c语言大学教程课件

第4章选择结构C语言大学实用教程南京审计学院计算机科学与技术系孙玉星

1本章学习内容算法的描述方法常用算法（累加累乘、统计、递推迭代、穷举）选择结构及相关控制语句（第四章）循环结构及相关控制语句（第五章）结构化程序设计的基本思想Skill:MapproblemtosolutioninflowchartandpseudocodeformsBeabletodevelopaprogramcontainingselectionandloopcontrolstructureConsiderthefollowing….Problem:

烤蛋糕（BakingaCake）Howtosolve:

Start

将烤箱预热

准备一个盘子

在盘子上抹上一些黄油

将面粉、鸡蛋、糖和香精混合在一起搅拌均匀将搅拌好的面粉团放在盘子上

将盘子放到烤箱内End实际生活中的算法

AlgorithminRealLife‘DivideandConquer’Strategy

（分治策略）inAlgorithmProblem:

准备早餐（PrepareaBreakfast）

1. Start2. 准备早餐3. End1.Start2.准备早餐

2.1准备一个金枪鱼三明治2.2准备一些薯条2.3冲一杯咖啡3.End‘DivideandConquer’Strategy

（分治策略）inAlgorithm1.Start2.准备早餐

2.1准备一个金枪鱼三明治2.1.1拿来两片面包2.1.2准备一些金枪鱼酱

2.2准备一些薯片2.3冲一杯咖啡3.End‘DivideandConquer’Strategy

（分治策略）inAlgorithm1.Start2.准备早餐2.1准备一个金枪鱼三明治2.1.1拿来两片面包2.1.2准备一些金枪鱼酱

2.2准备一些薯片

2.2.1将土豆切成片2.2.2油炸这些土豆片

2.3冲一杯咖啡3.End‘DivideandConquer’Strategy

（分治策略）inAlgorithm‘DivideandConquer’Strategy

（分治策略）inAlgorithm1.Start2.准备早餐2.1准备一个金枪鱼三明治2.1.1拿来两片面包2.1.2准备一些金枪鱼酱

2.2准备一些薯片2.2.1将土豆切成片2.2.2油炸这些土豆片

2.3冲一杯咖啡2.3.1烧些开水放入杯中2.3.2在水杯中加入一些咖啡和糖3.EndWhatistheconnectionbetweenthesereallifeprocessesandalgorithm?Somethingtoponder…算法（Algorithm）的概念

数据结构+算法=程序只对面向过程的语言(C)成立面向对象程序=对象+消息算法：为解决一个具体问题而采取的确定的有限的操作步骤，仅指计算机能执行的算法Aspecificandstep-by-stepsetofinstructionsforcarryingoutaprocedureorsolvingaproblem,usuallywiththerequirementthattheprocedureterminateatsomepoint算法的特性

有穷性在合理的时间内完成确定性，无歧义

如果x≥0，则输出Yes如果x≤0，则输出No有效性

能有效执行负数开平方没有输入或有多个输入

有一个或多个输出

算法的表示方法自然语言描述传统流程图（Flowchart）在1966年，Bohra

与Jacopini

提出N-S结构化流程图1973年，美国学者I.Nassi

和B.Shneiderman

提出伪码（Pseudocode）表示流程图（Flowchart）Flowchartrepresentsalgorithmgraphically.Start/EndSymbolSemanticProcessInput/OutputTestConnectorFlowofactivities问题求解步骤

（ProblemSolvingProcess）InputProcessOutputFirstidentifytheinputandoutputoftheproblem.Example1：买苹果，计算价钱Calculateanddisplaythepriceofanumberofapplesifthequantityinkgandpriceperkgaregiven.

quantity

pricePerkgpriceprice=quantity*pricePerkgInputProcessOutput流程图（Flowchart）:

CalculatePriceofApplesInputquantity

Startpricequantity*pricePperkgInputpricePerkgOutputpriceEndIfnecessary,use‘Divide&Conquer’strategytodecomposetheproblemintosmallerandmanageablesubproblems.main(){

scanf("%d",&quantity);

scanf("%d",&pricePerkg);price=quantity*pricePerkg;

printf("%d",price);}CProgram:CalculatePriceofApplesInputquantity

Startpricequantity*pricePperkgInputpricePerkgOutputpriceEndIt’snotcomplete!Declarethevariables…main(){

int

quantity,price_per_kg,price;

scanf("%d",&quantity);

scanf("%d",&pricePerkg);price=quantity*pricePerkg;

printf("%d",price);}CProgram:CalculatePriceofApplesInputquantity

Startpricequantity*pricePperkgInputpricePerkgOutputpriceEndWelldone!But…whatarethey?main(){

int

quantity,price_per_kg,price;

scanf("%d",&quantity);

scanf("%d",&pricePerkg);price=quantity*pricePerkg;

printf("%d",price);}CProgram:CalculatePriceofApplesInputquantity

Startpricequantity*pricePperkgInputpricePerkgOutputpriceEndStartDeclaration}InputProcessOutputEndAnoutlineofaprogram,writteninaformthatcaneasilybeconvertedintorealprogrammingstatements.Itresemblestheactualprogramthatwillbeimplementedlater.However,itcannotbecompilednorexecuted.伪码（Pseudocode）1.Start2.Readquantity3.Readprice_per_kg4.

price

quantity*price_per_kg5.Printprice6.EndPseudocodenormallycodesthefollowingactions:

InitialisationofvariablesAssignmentofvaluestothevariablesArithmeticoperationsRelationaloperations顺序（Sequence）结构的NS图给变量赋值赋值表达式语句

赋值表达式;

price=quantity*pricePerkg;输入输出数据标准库函数调用语句

scanf("%d",&pricePerkg);

printf("%d",price);NS图BAExample2：

CalculatetheMinimum计算两个数中的最小者.

num1num2min????InputProcessOutputABNY条件PBNAY条件P选择结构（分支结构）

（SelectionStructure）NS图传统流程图NoYesFlowchart:CalculatetheMinimumInputnum1Inputnum2Outputminnum1<num2?minnum2minnum1

StartEndscanf("%d%d",&num1,&num2);if(num1<num2)min=num1;elsemin=num2;printf("%d",min);NoYesInputnum1Inputnum2Outputminnum1<num2?minnum2minnum1

StartEndCProgram:CalculatetheMinimummain(){

intnum1,num2,min;

scanf("%d%d",&num1,&num2);

if(num1<num2) min=num1;

else min=num2;

printf("%d",min);}}

SelectionStructureCProgram:CalculatetheMinimumif-elseSingleSelectionDoubleSelectionMultipleSelectionifif–else-if选择结构（分支结构）

（SelectionStructure）单分支选择结构

（SingleSelection）StepaconditionStepmStepnStepbtruefalsestepaconditionstepmstepnstepbtruefalsePseudocodeStructurestepaif<conditionistrue>start stepm stepnend_ifstepbifStatementThestructureissimilartosingleselection(flowchart)Syntax:

if

(expression) statement;or

if

(expression){ statement1; statement2; }复合语句compoundstatement被当作一条语句看待ifStatementThestructureissimilartosingleselection(flowchart)Syntax:

if

(expression) statement;or

if

(expression){ statement1; statement2; }Don’tforgetthebraces

!!Don’tforgettheparentheses

!!if

StatementThesimilaritybetweensingleselectionstructureandif

statement:SingleSelectionPseudocode

:if<conditionistrue>start step1 step2 … stepkend_ifif

Statement:if(expression){ statement1 statement2 … statementk}表达式非0为真ifStatementmain(){

intnum1,num2,min;

printf(“Input2numbers:“);

scanf(“%d%d”,&num1,&num2);min=num1;

if(num1>num2) min=num2;

printf(“Smallest:%d\n”,min);}Input2numbers:_num2?num1?min?Input2

numbers:2015_20152015Input2numbers:2015Smallest:15_20>15?ifStatementExample:main(){

intmark;

scanf(“%d”,&mark); if(mark>=60)

printf(“Pass\n”);

printf(“Yourmarkis%d”,mark);}Whatwilltheoutputbeifthemarkis65?ifStatementExample:main(){

intmark;

scanf(“%d”,&mark); if(mark>=60)

printf(“Pass\n”);

printf(“Yourmarkis%d”,mark);}Whatwilltheoutputbeifthemarkis35?双分支选择结构

（DoubleSelection)PseudocodeStructureStepaif<conditionistrue>start Stepm Stepnend_ifelsestart Stepx Stepyend_elseStepzStepaconditionStepmStepnStepztruefalseStepxStepyStepaconditionStepmStepnStepztruefalseStepxStepyif-else

