programminginANSICChapterDecisionMakingand.ppt

1、Chapter 5Decision Making and Branching,PROGRAMMING IN ANSI C,9/6/2020,Question,Questions: How do we judge whether a student pass an examination according to his score? How do we decide his grade according to his score? In human nature language: If, then In C: Decision-making statement (branch statem

2、ent),9/6/2020,Chapter 5,In this chapter, we will learn: Decision-making statement: if Conditional operator: ? : Multiway decision-making statement: switch Assisted control statement: break,9/6/2020,3 Forms of if Statement,Form 1 the most simple form if ( test expression ) statement;,if ( score = 60

3、) printf(He passed this examination!);,9/6/2020,3 Forms of if Statement,Form 2 the most general form if ( test expression ) statement 1;elsestatement 2;,if ( score = 60 ) printf(He passed this examination!); else printf(He failed in this examination!);,9/6/2020,3 Forms of if Statement,Form 3 the nes

4、ted form if ( exp1 ) s1;else if ( exp2 ) s2; else elseif ( expn ) sn; else s;,if ( score = 90 ) grade = A; elseif ( score = 80 ) grade = B; elseif ( score = 70 ) grade = C; elseif ( score = 60) grade = D; else grade = E;,9/6/2020,if Statement,The value of the test expression may be any type.If it eq

5、uals zero, it is false, otherwise true.,if ( a=b ,if ( 3 ) printf ( OK );,OK,9/6/2020,if ( a = 2 ) printf ( %d, a ); else printf ( “Wrong );,if ( a = 0 ) printf ( %d, a ); else printf ( “Wrong );,if Statement,The value of the test expression may be any type.If it equals zero, it is false, otherwise

6、true.,2,Wrong,9/6/2020,if Statement,The statement following if or else may be a single statement or a compound statement.,main() int x, y; scanf( %d,%d, ,Compile Error!,Error 6: Misplaced else in function main,9/6/2020,if Statement Program 1,Input 2 real numbers, and output them in ascending order.

7、Step1: Read 2 real numbers into variable x and y. Step2: If x is greater than y, exchange them. Step3: Output x and y.,9/6/2020,if Statement Program 1,main() float x, y, temp; printf (Please input 2 numbers:n); scanf ( %f %f, ,Please input 2 numbers: 13.2 2.1 The 2 numbers are: 2.10, 13.20,9/6/2020,

8、if Statement Program 1,Input 2 real numbers, and output them in ascending order. Step1: Read 2 real numbers into variable x and y. Step2: If x is less than y, output x and y, or else output y and x.,9/6/2020,if Statement Program 1,main() float x, y; printf (Please input 2 numbers:n); scanf ( %f %f,

9、,9/6/2020,if Statement Program 1,Programming Exercises : Input 3 real numbers, and output them in ascending order.(Homework!),9/6/2020,if Statement Program 2,Read in one year, and judge whether it is a leap year or not. Step1: Read in the year. Step2: If the test expression (year%4=0 printf (Please

10、input the year:n); scanf (%d, ,Please input the year: 1900 1900 is not a leap year!,9/6/2020,if Statement Program 2,Step1: Set the flag variable isleap as 0. Step2: Read in the year. Step3: If the test expression (year%4=0 printf (Please input the year:n); scanf (%d, ,Please input the year: 2000 200

11、0 is a leap year!,9/6/2020,if Statement Program 3,Read in one character, and judge which kind of character it is: a figure, or a letter, or other. Step1: Read the character into variable ch. Step2: If (ch=0 printf (Please input the character:n); ch = getchar( ) ; if ( ch = 0 ,Please input the charac

12、ter: M M is a letter!,9/6/2020,Nesting of ifelse Statement,When a series of decision are involved, we may use more than one ifelse statement in nested form.,if (exp1) if (exp2) statement1; else statement2;else if (exp3) statement3; else statement4;,9/6/2020,Nesting of ifelse Statement,“else” matchin

13、g principle: An else is always linked to the closest non-terminated if.,if (exp1) if (exp2) statement1; else statement2;,if (exp1) if (exp2) statement1; elsestatement2;,9/6/2020,if Statement Program 4,According to the score, decide the grade. score = 80: grade A; score = 60 ,9/6/2020,if Statement Pr

14、ogram 4,main() float score; printf ( Please input the score:n ); scanf ( %f, ,Please input the score: 75.5 The grade is B!,9/6/2020,if Statement Program 4,main() float score; char grade; printf ( Please input the score:n ); scanf ( %f, ,9/6/2020,if Statement Program 4,main() float score; char grade;

15、 printf ( Please input the score:n ); scanf ( %f, ,9/6/2020,if Statement Program 4,main() float score;char grade; printf ( Please input the score:n ); scanf ( %f, ,9/6/2020,Conditional Operator ? :,Ternary operator: conditional exp ? exp1 : exp2,9/6/2020,Conditional Operator ? :,Ternary operator: co

16、nditional exp ? exp1 : exp2,if ( a b ) max = a; else max = b;,max = a b ? a : b;,if ( a b ) printf(%d, a); else printf(%d, b);,printf( %d, a b ? a : b);,9/6/2020,Conditional Operator ? :,Ternary operator: conditional exp ? exp1 : exp2 How to output the value of a + |b| ?,if ( b 0 )printf ( %f, a + b

17、 ); else printf ( %f, a b );,printf( %f, b 0 ? a + b : a b );,printf( %f, a + ( b 0 ? b : - b ) );,9/6/2020,Conditional Operator ? :,Ternary operator: conditional exp ? exp1 : exp2 Precedence ： higher than assignment operators lower than | Associativity： Right to left Conditional expression can be n

