3275: COP Chapter 06
JonathanC.L.Liu,Ph.D.
CISEDepartment
UniversityofFlorida,USA
Iteration Statements
Csiterationstatementsareusedtosetup
loops.
Aloopisastatementwhosejobisto
repeatedlyexecutesomeotherstatement
(theloopbody).
InC,everyloophasacontrolling
expression.
Eachtimetheloopbodyisexecuted(an
iterationoftheloop),thecontrolling
expressionisevaluated.
Iftheexpressionistrue(hasavaluethatsnot
zero)theloopcontinuestoexecute.
2
Iteration Statements
Cprovidesthreeiterationstatements:
Thewhilestatementisusedforloops
whosecontrollingexpressionistested
beforetheloopbodyisexecuted.
Thedostatementisusediftheexpression
istestedaftertheloopbodyisexecuted.
Theforstatementisconvenientforloops
thatincrementordecrementacounting
variable.
3
The while Statement
Usingawhilestatementistheeasiest
waytosetupaloop.
Thewhilestatementhastheform
while ( expression ) statement
expressionisthecontrollingexpression;
statementistheloopbody.
4
The while Statement
Exampleofawhilestatement:
while (i < n)
i = i * 2;
/* controlling expression */
/* loop body */
Whenawhilestatementisexecuted,the
controllingexpressionisevaluatedfirst.
Ifitsvalueisnonzero(true),theloopbodyis
executedandtheexpressionistestedagain.
Theprocesscontinuesuntilthecontrolling
expressioneventuallyhasthevaluezero.
5
The while Statement
Awhilestatementthatcomputesthesmallest
powerof2thatisgreaterthanorequaltoanumber
n:
i = 1;
while (i < n)
i = i * 2;
Atraceoftheloopwhennhasthevalue10:
i = 1;
Isi<n?
i=i*
Isi<n?
i=i*
Isi<n?
i=i*
Isi<n?
i=i*
Isi<n?
2;
2;
2;
2;
iisnow1.
Yes;continue.
iisnow2.
Yes;continue.
iisnow4.
Yes;continue.
iisnow8.
Yes;continue.
iisnow16.
No;exitfromloop.
6
The while Statement
Althoughtheloopbodymustbeasingle
statement,thatsmerelyatechnicality.
Ifmultiplestatementsareneeded,usebracesto
createasinglecompoundstatement:
while (i > 0) {
printf("T minus %d and counting\n", i);
i--;
}
Someprogrammersalwaysusebraces,even
whentheyrenotstrictlynecessary:
while (i < n) {
i = i * 2;
}
7
The while Statement
Thefollowingstatementsdisplaya
seriesofcountdownmessages:
i = 10;
while (i > 0) {
printf("T minus %d and
counting\n", i);
i--;
}
ThefinalmessageprintedisTminus1
andcounting.
8
The while Statement
Observationsaboutthewhilestatement:
Thecontrollingexpressionisfalsewhenawhile
loopterminates.Thus,whenaloopcontrolledbyi
>0terminates,imustbelessthanorequalto0.
Thebodyofawhileloopmaynotbeexecutedat
all,becausethecontrollingexpressionistested
beforethebodyisexecuted.
Awhilestatementcanoftenbewrittenina
varietyofways.Amoreconciseversionofthe
countdownloop:
while (i > 0)
printf("T minus %d and counting\n", i--);
9
Infinite Loops
Awhilestatementwontterminateifthe
controllingexpressionalwayshasanonzero
value.
Cprogrammerssometimesdeliberatelycreate
aninfiniteloopbyusinganonzeroconstantas
thecontrollingexpression:
while (1)
Awhilestatementofthisformwillexecute
foreverunlessitsbodycontainsastatementthat
transferscontroloutoftheloop(break,goto,
return)orcallsafunctionthatcausesthe
programtoterminate.
10
Printing a Table of Squares
Thesquare.cprogramusesawhile
statementtoprintatableofsquares.
Theuserspecifiesthenumberofentriesin
thetable:
This program prints a table of squares.
Enter number of entries in table: 5
1
1
2
4
3
9
4
16
5
25
11
square.c
/* Prints a table of squares using a while statement
*/
#include <stdio.h>
int main(void)
{
int i, n;
printf("This program prints a table of
squares.\n");
printf("Enter number of entries in table: ");
scanf("%d", &n);
i = 1;
while (i <= n) {
printf("%10d%10d\n", i, i * i);
i++;
}
return 0;
}
12
Summing a Series of Numbers
Thesum.cprogramsumsaseriesof
integersenteredbytheuser:
This program sums a series of
integers.
