C++程序控制結(jié)構(gòu)

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第2章程序控制結(jié)構(gòu)

2.1選擇控制

2.2循環(huán)控制

2.3判斷表達(dá)式的使用

2.4轉(zhuǎn)向語句

小結(jié)語句是程序的基本語法成分。程序設(shè)計(jì)語言的語句按功能可以分成三類：

聲明語句指示編譯器分配內(nèi)存，或者提供程序連接信息

操作語句描述對(duì)數(shù)據(jù)的處理

控制語句用于控制程序的執(zhí)行流程。所有程序都只能包含三種控制結(jié)構(gòu)：順序結(jié)構(gòu)、選擇結(jié)構(gòu)和循環(huán)結(jié)構(gòu)第2章程序控制結(jié)構(gòu)對(duì)給定的條件進(jìn)行判斷，并根據(jù)判斷的結(jié)果選擇不同的操作

2.1選擇控制條件運(yùn)算表達(dá)式

根據(jù)判斷條件，決定表達(dá)式的值不改變程序語句執(zhí)行流程

比較條件運(yùn)算1．if語句的形式和執(zhí)行流程if（表達(dá)式）語句；2.1.1if語句語句形式（1）執(zhí)行流程false(0)true(非0)

表達(dá)式

語句2.1.1if語句false(0)true(非0)

表達(dá)式

語句1．if語句的形式和執(zhí)行流程if（表達(dá)式）語句；2.1.1if語句語句形式（1）執(zhí)行流程

表達(dá)式

語句true(非0)2.1.1if語句false(0)true(非0)

表達(dá)式

語句1．if語句的形式和執(zhí)行流程if（表達(dá)式）語句；2.1.1if語句語句形式（1）執(zhí)行流程

表達(dá)式

false(0)2.1.1if語句2.1.1if語句35abmax例：

:max=a;if(b>a)max=b;cout<<"max="<<max<<endl;:2.1.1if語句2.1.1if語句35abmax3例：

:max=a;if(b>a)

max=b;cout<<"max="<<max<<endl;:2.1.1if語句2.1.1if語句35abmax5例：

:max=a;if(b>a)max=b;cout<<"max="<<max<<endl;:2.1.1if語句2.1.1if語句35abmax5

max=5輸出例：

:max=a;if(b>a)max=b;cout<<"max="<<max<<endl;:2.1.1if語句2.1.1if語句73abmax例：

:max=a;if(b>a)max=b;cout<<"max="<<max<<endl;:2.1.1if語句2.1.1if語句73abmax7例：

:max=a;if(b>a)max=b;cout<<"max="<<max<<endl;:2.1.1if語句2.1.1if語句73abmax7

max=7輸出例：

:max=a;if(b>a)max=b;cout<<"max="<<max<<endl;:2.1.1if語句2.1.1if語句1．if語句的形式和執(zhí)行流程語句形式（2）if（表達(dá)式）語句1

；

else語句2

；執(zhí)行流程false(0)true(非0)

表達(dá)式

語句1語句22.1.1if語句2.1.1if語句1．if語句的形式和執(zhí)行流程語句形式（2）if（表達(dá)式）語句1

；

else語句2

；執(zhí)行流程false(0)true(非0)

表達(dá)式

語句1語句2true(非0)

表達(dá)式

語句12.1.1if語句2.1.1if語句1．if語句的形式和執(zhí)行流程語句形式（2）if（表達(dá)式）語句1

；

else語句2

；執(zhí)行流程false(0)true(非0)

表達(dá)式

語句1語句2false(0)

表達(dá)式

語句22.1.1if語句2.1.1if語句例：

:if(b>a)max=b;elsemax=a;cout<<"max="<<max<<endl;:35abmax2.1.1if語句2.1.1if語句35abmax例：