18、ested. x 0 ? 1 : x 0 ? 1 : 0,9/6/2020,Conditional Operator ? :,Ternary operator: conditional exp ? exp1 : exp2 The types of conditional exp and exp1 and exp2 may not be the same. The type of the conditional expression is the same with the highest size type.x = 0 ? a : bx y ? 1 : 1.5,9/6/2020,Conditi

19、onal Operator ? : - Program,Read in a character, if it is an uppercase, output its lowercase equivalent. Otherwise output the read-in character. Step1: Read a character into variable ch. Step2: If ch is an uppercase, convert it into its lowercase equivalent. Step3: Output ch.,9/6/2020,Conditional Op

20、erator ? : - Program,main() char ch; printf ( Please input the charactor:n ); scanf ( %c, ,ch = ( ch = A ,printf ( The charactor is: %cn, ( ch = A ,9/6/2020,switch Statement,One general if statement controls 2 branches. In order to deal with more than 2 branches, we can use the nesting structure of

21、if statement. However, the more branches, the more nesting level, the longer program lines, the more difficultly to read. C supplies a multiway decision statement: switch.,9/6/2020,switch Statement,The general form:,switch (exp) case value1: statement group1; case value2: statement group2; . case va

22、luen: statement groupn; default: statement group; ,9/6/2020,switch Statement,According to the grade, output information. Grade A: output Excellent!. Grade B: output Good!. Grade C: output Pass!. Grade D: output Fail!. Others: output Data error!,switch ( grade ) case A: printf(Excellent!); case B: pr

23、intf(Good!); case C: printf(Pass!); case D: printf(Fail!); default: printf(Data error!); ,if grade=B,Good!Pass!Fail!Data error!,9/6/2020,switch Statement,According to the grade, output information. Grade A: output Excellent!. Grade B: output Good!. Grade C: output Pass!. Grade D: output Fail!. Other

24、s: output Data error!,switch ( grade ) case A: printf(Excellent!); case B: printf(Good!); case C: printf(Pass!); case D: printf(Fail!); default: printf(Data error!); ,switch ( grade ) case A: printf(Excellent!); break; case B: printf(Good!); break; case C: printf(Pass!); break; case D: printf(Fail!)

25、; break; default: printf(Data error!); ,Good!,if grade=B,9/6/2020,switch Statement,The general form:,switch (exp) case value1: statement group1; case value2: statement group2; . case valuen: statement groupn; default: statement group; ,should be integer or character type,If it is not integer or char

26、acter type expression, it will be converted to integer type.,For all that, you should not use real number expression as possible.,switch ( 2.8 ) case 1: printf(1); break; case 2:printf(2); break; case 3:printf(3); break; default:printf(0); ,2,9/6/2020,switch Statement,The general form:,switch (exp)

27、case value1: statement group1; case value2: statement group2; . case valuen: statement groupn; default: statement group; ,should be integer or character type,must be constants or constants expressions, and cant be real number,switch (2) case 1: printf(1);break; case 2:printf(2);break; case 3:printf(

28、3);break; default:printf(0); ,2,switch (2) case 1: printf(1);break; case 1+1:printf(2);break; case 3:printf(3);break; default:printf(0); ,2,int a = 1; switch (2) case 1: printf(1);break; case 1+a:printf(2);break; case 3:printf(3);break; default:printf(0); ,switch (2) case 1: printf(1);break; case 2.

29、0:printf(2);break; case 3:printf(3);break; default:printf(0); ,Error.: Constant expression required in function .,9/6/2020,switch Statement,The general form:,switch (exp) case value1: statement group1; case value2: statement group2; . case valuen: statement groupn; default: statement group; ,should

30、be integer or character type,must be constants or constants expressions, and cant be real number,switch (2) case 1: printf(1);break; case 2:printf(2);break; case 1+1:printf(3);break; default:printf(0); ,Each of these values must be unique within a switch statement.,Error.: Duplicate case in function

31、 .,9/6/2020,switch Statement,The general form:,switch (exp) case value1: statement group1; case value2: statement group2; . case valuen: statement groupn; default: statement group; ,can be zero or more statements, and neednt put braces around these statements.,In if statement, the statement after if

32、 or else must be a single statement or a compound statement.,switch (x+y) case 1: printf(1);printf(n); break; case 2:printf(2); printf(n); break; default:printf(0); printf(n); ,Pay attention to the usage of break.,9/6/2020,switch Statement,The general form:,switch (exp) case value1: statement group1

33、; case value2: statement group2; . case valuen: statement groupn; default: statement group; ,multiple case may share one statement group.,switch (x+y) case 1: case 2: case 3:printf(1); printf(n); break; default:printf(0); ,9/6/2020,switch Statement,The general form:,switch (exp) case value1: stateme

34、nt group1; case value2: statement group2; . case valuen: statement groupn; default: statement group; ,is an optional case, and can be placed anywhere but usually placed at the end.,If present, it will be executed when the expression does not match with any of the case values.,switch ( 0 ) case 1: pr

35、intf(1); break; case 2:printf(2); break; case 3:printf(3); break; default:printf(0); ,0,switch ( 0 ) case 1: printf(1); break; default:printf(0); case 2:printf(2); break; case 3:printf(3); break; ,02,9/6/2020,switch Statement Program 1,According to the score, decide the grade. score = 90: grade A; score = 70 ,Step 1: read in score (float type). Step 2: According to (int) score / 10 , decide and output the grade.,9/6/2020,switch Statement Program 1,main() float score; print