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第四章判定、分支和循环❖4.1C语句概述C语句:以“;”作分隔符,编译后产生机器指令.C语句分类❖表达式语句：表达式加分号构成。❖空语句：;❖程序控制语句(9种）：if()~else~switchfor()~while()~do~while()continuebreakgotore

turn分支循环辅助控制如total=total+limit;a=3;func();printf(“Hello,world!\n”);❖复合语句：用{…}括起来的一组语句一般形式：{[数据说明部分；]

执行语句部分；}✓“}”后不加分号✓语法上和单一语句相同✓复合语句可嵌套三种基本结构❖顺序结构ABAB流程图N-S图PAB真假PBA真假❖选择结构kA1A2AiAnk=k2k=k1k=knk=ki......⚫二分支选择结构⚫多分支选择结构❖循环结构⚫当型循环结构⚫直到型循环结构

PA假真当P为真AAP真假A直到P为真注：A,B,A1….An可以是一个简单语句，也可以是一个基本结构❖4.2分支简单if语句(p111~115)❖格式：if(expression)statement❖执行过程：exprstatement非0=0例：if(x>y)printf(“%d”,

x);例读入a,b,c,d四个整数，输出（a+b）/(c-d)。if(c-d!=0){ratio=(float)(a+b)/(c-d);printf(“Ratio=%f\n”,ratio);}/*absolutevalue*/#includ

e<stdio.h>main(){intx,y;printf("Enteraninteger:");scanf("%d",&x);y=x;if(y<0)y=-y;printf("\ninteger:%d-->absolutevalue:%d\n",x,y);}例求一个数的绝对值运行：Enter

aninteger:-12integer:-12--->absolutevalue:12abs(intn)labs(long)fabs(doublex)cabs(complexz)exprstatement1

statement2非0=0if…else语句p115❖格式：if(expression)statement1elsestatement2❖执行过程：例if(c-d!=0){ratio=(float)(

a+b)/(c-d);printf(“Ratio=%f\n”,ratio);}elseprintf(“c-diszero\n”);例if(x>y)max=x;elsemax=y;转为表达式：max=(x>y)?x:y这里引入了运算符——p129条件运算符？：条件运算符与表达式❖一般形式

：expr1?expr2:expr3❖执行过程❖功能：相当于条件语句，但不能取代一般if语句例if(a>b)printf(“%d”,a);elseprintf(“%d”,b);printf(“%d”,a>b?a:b);例求a+|b|pri

ntf(“a+|b|=%d\n”,b>0?a+b:a-b);expr1取expr2值取expr3值非0=0例(a==b)?’Y’:’N’(x%2==1)?1:0(x>=0)?x:-x(c>=‘a’&&c<=‘z’)?c-’a’+’A’:c❖条件运算符可

嵌套如x>0?1:(x<0?-1:0)❖优先级:13❖结合方向：自右向左如a>b?a:c>d?c:da>b?a:(c>d?c:d)❖expr1、expr2、expr3类型可不同，表达式值取较高的类型例x?‘a

’:‘b’//x=0,表达式值为‘b’;x‡0,表达式值为‘a’x>y?1:1.5//x>y,值为1.0;x<y,值为1.5elseif梯状p122❖格式：if(expr1)statement1elseif(

expr2)statement2elseif(expr3)statement3…...[elsestatementn]expr1statemnt1非0=0expr2expr3statemntnstatemnt3statemnt2非0非0=0=0❖执行过

程：if(value>1000)takeoff=0.25;elseif(value>800)takeoff=0.2;elseif(value>600)takeoff=0.15;elseif(value>400)takeoff=0.1;elsetakeoff=

0;例百分制成绩转换为五级制p12280to100Honours60to79FirstDivision50to59SecondDivision40to49ThirdDivision0to39Fail例累进电费p124例根据税前收入计算税后收入速算扣除额/02512537

513753375637510375153752000元起征0%不超过500的5%超过500~2000的部分10%超过2000~5000的部分15%超过5000~20000的部分20%超过20000~40000的部分25%超过4

