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Bit Manipulation

CS253 Bit Manipulation

Refresher

C++ can do bit manipulation:

• bitwise and (&), or (|), exclusive-or (^)
• shift left (<<), shift right (>>)
• assignment operator versions (&=, |=, ^=, <<=, >>=)
• octal constant (0377)
• hexadecimal constant (0x3FF)
• binary constant (C++14) (0b1010110111)

Displaying values in other bases

// Sorry, no bin
cout << (127 & 0xfe) << '\n';
cout << hex << (127 & 0xfe) << '\n';
cout << oct << (127 & 0xfe) << '\n';

126
7e
176

I/O manipulators such as hex and oct are “sticky”; they persist until changed.

Precedence matters

cout << (1|2) << '\n';

3

However, the binary bitwise operators are low precedence:

cout << 1|2 << '\n';

c.cc:1: error: no match for 'operator|' in '
   std::cout.std::basic_ostream<char>::operator<<(1) | (2 << 10)' (operand 
   types are 'std::basic_ostream<char>' and 'int')

Setting a bit

int n = 32;
cout << hex << n << '\n';
cout << hex << (n|02) << '\n';

20
22

Or, using assignment:

int n = 32;
cout << hex << n << '\n';
n |= 02;
cout << hex << n << '\n';

20
22

Clearing bits

cout << hex << 126 << '\n';
cout << hex << (126 & ~0xF) << '\n';

7e
70

Testing bits

for (char c='A'; c<'N'; c++)
    if (c & 0x01)
        cout << "'" << c << "' (" << int(c) << ") is odd.\n";
    else
        cout << "'" << c << "' (" << int(c) << ") is even.\n";

'A' (65) is odd.
'B' (66) is even.
'C' (67) is odd.
'D' (68) is even.
'E' (69) is odd.
'F' (70) is even.
'G' (71) is odd.
'H' (72) is even.
'I' (73) is odd.
'J' (74) is even.
'K' (75) is odd.
'L' (76) is even.
'M' (77) is odd.

Shifting

int n = 42;
cout << oct << n << '\n';
n >>= 1;
cout << n << '\n';
n >>= 2;
cout << n << '\n';
n >>= 1;
cout << n << '\n';
n >>= 1;
cout << n << '\n';
n >>= 1;
cout << n << '\n';

52
25
5
2
1
0