StatementThestructureissimilartodoubleselection(flowchart)Syntax:

if(expression)

statement;

else

statement;or

if

(expression){ statement1; statement2; }

else

statement3;or

if

(expression)

{ statement1; statement2;}

else

{ statement3; statement4;}if-else

StatementThesimilaritybetweendoubleselectionstructureandif-elsestatement:DoubleSelectionPseudocode:if<conditionistrue>start step1 … stepkend_ifelsestart step1 … stepnend_elseif

Statement:if(expression)

{ statement1 … statementk}else{ statement1 … statementn}if-elseStatementExample:if(num1<num2) min=num1;else min=num2;printf(“Smallest:%d\n”,min);num215num110min?10<15?_10Smallest:10_if-elseStatementExample:if(num1<num2) min=num1;else min=num2;printf(“Smallest:%d\n”,min);num215num120min?20<15?_15Smallest:15_if-elseStatementif(num1<num2){ min=num1; max=num2;}else{ min=num2; max=num1;}printf(“Min=%d,Max=%d\n”,min,max);num2125num1700min??700<125?_max??Min=125,Max=700_125700条件运算符计算最小值if(num1<num2) min=num1;else min=num2;printf(“Smallest:%d\n”,min);min=num1<num2

?

num1

:num2;printf(“Smallest:%d\n”,min);表达式1?表达式2:表达式3习题4.24.2选择题（1）在下面的条件语句中，只有一个在功能上与其他三个语句不等价，这个不等价的语句是

A）if(a)s1;elses2;B）if(!a)s2;elses1;C）if(a!=0)s1;elses2;D）if(a==0)s1;elses2;Let’srecap…SingleSelectionStatement stepa

if

(expression){ stepm stepn } stepbSingleSelectionLet’srecap…PseudocodeStructure stepa if<conditionistrue>start stepm stepn end_if stepbSingleSelectionDoubleSelectionStatement

stepa

if(expression){ stepm stepn }

else

{ stepx stepy } stepbDoubleSelectionGuess…howdoesmultipleselectionlooklike?多分支选择结构（MultipleSelection）Multi-wayifStepaif

(expression1){ Stepm}if

(expression2)

{ Stepn}StepzStepaexpression1StepmStepnStepztruefalseexpression2truefalseStepaexpression1StepmStepnStepztruefalseexpression2truefalse多分支选择结构（MultipleSelection）CascadedifStepaif

(expression1){ Stepm}elseif(expression2){ Stepn}

else

{ Stepx}StepzStepaexpression1StepmStepnStepztruefalseexpression2truefalseStepxStepaexpression1StepmStepnStepztruefalseexpression2truefalseStepxCascadedifStepaif

(表达式1){Step1}elseif(表达式2){Step2}elseif(表达式3){Step3}elseif(…){Step…}

else

{Stepn}Stepzexpr1step1非0=0expr2expr3StepnStep3Step2非0非0=0=0StepaStepz…例4.5：体型判断按“体指数”对肥胖程度进行划分：

体指数t=w/h2

（体重w单位为公斤，身高h单位为米）当t<18时，为低体重；当18≤t<25时，为正常体重；当25<t<27时，为超重体重；当t≥27时，为肥胖。编程从键盘输入你的身高h和体重w，判断你的体重属于何种类型。例4.5#include<stdio.h>main(){

floath,w,t; printf("Pleaseenterh,w:"); scanf("%f,%f",&h,&w); t=w/(h*h);

if(t<18) { printf("t=%f\tLowerweight!\n",t); }

if(t>=18&&t<25) { printf("t=%f\tStandardweight!\n",t); }

if(t>=25&&t<27) { printf("t=%f\tHigherweight!\n",t); }

if(t>=27) { printf("t=%f\tToofat!\n",t); }}当t<18时，为低体重；当18≤t<25时，为正常体重；当25<t<27时，为超重体重；当t≥27时，为肥胖。#include<stdio.h>main(){

floath,w,t; printf("Pleaseenterh,w:"); scanf("%f,%f",&h,&w); t=w/(h*h);

if(t<18)

printf("t=%f\tLowerweight!\n",t);

elseif(t<25) printf("t=%f\tStandardweight!\n",t);

elseif(t<27)

printf("t=%f\tHigherweight!\n",t);

else

printf("t=%f\tToofat!\n",t);}182527例4.5当t<18时，为低体重；当18≤t<25时，为正常体重；当25<t<27时，为超重体重；当t≥27时，为肥胖。if语句的嵌套if