0000~60000的部分30%超过60000~80000的部分35%超过80000~100000的部分40%超过100000的部分45%Actual=(Income-Base)*对应税率－对应速算扣除额如Income=8000500*5%1500*10%300

0*15%(8000-7000)*20%=(8000-2000)*20%-5000*20%如：if(a==b&&x==y)printf(“a=b,x=y”);if(3)printf(“OK”);if(‘a’)printf(“%d”,’a’);例考虑下面程序的输出结果:#inc

lude<stdio.h>main(){intx,y;scanf(“%d,%d”,&x,&y);if(x>y)x=y;y=x;elsex++;y++;printf(“%d,%d\n”,x,y);}CompileError!✓if后面的表达式类型任意✓语句可以是复合语句✓if(x)i

f(x!=0)✓if(!x)if(x==0)/*Beequalornot*/#include<stdio.h>main(){inta,b;printf("Enterintegera:");scanf("%d",&a);printf("Enterintegerb:");scanf("%d",&b)

;if(a==b)printf("a==b\n");elseprintf("a!=b\n");}例输入两个数并判断两数相等否运行：Enterintegera:12Enterintegerb:12a==b运行：Enterintegera:12Enteri

ntegerb:9a!=b/*chartype*/#include<stdio.h>main(){charc;printf("Enteracharacter:");c=getchar();if(c<0x20)printf("Thecharacterisacontrolcharacter\n"

);elseif(c>='0'&&c<='9')printf("Thecharacterisadigit\n");elseif(c>='A'&&c<='Z')printf("Thecharacterisacapitalletter\n"

);elseif(c>='a'&&c<='z')printf("Thecharacterisalowerletter\n");elseprintf("Thecharacterisothercharacter

\n");}例判断输入字符种类运行：Enteracharacter：Thecharacterisacontrolcharacter运行：Enteracharacter：8Thecharacterisadigit运行：En

teracharacter：DThecharacterisacapitalletter运行：Enteracharacter：hThecharacterisalowerletter运行：Enteracharacter：F1Thecharacterisothercharacter#includ

e"ctype.h"iscntrl()isalpha()isdigit()ispunct()isprint()islower()isupper()isalnum()if语句嵌套p118⚫一般形式：if(expr1)if(exp

r2)statement1elsestatement2elseif(expr3)statement3elsestatement4内嵌if内嵌ifif(expr1)if(expr2)statement1elsestatement2内嵌ifif(expr1)if(expr2)stateme

nt1elsestatement3内嵌ifif(expr1)statement1elseif(expr3)statement3elsestatement4内嵌if例输入两数并判断其大小关系/*GreaterorLess*/#include<stdio.h>

main(){intx,y;printf("Enterintegerx,y:");scanf("%d,%d",&x,&y);if(x!=y)if(x>y)printf("X>Y\n");elseprintf("X<

Y\n");elseprintf("X==Y\n");}运行：Enterintegerx,y:12,23X<YEnterintegerx,y:12,6X>YEnterintegerx,y:12,12X==Y✓if~else配对原则：缺省{}时，else总是和它上面

离它最近的未配对的if配对if(……)if(……)if(……)else…...else…...else…...例：if(a==b)if(b==c)printf(“a==b==c”);elseprintf(“a!=b”);修改：if(a==b){if(b==c)printf(

“a==b==c”);}elseprintf(“a!=b”);实现if~else正确配对方法：加{}例考虑下面程序输出结果:main(){intx=100,a=10,b=20;intv1=5,v2=0;if

(a<b)if(b!=15)if(!v1)x=1;elseif(v2)x=10;x=-1;printf(“%d”,x);}结果：-1❖4.3switch语句（开关分支语句）p125❖一般形式：❖执行过程:switch(表达式){caseE1:语句组1;break;c

aseE2:语句组2;break;…….caseEn:语句组n;break;[default:语句组;break;]}switch表达式语句组1语句组2语句组n语句组…...E1E2Endefaultca

se✓E1,E2,…En是常量表达式,且值必须互不相同✓语句标号作用，必须用break跳出✓case后可包含多个可执行语句，且不必加{}✓switch可嵌套✓多个case可共用一组执行语句……case‘A’:case‘B’:case‘C’:printf(

