Bit manipulation of unsigned long long

This is a discussion on Bit manipulation of unsigned long long within the C++ Programming forums, part of the General Programming Boards category; I am using an unsigned long long to do bit manipulation. When trying to tell if bits are set or ...

  1. #1
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    Oct 2009
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    48

    [SOLVED] Bit manipulation of unsigned long long

    I am using an unsigned long long to do bit manipulation. When trying to tell if bits are set or not, my code detects that bit 32 is set even when it is not. Here is my code, the if statement is where the problem is.

    Code:
    void recover(unsigned int branch_id)
    {
    	unsigned long long mask = gbm.get_younger_branches(branch_id);
    
            /* mask prints as: 0x7 here */
            printf("0x%llX\n", mask);
    
    	for (int i = 0; i < entries.size(); ++i)
    	{
    		for (int j = 0; j < sizeof(unsigned long long) * 8; ++j)
    		{
                      
                            /* this gets entered for j = 0, 1, 2, 32 */
    			if (mask & (1 << j))
    			{
    				entries[i]->release_branch_id(j);
    			}
    		}
    	}
    }
    Thanks.
    Last edited by BdON003; 10-08-2011 at 05:32 PM.

  2. #2
    Registered User
    Join Date
    Oct 2009
    Posts
    48
    I solved it.

    Code:
    void recover(unsigned int branch_id)
    {
    	unsigned long long mask = gbm.get_younger_branches(branch_id);
    
            /* mask prints as: 0x7 here */
            printf("0x%llX\n", mask);
    
    	for (int i = 0; i < entries.size(); ++i)
    	{
    		for (int j = 0; j < sizeof(unsigned long long) * 8; ++j)
    		{
                      
                            /* this gets entered for j = 0, 1, 2, 32 */
    			if (mask & (static_cast<unsigned long long>(1) << j))
    			{
    				entries[i]->release_branch_id(j);
    			}
    		}
    	}
    }

  3. #3
    Algorithm Dissector iMalc's Avatar
    Join Date
    Dec 2005
    Location
    New Zealand
    Posts
    6,264
    You "solved" it huh...
    There's the cleaner equivalent of exactly how you were doing it:
    Code:
                if (mask & (1ULL << j))
    And then there's the smarter way to do what you wanted which doesn't involve 64-bit constants at all:
    Code:
                if ((mask >> j) & 1)
    Don't mark your threads as solved.
    My homepage
    Advice: Take only as directed - If symptoms persist, please see your debugger

    Linus Torvalds: "But it clearly is the only right way. The fact that everybody else does it some other way only means that they are wrong"

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