Enter integers (0 to terminate): 8 23
71 5 0
The sum is: 107
Theprogramwillneedaloopthatuses
scanftoreadanumberandthenadds
thenumbertoarunningtotal.
13
sum.c
/* Sums a series of numbers */
#include <stdio.h>
int main(void)
{
int n, sum = 0;
printf("This program sums a series of
integers.\n");
printf("Enter integers (0 to terminate): ");
scanf("%d", &n);
while (n != 0) {
sum += n;
scanf("%d", &n);
}
printf("The sum is: %d\n", sum);
return 0;
}
14
The do Statement
Generalformofthedostatement:
do statement while ( expression ) ;
Whenadostatementisexecuted,the
loopbodyisexecutedfirst,thenthe
controllingexpressionisevaluated.
Ifthevalueoftheexpressionis
nonzero,theloopbodyisexecuted
againandthentheexpressionis
evaluatedoncemore.
15
The do Statement
Thecountdownexamplerewrittenasado
statement:
i = 10;
do {
printf("T minus %d and counting\n", i);
--i;
} while (i > 0);
Thedostatementisoftenindistinguishable
fromthewhilestatement.
Theonlydifferenceisthatthebodyofa do
statementisalwaysexecutedatleastonce.
16
The do Statement
Itsagoodideatousebracesinalldo
statements,whetherornottheyre
needed,becauseadostatement
withoutbracescaneasilybemistaken
forawhilestatement:
do
printf("T minus %d and counting\n", i--);
while (i > 0);
Acarelessreadermightthinkthatthe
wordwhilewasthebeginningofa
whilestatement.
17
Calculating the Number of Digits
Thenumdigits.cprogramcalculatesthe
numberofdigitsinanintegerenteredbythe
user:
Enter a nonnegative integer: 60
The number has 2 digit(s).
Theprogramwilldividetheusersinputby10
repeatedlyuntilitbecomes0;thenumberof
divisionsperformedisthenumberofdigits.
Writingthisloopasadostatementisbetter
thanusingawhilestatement,because
everyintegereven0hasatleastonedigit.
18
numdigits.c
/* Calculates the number of digits in an integer */
#include <stdio.h>
int main(void)
{
int digits = 0, n;
printf("Enter a nonnegative integer: ");
scanf("%d", &n);
do {
n /= 10;
digits++;
} while (n > 0);
printf("The number has %d digit(s).\n", digits);
return 0;
}
19
The for Statement
Theforstatementisidealforloopsthat
haveacountingvariable,butitsversatile
enoughtobeusedforotherkindsofloopsas
well.
Generalformoftheforstatement:
for ( expr1 ; expr2 ; expr3 ) statement
expr1,expr2,andexpr3areexpressions.
Example:
for (i = 10; i > 0; i--)
printf("T minus %d and counting\n", i);
20
The for Statement
Theforstatementiscloselyrelatedtothe
whilestatement.
Exceptinafewrarecases,aforloopcan
alwaysbereplacedbyanequivalentwhileloop:
expr1;
while ( expr2 ) {
statement
expr3;
}
expr1isaninitializationstepthatsperformed
onlyonce,beforetheloopbeginstoexecute.
21
The for Statement
expr2controlslooptermination(theloop
continuesexecutingaslongasthevalue
ofexpr2isnonzero).
expr3isanoperationtobeperformedat
theendofeachloopiteration.
Theresultwhenthispatternisappliedto
thepreviousforloop:
i = 10;
while (i > 0) {
printf("T minus %d and counting\n", i);
i--;
}
22
The for Statement
Studyingtheequivalentwhilestatementcan
helpclarifythefinepointsofaforstatement.
Forexample,whatifi--isreplacedby--i?
for (i = 10; i > 0; --i)
printf("T minus %d and counting\n", i);
Theequivalentwhileloopshowsthatthe
changehasnoeffectonthebehaviorofthe
loop:
i = 10;
while (i > 0) {
printf("T minus %d and counting\n", i);
--i;
}
23
The for Statement
Sincethefirstandthirdexpressionsina
forstatementareexecutedas
statements,theirvaluesareirrelevant
theyreusefulonlyfortheirsideeffects.
Consequently,thesetwoexpressions
areusuallyassignmentsor
increment/decrementexpressions.
