Bitwise operations in C

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In the C programming language, operations can be performed on a bit level using bitwise operators.

Bitwise operations are contrasted by byte-level operations which characterize the bitwise operators' logical counterparts, the AND, OR and NOT operators. Instead of performing on individual bits, byte-level operators perform on strings of eight bits (known as bytes) at a time. The reason for this is that a byte is normally the smallest unit of addressable memory (i.e. data with a unique memory address.)

This applies to bitwise operators as well, which means that even though they operate on only one bit at a time they cannot accept anything smaller than a byte as their input.

Video Bitwise operations in C

Bitwise operators

C provides six operators for bit manipulation.

Bitwise AND &

The bitwise AND operator is a single ampersand: &. It is just a representation of AND which does its work on the bits of the operands rather than the truth value of the operands. Bitwise binary AND does the logical AND (as shown in the table above) of the bits in each position of a number in its binary form.

For instance, working with a byte (the char type):

       11001000       & 10111000        --------      = 10001000  

The most significant bit of the first number is 1 and that of the second number is also 1 so the most significant bit of the result is 1; in the second most significant bit, the bit of second number is zero, so we have the result as 0.

Bitwise OR |

Similar to bitwise AND, bitwise OR only operates at the bit level. Its result is a 1 if one of the either bits is 1 and zero only when both bits are 0. Its symbol is | which can be called a pipe.

Bitwise XOR ^

The bitwise XOR (exclusive or) performs a logical XOR function, which is equivalent to adding two bits and discarding the carry. The result is zero only when we have two zeroes or two ones. XOR can be used to toggle the bits between 1 and 0. Thus i = i ^ 1 when used in a loop toggles its values between 1 and 0.

Bitwise NOT ~ / ones' complement (unary)

The ones' complement (~) or the bitwise complement gets us the complement of a given number. Thus we get the bits inverted, for every bit 1 the result is bit 0 and conversely for every bit 0 we have a bit 1. This operation should not be confused with logical negation !.

Maps Bitwise operations in C

Shift operators

There are two bitwise shift operators. They are

• Right shift (>>)
• Left shift (<<)

Right shift >>

The symbol of right shift operator is >>. For its operation, it requires two operands. It shifts each bit in its left operand to the right. The number following the operator decides the number of places the bits are shifted (i.e. the right operand). Thus by doing ch >> 3 all the bits will be shifted to the right by three places and so on.

Example:

If the variable ch contains the bit pattern 11100101, then ch >> 1 will produce the result 01110010, and ch >> 2 will produce 00111001.

Here blank spaces are generated simultaneously on the left when the bits are shifted to the right. When performed on an unsigned type, the operation performed is a logical shift, causing the blanks to be filled by 0s (zeros). When performed on a signed type, the result is technically undefined and compiler dependant, however most compilers will perform an arithmetic shift, causing the blank to be filled with the sign bit of the left operand.

Right shift can be used to divide a bit pattern by 2 as shown:

Right shift operator usage

Typical usage of a right shift operator in C can be seen from the following code.

Example:

The output of the above program will be

Left shift <<

The symbol of left shift operator is <<. It shifts each bit in its left-hand operand to the left by the number of positions indicated by the right-hand operand. It works opposite to that of right shift operator. Thus by doing ch << 1 in the above example we have 11001010. Blank spaces generated are filled up by zeroes as above.

Left shift can be used to multiply an integer by powers of 2 as in

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A simple addition program Example

The following program adds two operands using AND, XOR and left shift (<<).

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Bitwise assignment operators

C provides a compound assignment operator for each binary arithmetic and bitwise operation (i.e. each operation which accepts two operands). Each of the compound bitwise assignment operators perform the appropriate binary operation and store the result in the left operand.

The bitwise assignment operators are as follows:

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Logical equivalents

Four of the bitwise operators have equivalent logical operators. They are equivalent in that they have the same truth tables. However, logical operators treat each operand as having only one value, either true or false, rather than treating each bit of an operand as an independent value. Logical operators consider zero false and any nonzero value true. Another difference is that logical operators perform short-circuit evaluation.

The table below matches equivalent operators and shows a and b as operands of the operators.

!= has the same truth table as ^ but unlike the true logical operators, by itself != is not strictly speaking a logical operator. This is because a logical operator must treat any nonzero value the same. To be used as a logical operator != requires that operands be normalized first. A logical not applied to both operands won't change the truth table that results but will ensure all nonzero values are converted to the same value before comparison. This works because ! on a zero always results in a one and ! on any nonzero value always results in a zero.

Example:

The output of the above program will be

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See also

• Bit manipulation
• Bitwise operation
• Find first set
• Operators in C and C++
• Bitboard
• Boolean algebra (logic)
• XOR swap algorithm
• XOR linked list

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References

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External links

• Bitwise Operators

Source of the article : Wikipedia