is the limit

[Daved]

! is logical negation, ~ is bitwise negation. !0 must be 1.

[maxorator]

! is logical negation, ~ is bitwise negation. !0 must be 1.

So destructor is the negation of constructor