24
for Statement Idioms
Theforstatementisusuallythebestchoicefor
loopsthatcountup(incrementavariable)or
countdown(decrementavariable).
Aforstatementthatcountsupordownatotal
ofntimeswillusuallyhaveoneofthefollowing
forms:
Countingupfrom0ton1: for (i = 0; i < n; i++)
Countingupfrom1ton:for (i = 1; i <= n; i++)
Countingdownfromn1to0:for (i = n - 1; i >= 0; i--)
Countingdownfromnto1:for (i = n; i > 0; i--)
25
for Statement Idioms
Commonforstatementerrors:
Using<insteadof>(orviceversa)inthe
controllingexpression.Countinguploops
shouldusethe<or<=operator.Counting
downloopsshoulduse>or>=.
Using==inthecontrollingexpression
insteadof<,<=,>,or>=.
Offbyoneerrorssuchaswritingthe
controllingexpressionasi<=ninsteadof
i<n.
26
Omitting Expressions in a for Statement
Callowsanyoralloftheexpressionsthatcontrol
aforstatementtobeomitted.
Ifthefirstexpressionisomitted,noinitializationis
performedbeforetheloopisexecuted:
i = 10;
for (; i > 0; --i)
printf("T minus %d and counting\n", i);
Ifthethirdexpressionisomitted,theloopbodyis
responsibleforensuringthatthevalueofthe
secondexpressioneventuallybecomesfalse:
for (i = 10; i > 0;)
printf("T minus %d and counting\n", i--);
27
Omitting Expressions in a for
Whenthefirstandthirdexpressionsareboth
omitted,theresultingloopisnothingmore
thanawhilestatementindisguise:
for (; i > 0;)
printf("T minus %d and counting\n", i--);
isthesameas
while (i > 0)
printf("T minus %d and counting\n", i--);
Thewhileversionisclearerandtherefore
preferable.
28
Omitting Expressions in a for
Ifthesecondexpressionismissing,it
defaultstoatruevalue,sothefor
statementdoesntterminate(unless
stoppedinsomeotherfashion).
Forexample,someprogrammersuse
thefollowingforstatementtoestablish
aninfiniteloop:
for (;;)
29
for Statements in C99
InC99,thefirstexpressioninafor
statementcanbereplacedbya
declaration.
Thisfeatureallowstheprogrammerto
declareavariableforusebytheloop:
for (int i = 0; i < n; i++)
Thevariableineednothavebeen
declaredpriortothisstatement.
30
for Statements in C99
Avariabledeclaredbyaforstatement
cantbeaccessedoutsidethebodyof
theloop(wesaythatitsnotvisible
outsidetheloop):
for (int i = 0; i < n; i++) {
printf("%d", i);
/* legal; i is visible inside loop */
}
printf("%d", i);
***/
31
/*** WRONG
for Statements in C99
Havingaforstatementdeclareitsown
controlvariableisusuallyagoodidea:its
convenientanditcanmakeprogramseasier
tounderstand.
However,iftheprogramneedstoaccessthe
variableafterlooptermination,itsnecessary
tousetheolderformoftheforstatement.
Aforstatementmaydeclaremorethanone
variable,providedthatallvariableshavethe
sametype:
for i (int = 0, j = 0; i < n; i++)
32
The Comma Operator
Onoccasion,aforstatementmayneedto
havetwo(ormore)initializationexpressions
oronethatincrementsseveralvariableseach
timethroughtheloop.
Thiseffectcanbeaccomplishedbyusinga
commaexpressionasthefirstorthird
expressionintheforstatement.
Acommaexpressionhastheform
expr1 , expr2
whereexpr1andexpr2areanytwo
expressions.
33
The Comma Operator
Acommaexpressionisevaluatedintwosteps:
First,expr1isevaluatedanditsvaluediscarded.
Second,expr2isevaluated;itsvalueisthevalueofthe
entireexpression.
Evaluatingexpr1shouldalwayshaveaside
effect;ifitdoesnt,thenexpr1servesnopurpose.
Whenthecommaexpression++i,i+jis
evaluated,iisfirstincremented,theni+jis
evaluated.
Ifiandjhavethevalues1and5,respectively,the
valueoftheexpressionwillbe7,andiwillbe
incrementedto2.
34
The Comma Operator
Thecommaoperatorisleftassociative,
sothecompilerinterprets
i = 1, j = 2, k = i + j
as
((i = 1), (j = 2)), (k = (i + j))
Sincetheleftoperandinacomma
expressionisevaluatedbeforetheright
operand,theassignmentsi=1,j=2,
andk=i+jwillbeperformedfromleft
toright.
