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Testing the bit at poistion n in an int variable

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• Testing a bit
  • Recall: how to test a bit

    Bit position: 6543210 Starting bit pattern: ..............x............... Bit mask: 000000000000001000000000000000 ----------------------------------------- AND 00000000000000x000000000000000 The result is equal to 0 (000000000000000000000000000000) if bit x is 0 The result is not equal to 0 if bit x is 1

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• Making the bit mask
  • The bit mask 00000000000010000000000000... can be made with the following operation:

    i positions <--------------> 1 << i = 000000..1000000000000000

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• C statement to test bit position i in an int variable
  • C code:

    int x; int i; if ( (x & (1 << i )) != 0 ) { bit at position i is ONE; } else { bit at position i is ZERO; }

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• Macro definition for ISSET operation
  • C macro to test whether the bit at position i in a variable x is set:

    #define ISSET( x, i ) (( x & (1 << i)) != 0 )

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• Application of test bit: print in binary
  • We can use the test bit operation to print an int variable in binary.

    The following printBits( ) C function will print an int variable (parameter) out in binary:

    #include <stdio.h> void printBits(int x) { for (int i = 31; i >= 0; i--) { if ( x & (1 << i) /* i-th bit in x is 1 */ ) printf("1"); else printf("0"); } } int main( int argc, char* argv[] ) { int a = 7; printf("a = %d, in binary: ", a); printBits(a); printf("\n\n"); }

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set1/print-binary.c

    How to run the program:

    To compile:   gcc print-binary.c          To run: ./a.out