:if(b>a)

max=b;elsemax=a;cout<<"max="<<max<<endl;:2.1.1if語句2.1.1if語句例：

:if(b>a)

max=b;

elsemax=a;cout<<"max="<<max<<endl;:35abmax52.1.1if語句2.1.1if語句例：

:if(b>a)max=b;elsemax=a;cout<<"max="<<max<<endl;:35abmax5

max=5輸出2.1.1if語句2.1.1if語句例：

:if(b>a)

max=b;elsemax=a;cout<<"max="<<max<<endl;:73abmax2.1.1if語句2.1.1if語句例：

:if(b>a)max=b;

elsemax=a;cout<<"max="<<max<<endl;:73abmax72.1.1if語句2.1.1if語句例：

:if(b>a)max=b;elsemax=a;cout<<"max="<<max<<endl;:73abmax7

max=7輸出2.1.1if語句2.1.1if語句例：

:if(b>a)max=b;elsemax=a;cout<<"max="<<max<<endl;:73abmax7

max=7輸出試一試用條件表達(dá)式修改該程序2.1.1if語句2.1.1if語句2．if語句的嵌套

if語句中的執(zhí)行語句如果又是另一個(gè)if語句，稱為嵌套if語句

if與else的配對(duì)關(guān)系：C++規(guī)定，else總是與它接近的if配對(duì)使用復(fù)合語句，可以改變條件語句的執(zhí)行流程2.1.1if語句2.1.1if語句例2-1

編寫程序計(jì)算貨物運(yùn)費(fèi)。設(shè)貨物運(yùn)費(fèi)每噸單價(jià)p（元）與運(yùn)輸距離s（公里）之間有如下關(guān)系：p=輸入要托運(yùn)的貨物重量為w噸，托運(yùn)距離s公里，計(jì)算總運(yùn)費(fèi)t：

t=p*w*s2.1.1if語句2.1.1if語句控制流圖truefalsetruetruetruefalsefalsefalseS<100?S<200?S<300?S<400?P=30P=20P=22.5P=25P=27.5t=p*w*s2.1.1if語句2.1.1if語句truefalsetruetruetruefalsefalsefalseS<100?S<200?S<300?S<400?P=30P=20P=22.5P=25P=27.5t=p*w*s//例2-1計(jì)算貨物運(yùn)費(fèi)#include<iostream>#include<iomanip>usingnamespacestd;intmain(){doublet,p,w,s;

cout<<"Pleaseinputweight(ton):";

cin>>w;

cout<<"Pleaseinputdistsnce(kilometre):";

cin>>s;if(s<100)p=30;elseif(s<200)p=27.5; elseif(s<300)p=25; elseif(s<400)p=22.5; elsep=20;t=p*w*s;

cout<<"Thecostis:"<<setprecision(2)<<t<<'$'<<endl;}想一想：幾個(gè)else分支語句的次序可以改變嗎？2.1.1if語句//例2-1計(jì)算貨物運(yùn)費(fèi)#include<iostream>#include<iomanip>usingnamespacestd;intmain(){doublet,p,w,s;

cout<<"Pleaseinputweight(ton):";

cin>>w;

cout<<"Pleaseinputdistsnce(kilometre):";

cin>>s;if(s<100)p=30;elseif(s<200)p=27.5; elseif(s<300)p=25; elseif(s<400)p=22.5; elsep=20;t=p*w*s;

cout<<"Thecostis:"<<setprecision(2)<<t<<'$'<<endl;}truefalsetruetruetruefalsefalsefalseS<100?S<200?S<300?S<400?P=30P=20P=22.5P=25P=27.5t=p*w*s2.1.1if語句

例如：

cin>>s;if(s<300)p=25;elseif(s<100)p=30;elseif(s<400)p=22.5;elseif(s<200)p=27.5;elsep=20;t=p*w*s;

若輸入s：150

輸出t值等于多少？為什么？2.1.1if語句2.1.1if語句例2-2

輸入三個(gè)整數(shù)，按從小到大順序輸出。解法一

數(shù)據(jù)排序。先找出最小值，放在a中，然后找次小值，放在b中：

1.對(duì)a、b進(jìn)行比較，把小值放于a;

ifa>bab

//a<b2.

