111CMSC 212 – S07 (lect 6)AnnouncementsProgram #2– Due 2/27/07Reading– Chapter 11 (by Thursday)2CMSC 212 – S07 (lect 6)Bit OperationsNumbers are represented by a fixed number of bits– typically int = 32 bits, char = 8 bits, long = 32 or 64 bitsC permits direct manipulation of bits within a number– This is powerful: can get exactly what you want– This can be non-portable: easy to write programs that don't work on different types of computersNumbers as a series of bits:1 1 1 10 0 0 0left most bit right most bit223CMSC 212 – S07 (lect 6)Bit Shift OperatorC has operators to bit shift numbers– number of bits changed depends on size of variableLeft Shift (number << n)– Move each bit of number to the left by n bit positions– Leftmost n bits discarded– Rightmost n bits gets 0Right Shift (number >> n)– Move each bit of number to the right by n bit positions– Rightmost n bits discarded– Leftmost n bits gets 0 (or can replicate sign bit - so 1 if negative)• for unsigned gets 0• for signed, it is implementation dependent– There is no >>> operator in C.4CMSC 212 – S07 (lect 6)Examples of Bit Shiftunsigned int a, b;a = 0x0000 0010;b = a >> 1; /* b is now 0x0000 0008 */b = a >> 4; /* b is now 0x0000 0001 */b = a >> 5; /* b is now 0x0000 0000 */b = a << 1; /* b is now 0x0000 0020 */b = a << 4; /* b is now 0x0000 0100 */b = a << 5; /* b is now 0x0000 0200 */335CMSC 212 – S07 (lect 6)Bitwise OperatorsAnd (&)– for each bit in two numbers, "and" the bits togetherOr (|)– for each bit in two numbers, "or" the bits togetherXor (^)– for each bit in two numbers, "xor" the bits toegher0 1010 010And0 1010 111Or0 1010 101Xor6CMSC 212 – S07 (lect 6)Assignment OperatorAssignment is an operator and the operations specified take place in a sequence:– x = y + 3;– a = x = y + 3;– assignment is right associative, so• a = x = y + 3 is the same as a = (x = y + 3)– value of assignment operator is result of assignment• if truncation is applied, the truncated value is used• Consider: (reminder: char is only 8 bits)unsigned char x;unsigned int a, b;b = 0xabcd;a = x = b;at the end, a = 0x00cd;447CMSC 212 – S07 (lect 6)Compound AssignmentShorthand to combine binary operator and assignment– += add right operand the left operand and update left• a += 3 is equivalent to a = a + 3– Also: -=, *=, /=, %=, <<=, >>=, &=, ^=, |=Handy for complex expressions in LHS– a[ i * 2 + j - f(n)] = a[ i * 2 + j - f(n)] + 3;– a[i * 2 + j - f(n)] += 3;– assumes f(n) has no side effects and returns the same value on the two sequential calls to that same function8CMSC 212 – S07 (lect 6)Unary Operators! logical not operator– if the operand is true, result is false– if the operand is false, result is true– is really an integer operand• produces 0 for false • produces 1 for true– b = !(a == 3); b = (a != 3); b = (a < 3 || a > 3);~ bit-wise negation (ones complement)– flip each bit position – b = ~a;- negation (twos complement)– changes sign of a number– a = 3;– b = -a; /* b is now -3 */559CMSC 212 – S07 (lect 6)Unary Operators (cont.)& Returns the address of a variable– int a, *b;– a = 3;– b = & a; /* b now holds the location (address) of a */* Dereferences a pointer– int a, *b;– a = 3;– b = & a; /* b now holds the location (address) of a */– printf(“%d and %d\n”, a, *b); /* 3 printed twice */sizeof - return number of bytes in variable or type– int a;– sizeof int– sizeof(int)– sizeof(a)(<typeName>) - cast to a new type– float a; int c,d;– a = (float) c / d;10CMSC 212 – S07 (lect 6)Unary Operators (cont.)++ increment operator– can be prefix or postfix ++a - add one to a and use new value as result of expression.a++ - add one to a and use old value as result of expression.-- decrement operator– can be prefix or postfix --a - subtract one from a and use new value as result of expressiona-- - subtract one from a and use old value as result of expression.Examples:a = 10; a = 10; b = 2; c = 4; int c=2,a=10,*b;c = ++a; c = a++; a = ++b * c++; b = &a;c = *b++;6611CMSC 212 – S07 (lect 6)Relational OperatorsTake two operands, result is integer– 0 for false, 1 for true– > >= < <= != ==!= is not equal== is equalCan use variable in conditional– int a;– if (a) { … is the same as if (a != 0) { … [ note: different if (a==1)]– if (!a) { … is the same as if (a == 0) {12CMSC 212 – S07 (lect 6)Logical Operators&& - and– if both operands are non-zero result is 1– else result is 0– uses short-circuit evaluation• if the first operand is not true, second is not evaluated• if ((a++) && (--b)) { .. }|| or – if either operands is non-zero result is 1– else result is 1– also uses short-circuit evaluation• if the first operand is true, second is not evaluatedCaution: && is not the same as &– & performs a bitwise operationCaution: || is not the same as |– | performs a bitwise operation7713CMSC 212 – S07 (lect 6)Other Operatorstrinary conditional operator– expr1 ? expr2 : expr3– if expr1 is non-zero, expr2 is evaluated and is the result– if expr1 is zero, expr3 is evaluated and is the result– a = (2 > 3) ? 65 : 17;Comma operator– evaluates both operands, rightmost is the result– a = (1,3,2); vs. b = 1,3,2;• assignment is higher precedence than , (the comma)• a ends up 2 and b ends up 114CMSC 212 – S07 (lect 6)Type ConversionPromotion of char and short– in expressions, char and short are promoted to intchar a, b; int c;a = b + c;• b is converted to int, then the sum is truncated– even if a, b and c are all char – b and c are converted to intArithmetic Conversions– arithmetic can't be performed on operators of different types• converted to "higher" typelong doubledoublefloatunsigned long intlong intunsigned intintHighest Type8815CMSC 212 – S07 (lect 6)PrecedenceThere is a set of rules about precedence of operator– Most important precedence rule:• When in doubt, put in () to ensure correct order– Order (highest to lowest):• ()• Function call, subscript, postfix increment/decrement• rest of unary operators• type conversion• Arithmetic operators• Relational operators• Bit operators• Assignment operators• comma