CodeMocha » Code

Bit Operations

Chet — Sat, 10 Sep 2011 22:41:28 +0000

I got into a discussion with a fellow student today who worked at Microsoft. His product manager told him not to use bit operations because it’s important to keep code readable. Code should be self-documenting. I agree with this, but when you can perform functions with a couple lines of bit operations, it can be a beautiful thing.

Here’s how to determine if a number is a power of two:

1	(n & (n - 1)) == 0

Of course, this sets 0 to be a power of two as well. I found this site which recommends using this instead which fixes this.

1	v && !(v & (v - 1))

Beatitudes in C#

Chet — Sun, 05 Jul 2009 01:26:04 +0000

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class the_earth
{
...
};
class the_meek : public the_earth
{
...
};

From: StackOverflow answer

Bit Operations

Chet — Sat, 04 Jul 2009 18:31:41 +0000

For CS351, Intro to Systems, we had to do several operations using bit operators only. These took awhile, but are pretty cool to show off. We were graded by how few operations we used to return the right values for the generated test cases, including some difficult boundary ones.

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//NOR operation using only ~ and &
int bitNor(int x, int y) {
return (~x)&(~y);
}

The XOR operation is equivalent to the (x NAND y) AND (x OR y).

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//XOR operation using only ~ and &
int bitXor(int x, int y) {
return (~(x&y))&(~((~x)&(~y)));
}

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//Return true if x is not equal to y not using comparison operators.
int isNotEqual(int x, int y) {
return !!(x^y);
}

There are two types of shifts, arithmetic and logical. Left shifts are the same in both types, but right shifts are different. Logical shifts insert zeroes to fill in the empty spaces to keep the logic of the bits, but arithmetic shifts fill in the value of the sign bit to preserve the sign of the operand in twos-complement integers.

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int rightLogicalShift(int x, int n) {
int allOnes = ~0;
int y = ~((allOnes) << (32 + (~n + 1)));
return ((x >> n) & y);
}