35
The Comma Operator
Thecommaoperatormakesitpossibletoglue
twoexpressionstogethertoformasingle
expression.
Certainmacrodefinitionscanbenefitfromthe
commaoperator.
Theforstatementistheonlyotherplacewhere
thecommaoperatorislikelytobefound.
Example:
for (sum = 0, i = 1; i <= N; i++)
sum += i;
Withadditionalcommas,theforstatement
couldinitializemorethantwovariables.
36
Program: Printing a Table
of Squares (Revisited)
Thesquare.cprogram(Section6.1)
canbeimprovedbyconvertingits
whilelooptoaforloop.
37
square2.c
/* Prints a table of squares using a for
statement */
#include <stdio.h>
int main(void)
{
int i, n;
printf("This program prints a table of
squares.\n");
printf("Enter number of entries in table: ");
scanf("%d", &n);
for (i = 1; i <= n; i++)
printf("%10d%10d\n", i, i * i);
return 0;
}
38
Printing a Table of Squares (Revisited)
Cplacesnorestrictionsonthethreeexpressions
thatcontrolthebehaviorofaforstatement.
Althoughtheseexpressionsusuallyinitialize,test,
andupdatethesamevariable,theresno
requirementthattheyberelatedinanyway.
Thesquare3.cprogramisequivalentto
square2.c,butcontainsaforstatementthat
initializesonevariable(square),testsanother
(i),andincrementsathird(odd).
Theflexibilityoftheforstatementcansometimes
beuseful,butinthiscasetheoriginalprogram
wasclearer.
39
square3.c
/* Prints a table of squares using an odd method */
#include <stdio.h>
int main(void)
{
int i, n, odd, square;
printf("This program prints a table of squares.\n");
printf("Enter number of entries in table: ");
scanf("%d", &n);
i = 1;
odd = 3;
for (square = 1; i <= n; odd += 2) {
printf("%10d%10d\n", i, square);
++i;
square += odd;
}
}
return 0;
40
Exiting from a Loop
Thenormalexitpointforaloopisatthe
beginning(asinawhileorfor
statement)orattheend(thedo
statement).
Usingthebreakstatement,its
possibletowritealoopwithanexit
pointinthemiddleoraloopwithmore
thanoneexitpoint.
41
The break Statement
Thebreakstatementcantransfer
controloutofaswitchstatement,butit
canalsobeusedtojumpoutofa
while,do,orforloop.
Aloopthatcheckswhetheranumbern
isprimecanuseabreakstatementto
terminatetheloopassoonasadivisor
isfound:
for (d = 2; d < n; d++)
if (n % d == 0)
break;
42
The break Statement
Aftertheloophasterminated,anif
statementcanbeusetodetermine
whetherterminationwaspremature
(hencenisntprime)ornormal(nis
prime):
if (d < n)
printf("%d is divisible by %d\n",
n, d);
else
printf("%d is prime\n", n);
43
The break Statement
Thebreakstatementisparticularlyusefulfor
writingloopsinwhichtheexitpointisinthemiddle
ofthebodyratherthanatthebeginningorend.
Loopsthatreaduserinput,terminatingwhena
particularvalueisentered,oftenfallintothis
category:
for (;;) {
printf("Enter a number (enter 0 to stop): ");
scanf("%d", &n);
if (n == 0)
break;
printf("%d cubed is %d\n", n, n * n * n);
}
44
The break Statement
Abreakstatementtransferscontroloutofthe
innermostenclosingwhile,do,for,orswitch.
Whenthesestatementsarenested,thebreak
statementcanescapeonlyonelevelofnesting.
Example:
while () {
switch () {
break;
}
}
breaktransferscontroloutoftheswitchstatement,
butnotoutofthewhileloop.
45
The continue Statement
Thecontinuestatementissimilartobreak:
breaktransferscontroljustpasttheendofa
loop.
continuetransferscontroltoapointjustbefore
theendoftheloopbody.
Withbreak,controlleavestheloop;with
continue,controlremainsinsidetheloop.
Theresanotherdifferencebetweenbreak
andcontinue:breakcanbeusedin
switchstatementsandloops(while,do,
andfor),whereascontinueislimitedto
loops.