“score>60\n”);break;……..switch(m){case5:printf(“Verygood!”);case4:printf(“Good!”);case3:printf(“Pass!”);case2:printf(“Fail!”);default:printf(“dataer

ror!”);}运行结果：m为5时，输出：Verygood!Good!Pass!Fail!dataerror!例voidmain(){intx=1,y=0,a=0,b=0;switch(x){case1:switch(y){case0:a++;break;case1:b++;br

eak;}case2:a++;b++;break;case3:a++;b++;}printf(“\na=%d,b=%d”,a,b);}运行结果：a=2,b=1/*SelectLabel*/#include<stdio.h>main(){intc;printf("Entermor

norhorother:");c=getchar();switch(c){case'm':printf("\nGoodmorning!\n");break;case'n':printf("\nGoodnight!\n");break;case'h':printf("

\nHello!\n");break;default:printf("\n????????\n");break;}}例根据输入字母输出字符串例百分制成绩转换为五级制p12280to100Honours60to79FirstDivisio

n50to59SecondDivision40to49ThirdDivision0to39Fail用SwitchStatement.❖4.4循环概述C语言可实现循环的语句：用goto和if构成循环while语句do~while语句for语句goto语句一般格式

p132－转移作用goto语句标号；….…..标号：语句；标号：语句；….…..goto语句标号；✓不能用整数作标号✓只能出现在goto所在函数内,且唯一✓只能加在可执行语句前面✓限制使用goto语句例用if和got

o语句构成循环，求1+2+…+100/*loopusinggotostatement*/#include<stdio.h>main(){inti,sum=0;i=1;loop:if(i<=100){sum+=i;i++;gotoloop;}printf("%d",sum);}sum=0

+1sum==1+2=3sum=3+3=6sum=6+4……sum=4950+100=5050循环初值循环终值循环变量增值循环条件循环体例从键盘输入一组数据，以0结束输入，求数据和/*sumofdata

*/#include<stdio.h>main(){intnumber,sum=0;read_loop:scanf("%d",&number);if(!number)gotoprint_sum;sum+=number;gotoread_loop;print_s

um:printf("Thetotalsumis%d\n",sum);}例输入10个男孩的身高和体重，统计身高超过170体重少于50公斤的人数。例pp135RANGEofNumbers:Asurveyofthecomputermacketshow

thatpersonalcomputersaresoldatvaryingcostsbythevendors.Thefollowingisthelistofcosts(inhundreds)quotedbysomevendors:35.00,40.50,25.00,31.25,68

.15,47.00,26.65,29.00,53.45,62.50Determinetheaveragecostandtherangeofvalues.例pp136Pay-BillCalculations:Aman

ufacturingcompanyhasclassifieditsexecutivesintofourlevelsforthebenefitofcertainperks.Thelevelsandcorrespondingperksareshownbelow:PerksLevel-

-----------------------------------------------------------------ConveyanceEntertainmentallowanceallo

wance11000500275020035001004250----Anexecutive’sgrosssalaryincludesbasicpay,houserentallowanceat25%ofbasicpayandotherperks.Incometaxiswithhel

dfromthesalaryonapercentagebasisasfollows:GrosssalaryTaxrateGross<=2000Notaxdeduction2000<Gross<=40003%4000<Gross<=50005%Gr

oss>50008%Writeaprogramthatwillreadanexecutive’sjobnumber,levelnumber,andbasicpayandthencomputerthenetsalaryafterwithholdingincometax.Theproblemisd

etailedintheprogram.while语句p147❖一般形式：while(表达式)循环体语句；❖执行流程：expr循环体假(0)真(非0)while❖特点：先判断表达式，后执行循环体❖说明：⚫循环体有可能一次也不执行⚫循环体可为任意类型语句⚫下

