Selection Structures - SWITCH Statement - Gateway 选择结构开关语句网关

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1、Click Here to Add Title,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,Lect 9P.,*,Engineering H192 -Computer Programming,Gateway Engineering Education Coalition,Switch Case Structures,Lecture 9,1,Switch Multiple Selection Structure,A multiple selection structure i

2、s useful when an algorithm contains a series of decisions in which a variable or expression is tested separately for one of several possible integral values.,Each integral value represents a different action to be taken in the algorithm.,C provides the,switch,multiple selection structure to implemen

3、t this type of decision making.,2,Switch-Case Structures,The switch-case syntax is:,switch(,integer expression test value,),case,case _1_fixed_value,:,action(s),;,case,case_2_fixed_value,:,action(s),;,default:,action(s),;,Note use of colon!,3,Switch-Case Structures,The,switch,is the controlling expr

4、ession,Can only be used with constant integer expressions.,Remember,a single character is a small positive integer.,The expression appears in,(),The,case,is a label,The label must be followed by a,:,Braces,not required around statements,4,Switch-Case Structures,Unlike,if-else if-else,structures,when

5、 the value in a,case,matches the test value,all of the actions in the rest of the structure take place.,This is shown in the following program where the user enters a value that matches the first,case,and every action in the structure is executed.,5,A Sample Program to Illustrate Switch-Case,Problem

6、:,Write a program to ask the user to enter his/her letter grade and then respond with an appropriate message regarding his/her academic status.,6,A Sample Program to Illustrate Switch-Case,Algorithm:,1.Set up the environment,2.Prompt user to enter his/her letter grade,3.Get users response,4.If grade

7、 is a or A say“Good Job and go to 9,5.If grade is b or B say“Pretty good and go to 9,6.If grade is c or C say“Better get to work and go to 9,7 If grade is d or D say“You are in trouble and go to 9,8.Say“You are failing,9.Terminate program,7,A Sample Program to Illustrate Switch-Case,/*This program a

8、ssociates a letter grade with a,message appropriate to the score.*/,#include,void,main,(),char,grade,;,printf,(,Enter your current letter graden,);,grade=,getchar,();,8,A Sample Program to Illustrate Switch-Case,switch(,grade,),case,(,a,):,case,(,A,),:,printf,(,Good Job!n,);,case,(,b,),:,case,(,B,),

9、:,printf,(,Pretty good.n,),;,9,A Sample Program to Illustrate Switch-Case,case,(,c,),:,case,(,C,),:,printf,(,Better get to work.n,),;,case,(,d,),:,case,(,D,),:,printf,(,You are in trouble.n,),;,default,:,printf,(,You are failing!n,),;,/*End of switch-case structure*/,/*End of,main,program */,10,Swit

10、ch-Case StructuresResultant Output from Grade Program,/*The following results are produced when the user,enters an A as input to the program prompt.*/,Good Job!,Pretty good.,Better get to work.,You are in trouble.,You are failing!,11,Switch-Case Structures,break;,The problems with the previous progr

11、am can be corrected by use of the,break,statement.,It can be used in either a repetition structure or a selection structure to break out of(that is,to exit from)the structure.,The syntax is:,break;,The following program is the previous one with the addition of the,break,statements.,12,Fixed Program

12、using,Switch-Case Structures,#include,void main(),int,grade,;,printf,(,Enter your current letter graden,),;,while,(,(,grade=,getchar(),),!=,EOF),switch,(,grade,),case,(,a,),:,case,(,A,),:,printf,(,Good Job!n,),;,break,;,13,Fixed Program using,Switch-Case Structures,case,(,b,),:,case,(,B,),:,printf,(

13、,Pretty good.n,),;,break,;,case,(,c,),:,case,(,C,),:,printf,(,Better get to work.n,),;,break,;,case,(,d,),:,case,(,D,),:,printf,(,You are in trouble.n,),;,break,;,14,Fixed Program using,Switch-Case Structures,case,(,f,),:,case,(,F,):,printf,(,You are failing!n,),;,break,;,case,(,),:,case,(,n,):,brea

14、k,;,default,:,printf,(,Invalid grade.Try again.n,),;,/*End of switch/case*/,/*End of while loop */,/*End of main function*/,15,Comments on Last Example Program,Use of the,while,repetition structure-more discussion on repetition structures later this week.,Use of the end-of-file,or EOF,test.Note that

15、 EOF(a DEFINED constant)is a negative integral value,usually a-1 on most(but not all)systems.(EOF is actually defined in the header file.),Use of,int,s(instead of,char,s).Why?,From the keyboard,a generates an EOF signal on most UNIX systems.,16,Comments on Last Example Program,The statements:,case()

16、:case(n):,break;,were used to clear the keyboard input buffer.,Another way to clear it is with the statement:,fflush(stdin);,This statement can prove very useful in todays daily assignment.,17,Comments on Last Example Program,Example of use of,fflush,char,figure,;,float,size,;,printf,(,Enter figure type,),;,fflush(stdin);,scanf,(,%c,&figure),;,/or figure=getchar();,printf,(,Enter size of figure,);,fflush,(stdin);,scanf,(,%f,&size,);,18,Assignment E8,Use a switch-case structure to select from amo