cs202ch2.docx
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cs202ch2
COURSE:
CLanguage（CS102）
UNIT:
2.1
CONTROLSTRUCTURE
SEQUENCE
*Instructionsareexecutedoneafteranotherwithoutbranching.
*E.g.#include
main（）
{
inta,b;
printf（"Enterthefirstinteger"）;
scanf（"%d",&a）;
printf（"Enterthesecondinteger\n"）;
scanf（"%d",&b）;
printf（"Sumis%d\n",a+b）;
return0;
}
SELECTIONSTRUCTURE
Aselectionstructureisusedtochooseamongdifferentactions.
<1>SingleSelection（IFSelectionStructure）
*Itperformsanactiononlywhenaconditionistrue.
*Format:
if（condition）
statement
Problem:
printthewordPassedifthegradeisgreaterthan60
Coding:
if（grade>60）
printf（"Passed\n"）;（printonlyifgrade>60）
Warning:
if（grade>60）;->if（grade>60）;
printf（“Passed\n”）;printf（“Passed\n”）;
（printwithanygrade）
<2>DoubleSelection（IF/ELSESelectionStructure）
*Itperformsanactionwhenaconditionistrue,
performadifferentactionwhentheconditionisfalse
*Format:
if（condition）
statement-1
else
statement-2
Problem:
printthewordPassedifthegradeisgreaterthan60
printthewordFailedifthegradeisbeloworequal60
Coding:
if（grade>60）
printf（"Passed\n"）;
else
printf（"Failed\n"）;
<3>MultipleSelection
NestedIF/ELSESelectionStructure
*Format:
if（condition-1）
statement-1
elseif（condition-2）
statement-2
.........
*Problem
（1）:
printAforthemarksgreaterthanorequalto80.
printBforthemarkgreaterthanorequalto70.
printCforthemarkgreaterthanorequalto60.
AndFforothermarks
*Coding:
if（grade>=80）//80-100
printf（"A\n"）;
elseif（grade>=70）//70-79
printf（"B\n"）;
elseif（grade>=60）//60-69
printf（"C\n"）;
else//59orbelow
printf（"F\n"）;
*ifselectionstructureexpectsonlyonestatementinitsbody,toinclude
severalstatementsinthebody,enclosethesetofstatementsusing{}
*E.g.if（grade<60）
{
printf（"Failed\n"）;
printf（"Youmustworkhard\n"）;
}
elseif……..
*Problem
（2）:
Convertanintegerintothecorrespondingdayofaweek.
*Coding:
if（day==1）
printf（"TodayisMonday\n"）;
elseif（day==2）
printf（"TodayisTuesday\n"）;
elseif（day==3）
printf（"TodayisWednesday\n"）;
elseif（day==4）
printf（"TodayisThursday\n"）;
elseif（day==5）
printf（"TodayisFriday\n"）;
elseif（day==6）
printf（"TodayisSaturday\n"）;
elseprintf（“TodayisSunday\n”）
Switchstructure（multipleselection）
Problem:
Convertanintegerintothecorrespondingdayofaweek.
Coding:
main（）
{
intday;
printf（"Enteranintegerbetween1to7.Enter-1toquit:
"）;
scanf（"%d",&day）;
switch（day）
{
case1:
printf（"TodayisMonday\n\n"）;
break;
case2:
printf（"TodayisTuesday\n\n"）;
break;
case3:
printf（"TodayisWednesday\n\n"）;
break;
case4:
printf（"TodayisThursday\n\n"）;
break;
case5:
printf（"TodayisFriday\n\n"）;
break;
case6:
printf（"TodayisSaturday\n\n"）;
break;
case7:
printf（"TodayisSunday\n\n"）;
break;
default:
printf（"IncorrectNumberEntered!
"）;
break;
}
return0;
}
Enteranintegerbetween1to7.Enter-1toquit:
1
TodayisMonday
Enteranintegerbetween1to7.Enter-1toquit:
6
TodayisSaturday
Enteranintegerbetween1to7.Enter-1toquit:
9
IncorrectNumberEntered!
!
!
Enteranintegerbetween1to7.Enter-1toquit:
-1
Break
*Theswitchstructureisexitedimmediatelywiththebreakstatement.
*Ifbreakisnotused,evenamatchisfound,thestatementsforallthe
remainingcaseswillstillbeexecuted.
*breakcan/cannotbeusedindefaultstatement（laststatement）
Default
*Ifnomatchoccurs,thedefaultcaseisexecuted
*placethedefaultclauseattheendoftheswitchstructure.
Case
*Eachcasecanhaveoneormoreactionandbraces{}arenotrequired
aroundmultipleactioninacaseofaswitch
Switch
*switchstructurecanonlybeusedfortestingaconstant
integralexpression,thatis,anycombinationofcharacterconstants
andintegerconstantsthatevaluatestoaconstantintegervalue.
switch（day）
{
E.g.<1>case1:
..
case2:
..
E.g.<2>case'a':
..
case'b':
..
E.g.<3>case"abc":
..WRONGUSE!
!
!
!
!
case"123":
..
}
REPETITION（looping）STRUCTURE
*Anactionwillberepeatedwhilesomeconditionremainstrue.
*Theactionwillstoponlyiftheconditionbecomefalse.