對(duì)a、c進(jìn)行比較，把小值放于a;

ifa>cac

//a<c,a<b3.對(duì)

b、c進(jìn)行比較，把小值放于b;

ifb>cbc

//a<b&&b<c4.輸出a,b,c的值。2.1.1if語句2.1.1if語句例2-2

輸入三個(gè)整數(shù)，按從小到大順序輸出。a

2bca>b:ab

2bca

5bc

7a>c:ac

2b>c:bc

7bca<ba<b,a<ca<b&&b<c2.1.1if語句2.1.1if語句例2-2

輸入三個(gè)整數(shù)，按從小到大順序輸出。#include<iostream>usingnamespacestd;intmain(){inta,b,c,t;

cout<<"Pleaseinputthreeintegernumbers:";

cin>>a>>b>>c;if(a>b){t=a;a=b;b=t;}if(a>c){t=a;a=c;c=t;}if(b>c){t=b;b=c;c=t;}

cout<<a<<""<<b<<""<<c<<endl;}注意語句塊結(jié)構(gòu)2.1.1if語句2.1.1if語句例2-2

輸入三個(gè)整數(shù)，按從小到大順序輸出。#include<iostream>usingnamespacestd;intmain(){inta,b,c,t;

cout<<"Pleaseinputthreeintegernumbers:";

cin>>a>>b>>c;if(a>b)

{t=a;a=b;b=t;}if(a>c)

{t=a;a=c;c=t;}if(b>c)

{t=b;b=c;c=t;}

cout<<a<<""<<b<<""<<c<<endl;}注意語句塊結(jié)構(gòu)2.1.1if語句2.1.1if語句 if(E1) if(E2)S1

elseS2if(E1)

{if(E2)S1

}elseS2分析S1和S2的執(zhí)行條件：E1&&E2

執(zhí)行S1E1&&!E2

執(zhí)行S2E1&&E2

執(zhí)行S1

!E1

執(zhí)行S2注意括號(hào)的作用2．if語句的嵌套2.1.1if語句3．應(yīng)用舉例(1)

把輸入字符轉(zhuǎn)換為小寫字母。對(duì)輸入字符進(jìn)行判斷，如果是大寫字母，則轉(zhuǎn)換為小寫字母；否則，不轉(zhuǎn)換。//例2-3#include<iostream>usingnamespacestd;intmain(){charch;

cout<<"ch=";

cin>>ch;if(ch>='A'&&ch<='Z')ch+=32;

cout<<ch<<endl;}2.1.1if語句3．應(yīng)用舉例//例2-3

#include<iostream>usingnamespacestd;intmain(){charch;

cout<<"ch=";

cin>>ch;if(ch>='A'&&ch<='Z')

ch+=32;

cout<<ch<<endl;}輸入大寫字母(1)

把輸入字符轉(zhuǎn)換為小寫字母。對(duì)輸入字符進(jìn)行判斷，如果是大寫字母，則轉(zhuǎn)換為小寫字母；否則，不轉(zhuǎn)換。2.1.1if語句3．應(yīng)用舉例//例2-3#include<iostream>usingnamespacestd;intmain(){charch;

cout<<"ch=";

cin>>ch;if(ch>='A'&&ch<='Z')ch+=32;

cout<<ch<<endl;}計(jì)算ASCII碼偏移值(1)

cout<<"ch=";

cin>>ch;if(ch>='A'&&ch<='Z')ch+=32;

cout<<ch<<endl;}(1)

cout<<"ch=";

cin>>ch;

if(ch>='A'&&ch<='Z')ch+=32;

cout<<ch<<endl;}改寫為條件表達(dá)式ch=(ch>='A'&&ch<='Z')?ch+32:ch;(1)

把輸入字符轉(zhuǎn)換為小寫字母。對(duì)輸入字符進(jìn)行判斷，如果是大寫字母，則轉(zhuǎn)換為小寫字母；否則，不轉(zhuǎn)換。2.1.1if語句3．應(yīng)用舉例

(2)

