Two problems on programming

This is a discussion on Two problems on programming within the C++ Programming forums, part of the General Programming Boards category; First, I found some coding like this: Code: int current01; current01 = !current01 what does !current01 mean? What will the ...

  1. #1
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    Cool Two problems on programming

    First, I found some coding like this:
    Code:
     
    int current01;
    current01 = !current01
    what does !current01 mean? What will the value of !current01 be?


    Here is another weird code:

    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:

    Code:
    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?
    Last edited by rosicky2005; 11-06-2006 at 08:39 PM.

  2. #2
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    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.

  3. #3
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    But the code
    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?

  4. #4
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    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.

  5. #5
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    Quote Originally Posted by 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.
    So, it is swap of 1 and 0 if I run several times!!
    Right??

  6. #6
    and the hat of wrongness Salem's Avatar
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    Yes, it will alternate 0 1 0 1 0 1
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    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?
    Code:
    #include <stdio.h>
    
    void J(char*a){int f,i=0,c='1';for(;a[i]!='0';++i)if(i==81){
    puts(a);return;}for(;c<='9';++c){for(f=0;f<9;++f)if(a[i-i%27+i%9
    /3*3+f/3*9+f%3]==c||a[i%9+f*9]==c||a[i-i%9+f]==c)goto e;a[i]=c;J(a);a[i]
    ='0';e:;}}int main(int c,char**v){int t=0;if(c>1){for(;v[1][
    t];++t);if(t==81){J(v[1]);return 0;}}puts("sudoku [0-9]{81}");return 1;}

  8. #8
    (?<!re)tired Mario F.'s Avatar
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    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
    The programmer’s wife tells him: “Run to the store and pick up a loaf of bread. If they have eggs, get a dozen.”
    The programmer comes home with 12 loaves of bread.


    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  9. #9
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    ! is logical negation, ~ is bitwise negation. !0 must be 1.
    Last edited by Daved; 11-07-2006 at 09:05 AM.

  10. #10
    Reverse Engineer maxorator's Avatar
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    Quote Originally Posted by Daved
    ! is logical negation, ~ is bitwise negation. !0 must be 1.
    So destructor is the negation of constructor
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