<1>WHILERepetitionStructure
*Format:
while（condition）
statement
*Problem
（1）:
sumalltheintegerfrom1to10
*Coding:
main（）
{
intsum,number;
sum=0;
number=1;
while（number<=10）
{
sum=sum+number;
number++;
}
printf（“Sumis%d”,sum）;
}
*Therewillbeaninfiniteloopiftheconditionalwaystrue.
*Mustbeanactionthateventuallycausetheconditiontobecomefalse
*Probem
（2）:
Determinetheclassaverageofatesttakenby5student.
*Coding:
main（）
{
intstudent_counter,total_mark,mark;
doubleaverage;
total_mark=0;
student_counter=1;
while（student_counter<=5）
{
printf（"Pleaseenterthemark:
"）;
scanf（"%d",&mark）;
total_mark=total_mark+mark;
student_counter=student_counter+1;
}
average=total_mark/5;
printf（"Theclassaverageis%lf\n",average）;
return0;
}
*OUTPUT:
Pleaseenterthemark:
50
Pleaseenterthemark:
10
Pleaseenterthemark:
90
Pleaseenterthemark:
93
Pleaseenterthemark:
50
Theclassaverageis58.000000
*Actualaverageshouldbe（50+50+10+90+93）/558.6
Why%fprints58.000000insteadof58.6?
?
?
?
NOTE:
average=total_mark/10;
（float）（int）（int）
Integerdivisionyieldsanintegerresult
Significantdigitsarecut.
*Problem
（2）RevisedProgram
*Coding:
main（）
{intstudent_counter,mark;
doubleaverage,total_mark;
total_mark=0;
student_counter=0;
printf（"Enterthemark,orenter-1toend:
"）;
scanf（"%d",&mark）;
while（mark!
=-1）
{
total_mark=total_mark+mark;
student_counter=student_counter+1;
printf（"Plsenterthenextmark,orenter-1toend:
"）;
scanf（"%d",&mark）;
}
if（student_counter!
=0）
{
average=total_mark/student_counter;
printf（"Theclassaverageis%.3lf\n",average）;
}
else
printf（"Nomarkswereentered\n"）;
return0;
}
*OUTPUT<1>
Enterthemark,orenter-1toend:
50
Pleaseenterthenextmark,orenter-1toend:
10
Pleaseenterthenextmark,orenter-1toend:
90
Pleaseenterthenextmark,orenter-1toend:
93
Pleaseenterthenextmark,orenter-1toend:
50
Pleaseenterthenextmark,orenter-1toend:
-1
Theclassaverageis58.600
*OUTPUT<3>
Enterthemark,orenter-1toend:
-1
Nomarkswereentered
*Formatstring"%.3f"
.3->afloating-pointwith3decimaldigitstotherightofdecimalpoint.
eg.printf（"%.2f",9.348）;9.35
eg.printf（"%.1f",9.348）;9.3
eg.printf（"%.0f",9.348）;9
Thedefaultprecisionis（%.6f==%f）
*Precisionareonlyusedinprintf,notscanf.
eg.scanf（“%.5lf\n”,&a）wrong!
!
!
scanf（“%lf”,&a）
*Donotcomparefloating-pointvalueforequality.
<2>DO/WHILEStructure
Ittesttheloop-continuationconditionaftertheloopbodyisperformed,soloop
bodywillbeexecutedatleastonce.
*Format:
do{
statements;
}
while（condition）;
*Problem:
Findtheno.oftimesthestatementshasbeenexecutedintheloop.
*CodingUsingDo/WhileLoop:
main（）
{intloop_count,number;
loop_count=0;
number=0;
do{
loop_count++;
number--;
}
while（number>0）;
printf（"Loop_countis%d\n",loop_count）;
printf（"Numberis%d\n",number）;
return0;
}
Numberis-1
*CodingUsingWhileLoop:
main（）
{intloop_count,number;
loop_count=0;
number=0;
while（number>0）
{
loop_count++;
number--;
}
printf（"Loop_countis%d\n",loop_count）;
printf（"Numberis%d\n",number）;
return0;
}
Numberis0
<3>FORStructure
*Problem:
Print1to10
1
*Coding
（1）:
main（）2
{intcounter;3
4
for（counter=1;counter<=10;counter++）5
{6
printf（"%d",counter）;:
printf（"\n"）;:
}
return0;
}
*Coding
（2）:
main（）
{intcounter=1;
for（;counter<11;counter++）
printf（"%d",counter）;
printf（“\n”）;
return0;
}
*
*for（counter=1;counter<=10;counter++）
Initialization,repetitioncondition,incrementareallputintheheader.
-Initializationoccursonlyonce,
-Incrementoccursafterthebodystatementisperformed.
124
for（counter=1;counter<=10;counter++）
{
printf（"%d",counter）;
3printf（"\n"）;
}
1234
*Thecondition（counter<=10）or（counter<11）isOK
But（counter<10）willrunonly9timeandcausedoff-by-oneerror.
*ForLoopWhileLoop
main（）main（）
{{
intcount;intcount;
for（count=1;count<=10;count++）count=1;
printf（"%d",count）;while（count<=10）
printf（“\n”）;{
return0;printf（"%d",count）;
}count++;
}
printf（“\n”）;
return0;
}
*Initialization,condition,and
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