列情况，退出while循环◆条件表达式不成立（为零）◆循环体内遇break,return,goto⚫无限循环:while(1)循环体;例用while循环求1+2+…+100/*sumof1to100*/#include<

stdio.h>main(){inti,sum=0;i=1;while(i<=100){sum=sum+i;i++;}printf("%d",sum);}循环初值循环终值循环变量增值循环条件循环体例显示1~10的平方

/*squareofeverynumber*/#include<stdio.h>main(){inti=1;while(i<=10){printf("%d*%d=%d\n",i,i,i*i);i++;}}运行结果：1*1=12*2=43*3=94*4=1

65*5=256*6=367*7=498*8=649*9=8110*10=100do~while语句p150❖一般形式：do循环体语句；while(表达式);❖执行流程：do循环体expr假(0)真(非0)while❖特点：先执行循环体，后判断表达式expr循环体假(0)真(非0)循环体

While循环✓至少执行一次循环体✓do~while可转化成while结构例用do~while循环求1+2+…+100/**/#include<stdio.h>main(){inti,sum=0;i=1;do{sum+=i;i++;}while(

i<=100);printf("%d",sum);}例while和do~while比较#include<stdio.h>main(){inti,sum=0;scanf("%d",&i);do{sum+=i;i++;}while(i<=10);printf("%d",sum);}#i

nclude<stdio.h>main(){inti,sum=0;scanf("%d",&i);while(i<=10){sum+=i;i++;}printf("%d",sum);}for语句p152❖一般形式：for([expr1];[expr2];

[expr3])循环体语句；❖执行流程：expr2循环体假(0)真(非0)forexpr1expr3❖for语句一般应用形式：for(循环变量赋初值；循环条件；循环变量增值){循环体语句；}expr1;while(expr2){循环

体语句；expr3;}例用for循环求1+2+…+100#include<stdio.h>main(){inti,sum=0;for(i=1;i<=100;i++)sum+=i;printf("%d",sum);}✓for语句中expr1,expr2,expr3

类型任意，都可省略，但分号；不可省✓无限循环:for(;;)✓for语句可以转换成while结构“温故”逗号运算符p154例用for循环生成10个随机数并累加#include<stdio.h>#include<time.h>#inclu

de<stdlib.h>main(){inti,tmp,sum=0;/*Seedtherandom-numbergeneratorwithcurrenttimesothatthenumberswillbedifferenteverytimewe

run.*/srand((unsigned)time(NULL));for(i=1;i<=10;i++){tmp=rand();sum+=tmp;}printf("%d\n\n",sum);}#include<stdio.h>#includ

e<time.h>#include<stdlib.h>main(){inti,sum;srand((unsigned)time(NULL));for(sum=0,i=1;i<=100;i++)sum+=rand();printf("%d\n\n",sum)

;}各人各次运行结果不一逗号运算符和表达式❖形式：表达式1,表达式2,……表达式n❖结合性:从左向右❖优先级:15❖逗号表达式的值：等于表达式n的值❖用途：常用于循环for语句中例a=3*5,a*4a=3*5,a*4,a+

5例x=(a=3,6*3)x=a=3,6*a例a=1;b=2;c=3;printf(“%d,%d,%d”,a,b,c);printf(“%d,%d,%d”,(a,b,c),b,c);//a=15,表达式值60//a=15,表达式值20//赋值表达式

，表达式值18，x=18//逗号表达式,表达式值18,x=3//1,2,3//3,2,3例:#include<stdio.h>main(){intx,y=7;floatz=4;x=(y=y+6,y/z);printf("x=%d

\n",x);}运行结果：x=3例：#include<stdio.h>main(){inti;for(i=0;i<10;i++)putchar(‘a’+i);}运行结果：abcdefghij例：#include<stdio.h>main(){inti=0;for(;i<10;i++

)putchar(‘a’+i);}例：#include<stdio.h>main(){inti=0;for(;i<10;)putchar(‘a’+(i++));}例：#include<stdio.h>main(){inti=0;for(;i<10;putchar(‘a’+i),i

