Bitwise Operators 11 19 2007 1 Opening Discussion Let s look at solutions to the interclass problem Do you have any questions about the assignment 2 Motivating Bitwise Operators There are a number of situations in which being able to play directly with the bits in numbers can be helpful Consider an application where you must keep a large number of boolean values Another is when you need to build a single number from several other values often booleans Packing booleans Packing RGB values Passing multiple flags to a function 3 and There are three binary bitwise operators That is three operators that work on the individual bits in numbers and take two operands is bitwise and A bit is only turned on in the result if the bit in that position in both operands is on is bitwise or A bit will be on in the result if either of the bits in that position in the operands are on is bitwise xor exculsive or A bit in the result will be on if it is on in only one of the two operands but not both 4 This is the bitwise not operator It is unary Applied to an integer value it flips the states of all the bits in that value Note that 0 gives you a number with all the bits on regardless of the number of bits 5 and These are bit shifting operators They are binary and move the bits in the first argument either up or down by the number of places specified in the second argument The upshift basically works like multiplying by a power of two while downshift is basically dividing by a power of two 6 Combining Operators Masks Combining the different bitwise operators allows us to specifically check or set any combination of bits in a number that we want to This is the real power of bitwise operators These are often combined with hex literals for masking off different parts of numbers allows you to swap ints without a temporary Don t bother doing this It s just a fun trick to know 7 Assignment Operators All the binary bitwise operators can be used in an assignment form by putting them before in an assignment So for integers a 2 is the same as a 1 I find that I use and fairly often when going through loops that build values through bitwise operations 8 Minute Essay For the fun class would you rather do cover ASCII based graphics or X11 based graphics Interclass Problem Do problem 46 on page 922 9
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