求一元二次方程ax2+bx+c=0的根。求根公式：①當(dāng)a=0時(shí)，方程不是二次方程②當(dāng)b2-4ac=0時(shí),有兩個(gè)相同的實(shí)根：③當(dāng)b2-4ac>0時(shí),有兩個(gè)不同的實(shí)根：④當(dāng)b2-4ac<0時(shí),有兩個(gè)共軛復(fù)根：2.1.1if語句#include<iostream> //例2-4#include<cmath>usingnamespacestd;intmain(){doublea,b,c,d,x1,x2,rp,ip;

cout<<"a,b,c=";cin>>a>>b>>c;if(fabs(a)<=1e-8)

cout<<"Itisnotquadratic."<<endl;else{d=b*b-4*a*c;if(fabs(d)<=1e-8)

cout<<"Ithastwoequalrealroots:"<<-b/(2*a)<<endl;elseif(d>1e-8) {x1=(-b+sqrt(d))/(2*a);x2=(-b-sqrt(d))/(2*a);

cout<<"Ithastwodistinctrealroots:"<<x1<<"and"<<x2<<endl; } else{rp=-b/(2*a);ip=sqrt(-d)/(2*a);

cout<<"Ithastwocomplexroots:"<<endl;

cout<<rp<<"+"<<ip<<"i"<<endl;

cout<<rp<<"-"<<ip<<"i"<<endl; } }}2.1.1if語句#include<iostream> //例2-4#include<cmath>usingnamespacestd;intmain(){doublea,b,c,d,x1,x2,rp,ip;

cout<<"a,b,c=";cin>>a>>b>>c;if(fabs(a)<=1e-8)

cout<<"Itisnotquadratic."<<endl;else{d=b*b-4*a*c;if(fabs(d)<=1e-8)

cout<<"Ithastwoequalrealroots:"<<-b/(2*a)<<endl;elseif(d>1e-8) {x1=(-b+sqrt(d))/(2*a);x2=(-b-sqrt(d))/(2*a);

cout<<"Ithastwodistinctrealroots:"<<x1<<"and"<<x2<<endl; } else{rp=-b/(2*a);ip=sqrt(-d)/(2*a);

cout<<"Ithastwocomplexroots:"<<endl;

cout<<rp<<"+"<<ip<<"i"<<endl;

cout<<rp<<"-"<<ip<<"i"<<endl; } }}數(shù)據(jù)說明2.1.1if語句#include<iostream> //例2-4#include<cmath>usingnamespacestd;intmain(){doublea,b,c,d,x1,x2,rp,ip;

cout<<"a,b,c=";cin>>a>>b>>c;if(fabs(a)<=1e-8)

cout<<"Itisnotquadratic."<<endl;else{d=b*b-4*a*c;if(fabs(d)<=1e-8)

cout<<"Ithastwoequalrealroots:"<<-b/(2*a)<<endl;elseif(d>1e-8) {x1=(-b+sqrt(d))/(2*a);x2=(-b-sqrt(d))/(2*a);

cout<<"Ithastwodistinctrealroots:"<<x1<<"and"<<x2<<endl; } else{rp=-b/(2*a);ip=sqrt(-d)/(2*a);

cout<<"Ithastwocomplexroots:"<<endl;

cout<<rp<<"+"<<ip<<"i"<<endl;

cout<<rp<<"-"<<ip<<"i"<<endl; } }}提示并輸入系數(shù)2.1.1if語句#include<iostream> //例2-4#include<cmath>usingnamespacestd;intmain(){doublea,b,c,d,x1,x2,rp,ip;

cout<<"a,b,c=";cin>>a>>b>>c;

if(fabs(a)<=1e-8)

cout<<"Itisnotquadratic."<<endl;else{d=b*b-4*a*c;if(fabs(d)<=1e-8)

cout<<"Ithastwoequalrealroots:"<<-b/(2*a)<<endl;elseif(d>1e-8) {x1=(-b+sqrt(d))/(2*a);x2=(-b-sqrt(d))/(2*a);

cout<<"Ithastwodistinctrealroots:"<<x1<<"and"<<x2<<endl; } else{rp=-b/(2*a);ip=sqrt(-d)/(2*a);

cout<<"Ithastwocomplexroots:"<<endl;

cout<<rp<<"+"<<ip<<"i"<<endl;

cout<<rp<<"-"<<ip<<"i"<<endl; } }}不是二次方程2.1.1if語句#include<iostream> //例2-4#include<cmath>usingnamespacestd;intmain(){doublea,b,c,d,x1,x2,rp,ip;

