Bitwise Operators in C/C++
In C, the following 6 operators are bitwise operators (work at bit-level)
- The & (bitwise AND) in C or C++ takes two numbers as operands and does AND on every bit of two numbers. The result of AND is 1 only if both bits are 1.
- The | (bitwise OR) in C or C++ takes two numbers as operands and does OR on every bit of two numbers. The result of OR is 1 if any of the two bits is 1.
- The ^ (bitwise XOR) in C or C++ takes two numbers as operands and does XOR on every bit of two numbers. The result of XOR is 1 if the two bits are different.
- The << (left shift) in C or C++ takes two numbers, left shifts the bits of the first operand, the second operand decides the number of places to shift.
- The >> (right shift) in C or C++ takes two numbers, right shifts the bits of the first operand, the second operand decides the number of places to shift.
- The ~ (bitwise NOT) in C or C++ takes one number and inverts all bits of it
a = 5, b = 9 a&b = 1 a|b = 13 a^b = 12 ~a = 250 b<<1 = 18 b>>1 = 4
Interesting facts about bitwise operators
- The left shift and right shift operators should not be used for negative numbers. If the second operand(which decides the number of shifts) is a negative number, it results in undefined behaviour in C. For example results of both 1 <<- 1 and 1 >> -1 is undefined. Also, if the number is shifted more than the size of the integer, the behaviour is undefined. For example, 1 << 33 is undefined if integers are stored using 32 bits. Another thing is, NO shift operation is performed if additive-expression(operand that decides no of shits) is 0. See this for more details.
Note: In C++, this behavior is well-defined.
- The bitwise XOR operator is the most useful operator from a technical interview perspective. It is used in many problems. A simple example could be “Given a set of numbers where all elements occur even a number of times except one number, find the odd occurring number” This problem can be efficiently solved by just doing XOR of all numbers.
The odd occurring element is 90
- The following are many other interesting problems using XOR operator.
- Find the Missing Number
- swap two numbers without using a temporary variable
- A Memory Efficient Doubly Linked List
- Find the two non-repeating elements.
- Find the two numbers with odd occurences in an unsorted-array.
- Add two numbers without using arithmetic operators.
- Swap bits in a given number/.
- Count number of bits to be flipped to convert a to b .
- Find the element that appears once.
- Detect if two integers have opposite signs.
- The bitwise operators should not be used in place of logical operators. The result of logical operators (&&, || and !) is either 0 or 1, but bitwise operators return an integer value. Also, the logical operators consider any non-zero operand as 1. For example, consider the following program, the results of & and && are different for same operands.
- The left-shift and right-shift operators are equivalent to multiplication and division by 2 respectively. As mentioned in point 1, it works only if numbers are positive.
x << 1 = 38 x >> 1 = 9
- The & operator can be used to quickly check if a number is odd or even. The value of expression (x & 1) would be non-zero only if x is odd, otherwise the value would be zero.
- The ~ operator should be used carefully. The result of ~ operator on a small number can be a big number if the result is stored in an unsigned variable. And the result may be a negative number if the result is stored in a signed variable (assuming that the negative numbers are stored in 2’s complement form where the leftmost bit is the sign bit)
Signed Result -2 Unsigned Result 4294967294d
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