(p1)

if(p2)A

elseBelse

if(p4)C

elseDif

(p1)Aelse

if(p2)C

elseDif

(p1)

if(p2)A

elseBelse

C

if

(p1)

if(p2)Aelseif(p4)C

elseDif

(p1)

if(p2)AelseC

else与哪个if配套？{}{}多路选择(switch)

and

break

Thestructureissimilartomultipleselection(flowchart)switch(expression){

case

value1:

statement1; break;

case

value2:

statement2; break;

…

default:

statementX; break;}Don’tforgetthebraces!!Don’tforgetthecolons!!Don’tforgettheblank!!

ImportantRule!!switch(expression){

case

value1:

statement1; break;

case

value2:

statement2; break;

…

default:

statementX; break;}Mustbeintorchar!多路选择(switch)

and

break

Example:switch(month){ case1:

printf(“January\n”); break; case2:

printf(“February\n”); break; case3:

printf(“March\n”); break; default:

printf(“Others\n”); break; }

printf(“End”);Assumemonth=1,so……thisstepwillbeexecuted.Later……caseisterminatedhere.Jumpto…January_JanuaryEnd_多路选择(switch)

and

break

Example:switch(month){ case1:

printf(“January\n”); break; case2:

printf(“February\n”); break; case3:

printf(“March\n”); break; default:

printf(“Others\n”); break; }

printf(“End”);…thisstepwillbeexecuted.Later…March_MarchEnd_Assumemonth=3,so……caseisterminatedhere.Jumpto…多路选择(switch)

and

break

Example:switch(month){ case1:

printf(“January\n”); break; case2:

printf(“February\n”); break; case3:

printf(“March\n”); break; default:

printf(“Others\n”); break;}

printf(“End”);Now…whatwillhappenifthisbreakistakenoutfromtheprogram?多路选择(switch)

and

break

Example:switch(month){ case1:

printf(“January\n”); break; case2:

printf(“February\n”);

case3:

printf(“March\n”); break; default:

printf(“Others\n”); break; }

printf(“End”);Nomore!!多路选择(switch)

and

break

Example:switch(month){ case1:

printf(“January\n”); break; case2:

printf(“February\n”);

case3:

printf(“March\n”); break; default:

printf(“Others\n”); break; }

printf(“End”);…thisstepwillbeexecuted.Later…February_March_Assumemonth=2,so……caseisterminatedhere.Jumpto…End_…executioncontinues.Thus,thisstepisexecuted.So…多路选择(switch)

and

break

Example:switch(month){ case1:

printf(“January\n”); break; case2:

printf(“February\n”);

case3:

printf(“March\n”); break; default:

printf(“Others\n”); break;}

printf(“End”);Now…whatwillhappenifthesebreaksaretakenoutfromtheprogram?And…ifmonth=1?And…ifmonth=34?多路选择(switch)

and

break

最好不省略4.2（2）设有声明语句inta=1,b=0;，则执行以下语句后输出结果为

。

switch(a){case1:switch(b){case0:printf("**0**");break;case1:printf("**1**");break;}case2:printf("**2**");break;}A）**0** B）**0****2** C）**0****1****2** D）有语法错误例4.8：计算器程序编程设计一个简单的计算器程序，要求根据用户从键盘输入如下形式的表达式：操作数1运算符op操作数2

然后，计算并输出表达式的值指定的运算符为 加（+） 减（-） 乘（*） 除（/）

main(){

intdata1,data2;/*定义两个操作符*/

charop;/*定义运算符*/ printf("Pleaseentertheexpression:"); scanf("%d%c%d",&data1,&op,&data2);/*输入运算表达式*/

switch(op) {

case'+':/*处理加法*/ printf("%d+%d=%d\n",data1,data2,data1+data2);

break;

case'-':/*处理减法*/ printf("%d-%d=%d\n",data1,data2,data1-data2);

break;

case'*':