cout<<"a,b,c=";cin>>a>>b>>c;if(fabs(a)<=1e-8)

cout<<"Itisnotquadratic."<<endl;else{d=b*b-4*a*c;if(fabs(d)<=1e-8)

cout<<"Ithastwoequalrealroots:"<<-b/(2*a)<<endl;elseif(d>1e-8) {x1=(-b+sqrt(d))/(2*a);x2=(-b-sqrt(d))/(2*a);

cout<<"Ithastwodistinctrealroots:"<<x1<<"and"<<x2<<endl; } else{rp=-b/(2*a);ip=sqrt(-d)/(2*a);

cout<<"Ithastwocomplexroots:"<<endl;

cout<<rp<<"+"<<ip<<"i"<<endl;

cout<<rp<<"-"<<ip<<"i"<<endl; } }}a等于0浮點(diǎn)數(shù)的誤差判斷2.1.1if語句#include<iostream> //例2-4#include<cmath>usingnamespacestd;intmain(){doublea,b,c,d,x1,x2,rp,ip;

cout<<"a,b,c=";cin>>a>>b>>c;if(fabs(a)<=1e-8)

cout<<"Itisnotquadratic."<<endl;

else

{d=b*b-4*a*c;if(fabs(d)<=1e-8)

cout<<"Ithastwoequalrealroots:"<<-b/(2*a)<<endl;elseif(d>1e-8) {x1=(-b+sqrt(d))/(2*a);x2=(-b-sqrt(d))/(2*a);

cout<<"Ithastwodistinctrealroots:"<<x1<<"and"<<x2<<endl; } else{rp=-b/(2*a);ip=sqrt(-d)/(2*a);

cout<<"Ithastwocomplexroots:"<<endl;

cout<<rp<<"+"<<ip<<"i"<<endl;

cout<<rp<<"-"<<ip<<"i"<<endl; } }}有根2.1.1if語句#include<iostream> //例2-4#include<cmath>usingnamespacestd;intmain(){doublea,b,c,d,x1,x2,rp,ip;

cout<<"a,b,c=";cin>>a>>b>>c;if(fabs(a)<=1e-8)

cout<<"Itisnotquadratic."<<endl;else{d=b*b-4*a*c;if(fabs(d)<=1e-8)

cout<<"Ithastwoequalrealroots:"<<-b/(2*a)<<endl;elseif(d>1e-8) {x1=(-b+sqrt(d))/(2*a);x2=(-b-sqrt(d))/(2*a);

cout<<"Ithastwodistinctrealroots:"<<x1<<"and"<<x2<<endl; } else{rp=-b/(2*a);ip=sqrt(-d)/(2*a);

cout<<"Ithastwocomplexroots:"<<endl;

cout<<rp<<"+"<<ip<<"i"<<endl;

cout<<rp<<"-"<<ip<<"i"<<endl; } }}求判別式2.1.1if語句#include<iostream> //例2-4#include<cmath>usingnamespacestd;intmain(){doublea,b,c,d,x1,x2,rp,ip;

cout<<"a,b,c=";cin>>a>>b>>c;if(fabs(a)<=1e-8)

cout<<"Itisnotquadratic."<<endl;else{d=b*b-4*a*c;

if(fabs(d)<=1e-8)

cout<<"Ithastwoequalrealroots:"<<-b/(2*a)<<endl;elseif(d>1e-8) {x1=(-b+sqrt(d))/(2*a);x2=(-b-sqrt(d))/(2*a);

cout<<"Ithastwodistinctrealroots:"<<x1<<"and"<<x2<<endl; } else{rp=-b/(2*a);ip=sqrt(-d)/(2*a);

cout<<"Ithastwocomplexroots:"<<endl;

cout<<rp<<"+"<<ip<<"i"<<endl;

cout<<rp<<"-"<<ip<<"i"<<endl; } }}有兩個(gè)相同的實(shí)根2.1.1if語句#include<iostream> //例2-4#include<cmath>usingnamespacestd;intmain(){doublea,b,c,d,x1,x2,rp,ip;

cout<<"a,b,c=";cin>>a>>b>>c;if(fabs(a)<=1e-8)