++);}灵活应用for语句main(){inti,j,k;for(i=0,j=100;i<=j;i++,j--){k=i+j;printf("%d+%d=%d\n",i,j,k);}}#include<stdio.h>main(){cha

rc;for(;(c=getchar())!='\n';)printf("%c",c);}#include<stdio.h>main(){inti,c;for(i=0;(c=getchar())!='\n';i+=3)printf("%c",i+

c);}例p161求1/(1-x)=1+x+x2+x3+……+xn,x<1,算到通项小于0.0001#include"stdio.h"intmain(){floatsum=1,x,term;printf

("Inputthevalueofx:");scanf("%f",&x);term=x;for(;term>=0.0001;){sum+=term;term*=x;}printf("term=%.5f,Sum=%.5f\n\n",term,sum

);return0;}这个例子若x的值超过1，则出现列循环。例梯形法求数值积分0yxaa+ha+iha+(i+1)hbf(x));1;1()*(*))()((**5.0)())()((2)))1(()((2)))1(()((21110++-<==++

=+=+++=++++=++++=-=-=-=iniihiafhssbfafhsihafhbfafhhiafihafhShiafihafhsnabhninii迭代：循环的嵌套p156❖三种循环可互相嵌套,层数不限❖外层循环可包含两个以上内循环,但不能相互交叉❖嵌套循环的执行流程(

1)while(){……while(){……}…...}(2)do{……do{……}while();…...}while();(3)while(){……do{……}while();…….}(4)for(;;){……do{……}while();……while(){…

…}…...}内循环外循环内循环❖嵌套循环的跳转禁止：⚫从外层跳入内层⚫跳入同层的另一循环⚫向上跳转例循环嵌套，输出九九表p151123456789123456789246810121416183691215182124

2791827364554637281……………..ij/*MultiplicationTable*/#include<stdio.h>main(){inti,j;for(i=1;i<10;i++)printf("%4d",i);printf("\n---

------------------------------------\n");for(i=1;i<10;i++)for(j=1;j<10;j++)printf((j==9)?"%4d\n":"%4d",i*j);}i<10printf假

(0)真(非0)i=1j++j=1j<10真(非0)假(0)i++for(i=1;i<10;i++)for(j=1;j<10;j++)printf((j==9)?"%4d\n":"%4d",i*j);内循环下面这个例子既是循环语句的应用，也比较充分体现计算机的

强大的逻辑运算能力——例有红、黄、兰、白、紫色的珠子分别放在５个包里。甲说：第二个包是紫色珠子,第四个包是黄色珠子.乙说：第一个包是红色珠子,第五个包是白色珠子.丙说：第三个包是白色珠子,第四个包是兰色珠子.丁说：第二个包是黄色珠子,第五

个包是紫色珠子.每个人都说对一句,说错一句.各个包里的珠子分别是什么颜色的?编制求解程序.❖4.5辅助控制语句p159~165break语句p159❖功能：在循环语句和switch语句中,终止并跳出循环体或开关体✓break只能终止并跳出

最近一层的结构✓break不能用于循环语句和switch语句之外的任何其它语句之中expr……break;……假(0)真(非0)whiledo……break;…...expr假(0)真(非0)whileexpr2……break;…...假(0)真(非0)forex

pr1expr3switchexpr语句组1break;语句组2break;语句组nbreak;语句组break;…...const1const2constndefaultcase例break举例：输出圆面积，面积大于10

0时停止#definePI3.14159main(){intr;floatarea;for(r=1;r<=10;r++){area=PI*r*r;if(area>100)break;printf("r=%d,area=

%.2f\n",r,area);}}例break举例：小写字母转换成大写字母,直至输入非字母字符#include<stdio.h>main(){inti,j;charc;while(1){c=getchar();if(

c>='a'&&c<='z')putchar(c-'a'+'A');elsebreak;}}#include"stdio.h"/*compute1/(1-x)*/intmain(){floatsum,x,term;intn;printf("Inputthevalueofx:

");scanf("%f",&x);for(term=1,sum=0,n=1;n<100;n++){if(term<0.0001)gotooutput;sum+=term;term*=x;}printf("NotR

eachtheDesireAccuracy!\n\n");gotoend;output:printf("ExitfromLoop\n");printf("term=%.5f,Sum=%.5f\n\n",term,sum);end:

return0;}break用到前面的例子(为解决列循环)却不方便：求1/(1-x)=1+x+x2+x3+……+xn,x<1,算到通项小于0.0001continue语句p163❖功能：结束本次循环，跳过循环体中一部分尚未执行的语句，进行下一次是否执行循环体的判断❖仅用于循环语句中e

xpr……continue;……假(0)真(非0)while真(非0)do……continue;…...expr假(0)whileexpr2……continue;…...假(0)真(非0)forexpr1expr3例求输入的十个整数中正数的个数及其平均值/*usingcontinu

estatement*/#include<stdio.h>main(){inti,num=0,a;floatsum=0;for(i=0;i<10;i++){scanf("%d",&a);if(a<=0)continue;num++;sum+=a;}printf("%dplusinteger's

sum:%6.0f\n",num,sum);printf("Meanvalue:%6.2f\n",sum/num);}程序举例t=1,pi=0,n=1.0,s=1当|t|1e-6pi=pi+tn=n+2s=-st=s/npi=pi*4输出pi分子：1，-1，1，-1…分

母：1，3，5，7，...为止最后一项的绝对值小于的近似值，直到公式求例用61071513114-+-+-ppLL例求Fibonacci数列：1，1，2，3，5，8，……的前40个数f1=1,f2=1fori=1to20输出f1,f2f1=f1+f2f

2=f2+f11534233159710946750255142293524578241578171855377258417711121393832040570288739088169213896104181286571964181346269922746563245986321144987

676546368317811217830914930352102334155)3()2(12)1(1121+=====--nFFFnFnFnnn例判断m是否素数读入mk=mi=2当ikm被i整除真假用break结束循环i=i+1ik+1真假输出:m”是

素数”输出:m”不是素数”例简单密码系统例如Hello,world!译成密码：Lipps,asvph!解码：例pp168BonomialCoeffcientsareusedinthestudyofbonomialdistributi

ons.ItisgivenbyB(m,x)=m!/(x!(m-x)!),m>=xAtableofbonomialCoeffcientsisrequiredtobeprintedasfollows:Mx012345678910-------------

---------------------------------------------------------------0111121213133141464151510105161615201561717213535217181828567056288191936

84126126843691101104512021025221012045101ProblemAnalysis:B(m,0)=1;B(m,x)=B(m,x-1)[(m-x+1)/x],x=1,2,3,...,mB(0,0)=1;例pp169Inanorganiz

ation,theemployeesaregroupedaccordingtotheirbasicpayforpurposeofcertainperks.Thepay-rangeandthenumberofemployeesineachgroupa

reasfollows:GROUPPAY-RANGENUMBEROFEMPLOYEES1750-15001221501-30002333001-45003544501-6000205above600011Drawahistogramtohighlightthegroupsizes.

例pp171MinimumCost:ThecostofoperationofaunitconsistsoftwocomponentsC1andC2whichcanbeexpressedasfunctionsofaparamet

erpasfollows:C1=30-8pC2=10+p2Theparameterprangesfrom0to10.Determinethevalueofpwithanaccuracyof+0.1wherethecostofoperationwouldbe

minimum.ProblemAnalysis:Totalcost=C1+C2=40-8p+p2Thecostis40whenp=0,and33whenp=1and60whenp=10.Thecost,therefore,decreasesfir

standthenincreases.例pp172PlottingofTwoFunctions:y1=exp(-ax)y2=exp(-ax2/2)xvaryingfrom0to5.0ProgramAnalys

is:Initiallywhenx=0,y1=y2=1andthegraphsstartfromthesamepoint.Thecurvescrosswhentheyareagainequalatx=2.0.Theprograms

houldhaveappropriatebranchstatementstoprintthegraphpointsatthefollowingthreecondition:1.y1>y22.y1<y23.y1=y2Thefunctionsy1andy2arenormaliz

edandconvertedtointgersasfollows:y1=50exp(-ax)+0.5y2=50exp(-ax2/2)+0.5