不同的逻辑意义的段落之间应该空行比如头文件包含部分

1、Lesson 8 Flow of control Selection2006/10/172About homework program stylen不同的逻辑意义的段落之间应该空行。比如：头文件包含部分、宏定义部分、主函数部分、变量声明、变量初始化、程序主体、函数返回等等n空格。表达式中，运算符与变量/常量之间应该空格，函数的参数之间也最好空格。如：distance = (double)Speed * 3600 * 24 * 365;比 distance=(double)Speed*3600*24*365;好些 printf(“%d”, a)比printf(“%d”,a)好些n常量也有类型。如

2、：2.13表示一个double型的常量，2.13f表示float型常量3ObjectivesnRelational ExpressionsnThe if and if-else StatementsnThe if-else ChainnThe switch StatementnCase Study: Data Validation4Introduction: Flow of control nrefers to the order in which a programs statements are executednfour kind of control structures:qseque

3、ntialqSelectionqRepetitionqInvocation5Introduction: Selectionnif structure is a single-entry/single-exit structuretruefalseexpressionprint “Passed”A decision can be made on any expression. zero - false nonzero - trueExample:3 - 4 is true6if (grade = 60) printf(“passed”); truefalsegrade = 60printf(“p

4、assed”); if()7Introduction: Selection (continue)8Relational Expressionsinteger (char)floatdoubleSame Type0 false1 true9Relational Expressions (continued)10Relational Expressions (continued)nRelational expressions are also known as conditionsnA relational expression evaluates to 1 (true) or 0 (false)

5、qThe expression 3 3.3 has a value of 0qThe value of hours 0 depends on the value of hours11Relational Expressions (continued)12Relational Expressions (continued)char key = m;int i = 5, j = 7, k = 12;double x = 22.5;13Logical Operations 逻辑运算OperatorSymbol Formlogical AND & * a & blogical OR |

6、 + a | blogical negation ! - ! a14Logical Operators (continued)nThe evaluation feature for the & and | operators that makes the evaluation of an expression stop as soon as it is determined that an expression is false is known as short-circuit evaluation(6 * 3 = 38 / 2) & (13 3 * 3 + 4)|!(6-2

7、 (18 = 19) & (13 9 + 4) | !(4 0 & x = 015Truth Table （真值表） Operand Operator Operand Result 0 1 0 1 & 0 0 1 1 0 0 0 1 0 1 0 1 | 0 0 1 1 0 1 1 1 ! 0 1 1 0 16Precedence (运算的优先级)OperatorsSymbolparentheses unarymultiplicativeadditiverelationalequalitylogical ANDlogical ORconditionalassignment

8、comma( )+ , - , + , - , !* , / , %+ , - , , = , !=&|?:= , += , -= , *= , /= , %=, 17Exp：Given the following declarations :int a = 5 , b = 2 , c = 4 , d=5;b*d+4 = c*c0!(-d+a=1|c5|c%b*d=60) printf (“pass”); else printf (“fail”);truefalsegrade = 60if()print “fail”print “pass”else23nC code:if ( grad

9、e = 60 ) printf( Passedn);else printf( Failedn); warning = 1; For very short statements, you can code a complete if statement placed on a single line24Exp： #include int main () int score; printf(Please input your score:); scanf(%d,&score); if (score=60) printf(Passed!n); else printf(Sorry you fa

10、iled!n); return 0;25The if-else ChainnNested if statement:if (expression1) statement1;else if (expression2) statement2; else statement3;nWhether the indentation exists or not, the compiler will, by default, associate an else with the closest previous unpaired if, unless braces are used to alter this

11、 default pairing26The if/else Selection StructureqIf students grade is greater than or equal to 90Print “A”else If students grade is greater than or equal to 80 Print “B”else If students grade is greater than or equal to 70 Print “C” else If students grade is greater than or equal to 60 Print “D” el

12、se Print “F”27The if-else Chain (continued)28Exp1: sgn( x ) int x,y; printf(input x：); scanf (%d,&x); if (x=0) if (x0) y = 1 ; else y = 0 ; else y = - 1 ; printf( y=%dn,y); 29Exp2: decoder ( x1,x2 ) int x1,x2; printf(input x1,x2: ); scanf (%d%d,&x1,&x2); if (x1=0) if (x2=0) printf(“selec

13、t 3”); else printf(“select 2”); else if (x2=0) printf(“select 1”); else printf(“select 0”); 30The if-else Chain (continued)31The switch StatementTerminated with a colondefault is optionalIf the break statement was omitted, the following case would be executed32The switch Statement (continued)33Exp:3

14、4Case Study: Data ValidationnDefensive programming is a technique where the program includes code to check for improper data before an attempt is made to process it furtherqChecking user input data for erroneous or unreasonable data is called input data validationnRequirements:qWrite a program to ca

15、lculate the square root and the reciprocal of a user-entered number. Validate that the number is not negative before attempting to take its square root, and that the number is not 0 before calculating the numbers reciprocal value.35Case Study: Data Validation (continued)36Common Programming ErrorsnU

16、sing the assignment operator, =, in place of the relational operator, =nNesting if statements without including braces to clearly indicate the desired structurenUsing a single & or | in place of the logical & and logical | operators, respectively37SummarynRelational expressions, which are al

17、so called simple conditions, are used to compare operandsnConditions can be constructed from relational expressions using Cs logical operators, &, |, and !nA one-way if statement has the general formif (expression) statement;nA compound statement consists of any number of individual statements enclosed within bracesnAn if-else selects between two alternative statements based on the value of an expression38Summary (continued)nAn if-else statement can contain other if-else statementsnThe if-else chain is a multiway selection statementnThe switch statement is a multiway selection statement;