Operators and Expressions BY Nandini N.Gaikwad Assistant Professor Department of IT

Contents Objective Introduction Arithmetic operations Arithmetic expressions Relational operators Logical operators Assignment operators Increment and decrement operators Conditional operators Type conversions in expressions

To be able to construct and evaluate expressions. Objectives To be able to construct and evaluate expressions. To master operator precedence and associativity To understand implicit type conversion and explicit type conversion. 能够书写表达式，计算表达式的值； 掌握运算符的优先级和结合性； 掌握类型的隐式转换和显式转换

3.1 Introduction An operator is a symbol that tells the computer to perform certain manipulations. An expression is a sequence of operands and operators that reduces to a single value. C operators can be classified into a number of categories. Arithmetic operators Relational operators Logical operators Assignment operators Increment and decrement operators Conditional operators Bitwise operators Special operators

3.2 Arithmetic operators The arithmetic operators in C Operator meaning + Addition or unary plus - Subtraction or unary minus * Multiplication / Division % modulo division

3.2 Arithmetic operators Note:, Integer division truncates remainder The % operator cannot be applied to a float or double. The precedence of arithmetic operators Unary + or - * / % + - 注意： 整数除法舍去余数； %运算符只能用于整数 算术运算符的优先级

3.10 Arithmetic expressions An arithmetic expression is a combination of variables, constants, and operators. For example, a*b-c  a*b-c (m+n)(x+y)  (m+n)*(x+y) ax2+bx+c  a*x*x+b*x+c

3.3 Relational Operators The relational operators in C are : Operator Meaning < less that <= less than or equal to > greater than >= greater than or equal to == equal to != not equal to

Relational Operators A relational expression yields a value of 1 or 0. 5 < 6 1 -34 + 8 > 23 - 5 0 if a=3, b=2, c =1; then a > b > c is ? the associativity of relational operators is left  right 关系表达式的结果为1或0 注意计算关系表达式的值与判断关系表达式值的真假在表示上的差别 计算时，若关系成立，结果为1，否则结果为0； 判断关系表达式值的真假时，只要表达式的值为非0，就表示关系成立；为0则表示关系不成立。 经常采用简化写法，如n%2!=0可写为n%2 关系运算符的结合型：左-〉右

3.4 Logical operators C has the following three logical operators && meaning logical and || meaning logical or ! meaning logical not ( unary operator ) Expressions connected by && or || are evaluated left to right, and evaluation stops as soon as the truth or falsehood of the result is known.

3.5 Assignment operators The use of shorthand assignment operators has three advantages: 1. What appears on the left-hand side need not be repeated and therefore it becomes easier to write. 2. The statement is more concise and easier to read. 3. The statement is more efficient. 使用复合的赋值运算符有三点好处： 容易书写 语句更简洁、易读 语句效率高

3.6 Increment and decrement operators C provides two unusual operators for incrementing and decrementing variables. The increment operator ++ adds 1 to its operand, while the decrement operator -- subtracts 1. The unusual aspect is that ++ and -- may be used either as prefix operators (before the variable, as in ++n), or postfix operators (after the variable: n++). In both cases, the effect is to increment n. But the expression ++n increments n before its value is used, while n++ increments n after its value has been used. 自增自减运算符 自增自减运算符“前置”和“后置”的区别 “前置” 变量先增值(或先减值),后被引用 “后置” 变量先被引用,后再增值(或后减值) 两种情况下，变量的值都加1

Consider the expression m = - n++ ; The increment and decrement operators can be used in complex statements. Example: m=n++ -j +10; Consider the expression m = - n++ ; The precedence of ++ and – operators are the same as those of unary + and -. The associatively of them is right to left. m = - n++; is equivalent to m = - (n++) ++和—的优先级同单目的+，- 相同，右结合

3.7 Conditional operator a ternary operator pair “? : ” is available in C to construct conditional expressions of the form expr1 ? expr2 : expr3 the expression expr1 is evaluated first. If it is non-zero (true), then the expression expr2 is evaluated, and that is the value of the conditional expression. Otherwise expr3 is evaluated, and that is the value. Only one of expr2 and expr3 is evaluated. 条件运算符是C语言中唯一的三目运算符

z = (a > b) ? a : b; /* z = max(a, b) */ 相当于 if ( a>b) z=a; else z=b;

3.9 Special operators 1. The Comma Operator The comma operator can be used to link the related expressions together. A comma-linked list of expressions is evaluated left to right and the value of right-most expression is the value of the combined expression. For example, the statement value = (x=10, y=5, x+y); first assigns the value 10 to x, then assigns 5 to y, and finally assigns 15 to value. Since comma operator has the lowest precedence of all operators, the parentheses are necessary. 逗号运算符的运算顺序是从左到右，表达式的值为最右端表达式的值 逗号运算符的优先级最低

3.14 Type conversions in expressions 1. Implicit Type Conversion C permits mixing of constants and variables of different types in an expression. C automatically converts any intermediate values to the proper type so that the expression can be evaluated without loosing any significance. This automatic conversion is known as implicit type conversion. The rule of type conversion: the lower type is automatically converted to the higher type. 类型的隐式转换：如果一个表达使用有多种类型，C自动由低类型向高类型转换

3.13 Some Computational Problems When expressions include real values, then it is important to take necessary precautions to guard against certain computational errors. For example, consider the following statements: a = 1.0 / 3.0; b = a * 3.0; There is no guarantee that the value of b will equal 1. Another problem is division by zero. The third problem is to avoid overflow and underflow errors. 表达式运算过程中可能出现一些运算错误 浮点数运算 0作除数 数据溢出

3.15 Operator precedence and Associativity Rules of Precedence and Associativity (1)Precedence rules decides the order in which different operators are applied. (2)Associativity rule decide the order in which multiple occurrences of the same level operator are applied. Table3.8 on page71 shows the summary of C Operators. for example, a = i +1== j || k and 3 != x 优先级和结合性的应用： 优先级决定了同一表达式中不同操作符的运算次序； 结合性决定表达式中同一优先级操作符的运算次序

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