cout<<"Itisnotquadratic."<<endl;else{d=b*b-4*a*c;if(fabs(d)<=1e-8)

cout<<"Ithastwoequalrealroots:"<<-b/(2*a)<<endl;elseif(d>1e-8) {x1=(-b+sqrt(d))/(2*a);x2=(-b-sqrt(d))/(2*a);

cout<<"Ithastwodistinctrealroots:"<<x1<<"and"<<x2<<endl; } else{rp=-b/(2*a);ip=sqrt(-d)/(2*a);

cout<<"Ithastwocomplexroots:"<<endl;

cout<<rp<<"+"<<ip<<"i"<<endl;

cout<<rp<<"-"<<ip<<"i"<<endl; } }}有兩個(gè)不相同的實(shí)根2.1.1if語句#include<iostream> //例2-4#include<cmath>usingnamespacestd;intmain(){doublea,b,c,d,x1,x2,rp,ip;

cout<<"a,b,c=";cin>>a>>b>>c;if(fabs(a)<=1e-8)

cout<<"Itisnotquadratic."<<endl;else{d=b*b-4*a*c;if(fabs(d)<=1e-8)

cout<<"Ithastwoequalrealroots:"<<-b/(2*a)<<endl;elseif(d>1e-8) {x1=(-b+sqrt(d))/(2*a);x2=(-b-sqrt(d))/(2*a);

cout<<"Ithastwodistinctrealroots:"<<x1<<"and"<<x2<<endl; } else{rp=-b/(2*a);ip=sqrt(-d)/(2*a);

cout<<"Ithastwocomplexroots:"<<endl;

cout<<rp<<"+"<<ip<<"i"<<endl;

cout<<rp<<"-"<<ip<<"i"<<endl; } }}有兩個(gè)共軛復(fù)根2.1.1if語句#include<iostream> //例2-4#include<cmath>usingnamespacestd;intmain(){doublea,b,c,d,x1,x2,rp,ip;

cout<<"a,b,c=";cin>>a>>b>>c;if(fabs(a)<=1e-8)

cout<<"Itisnotquadratic."<<endl;else{d=b*b-4*a*c;if(fabs(d)<=1e-8)

cout<<"Ithastwoequalrealroots:"<<-b/(2*a)<<endl;elseif(d>1e-8) {x1=(-b+sqrt(d))/(2*a);x2=(-b-sqrt(d))/(2*a);

cout<<"Ithastwodistinctrealroots:"<<x1<<"and"<<x2<<endl; } else{rp=-b/(2*a);ip=sqrt(-d)/(2*a);

cout<<"Ithastwocomplexroots:"<<endl;

cout<<rp<<"+"<<ip<<"i"<<endl;

cout<<rp<<"-"<<ip<<"i"<<endl; } }}a,b,c=021Itisnotquadratic.a,b,c=121Ithastwoequalrealroots:-1a,b,c=151Ithastwodistinctrealroots:-0.208712and–4.791288a,b,c=234Ithastwocomplexroots:-0.75+1.198958i-0.75-1.198958i2.1.1if語句根據(jù)一個(gè)整型表達(dá)式的值決定程序分支2.1.2switch語句一般形式：

switch(表達(dá)式){case常量表達(dá)式1:語句1

case常量表達(dá)式2:語句2

…

case常量表達(dá)式n:語句ndefault:語句n+1}注：

表達(dá)式類型為非浮點(diǎn)型

各常量表達(dá)式類型要與之匹配各常量表達(dá)式要求各不相等default子句可選。缺省時(shí)，沒有匹配值switch語句為空語句標(biāo)號(hào)2.1.2switch語句根據(jù)一個(gè)整型表達(dá)式的值決定程序分支2.1.2switch語句表達(dá)式語句1語句2語句3語句n語句n+1=常量1=常量2=常量3=常量n=常量n+1執(zhí)行流程2.1.2switch語句2.1.2switch語句例2-5根據(jù)考試成績(jī)的等級(jí)打印出百分制分?jǐn)?shù)段。#include<iostream>usingnamespacestd;intmain(){chargrade;