46
The continue Statement
Aloopthatusesthecontinue
statement:
n = 0;
sum = 0;
while (n < 10) {
scanf("%d", &i);
if (i == 0)
continue;
sum += i;
n++;
/* continue jumps to here */
}
47
The continue Statement
Thesameloopwrittenwithoutusing
continue:
n = 0;
sum = 0;
while (n < 10) {
scanf("%d", &i);
if (i != 0) {
sum += i;
n++;
}
}
48
The goto Statement
Thegotostatementiscapableofjumpingtoany
statementinafunction,providedthatthestatement
hasalabel.
Alabelisjustanidentifierplacedatthebeginningof
astatement:
identifier : statement
Astatementmayhavemorethanonelabel.
Thegotostatementitselfhastheform
goto identifier ;
ExecutingthestatementgotoL;transferscontrolto
thestatementthatfollowsthelabelL,whichmustbe
inthesamefunctionasthegotostatementitself.
49
The goto Statement
IfCdidnthaveabreakstatement,a
gotostatementcouldbeusedtoexit
fromaloop:
for (d = 2; d < n; d++)
if (n % d == 0)
goto done;
done:
if (d < n)
printf("%d is divisible by %d\n",
n, d);
else
printf("%d is prime\n", n);
50
The goto Statement
Thegotostatementisrarelyneededin
everydayCprogramming.
Thebreak,continue,andreturn
statementswhichareessentially
restrictedgotostatementsandthe
exitfunctionaresufficienttohandle
mostsituationsthatmightrequirea
gotoinotherlanguages.
Nonetheless,thegotostatementcan
behelpfulonceinawhile.
51
The goto Statement
Considertheproblemofexitingaloopfromwithina
switchstatement.
Thebreakstatementdoesnthavethedesired
effect:itexitsfromtheswitch,butnotfromtheloop.
Agotostatementsolvestheproblem:
while () {
switch () {
goto loop_done;
}
}
loop_done:
/* break won't work here */
Thegotostatementisalsousefulforexitingfrom
nestedloops.
52
Balancing a Checkbook
Manysimpleinteractiveprogramspresentthe
userwithalistofcommandstochoosefrom.
Onceacommandisentered,theprogram
performsthedesiredaction,thenpromptsthe
userforanothercommand.
Thisprocesscontinuesuntiltheuserselects
anexitorquitcommand.
Theheartofsuchaprogramwillbealoop:
for (;;) {
promptusertoentercommand;
readcommand;
executecommand;
}
53
Balancing a Checkbook
Executingthecommandwillrequireaswitch
statement(orcascadedifstatement):
for (;;) {
promptusertoentercommand;
readcommand;
switch (command) {
case command1: performoperation1; break;
case command2: performoperation2; break;
.
.
.
case commandn: performoperationn; break;
default: printerrormessage; break;
}
}
54
Balancing a Checkbook
Thechecking.cprogram,which
maintainsacheckbookbalance,usesa
loopofthistype.
Theuserisallowedtocleartheaccount
balance,creditmoneytotheaccount,
debitmoneyfromtheaccount,display
thecurrentbalance,andexitthe
program.
55
Balancing a Checkbook
*** ACME checkbook-balancing program ***
Commands: 0=clear, 1=credit, 2=debit, 3=balance,
4=exit
Enter command: 1
Enter amount of credit: 1042.56
Enter command: 2
Enter amount of debit: 133.79
Enter command: 1
Enter amount of credit: 1754.32
Enter command: 2
Enter amount of debit: 1400
Enter command: 2
Enter amount of debit: 68
Enter command: 2
Enter amount of debit: 50
Enter command: 3
Current balance: $1145.09
Enter command: 4
56
checking.c
/* Balances a checkbook */
#include <stdio.h>
int main(void)
{
int cmd;
float balance = 0.0f, credit, debit;
printf("*** ACME checkbook-balancing program
***\n");
printf("Commands: 0=clear, 1=credit, 2=debit, ");
printf("3=balance, 4=exit\n\n");
for (;;) {
printf("Enter command: ");
scanf("%d", &cmd);
switch (cmd) {
case 0:
balance = 0.0f;
break;
57
}
case 1:
printf("Enter amount of credit: ");
scanf("%f", &credit);
balance += credit;
break;
case 2:
printf("Enter amount of debit: ");
scanf("%f", &debit);
balance -= debit;
break;
case 3:
printf("Current balance: $%.2f\n", balance);
break;
case 4:
return 0;
default:
printf("Commands: 0=clear, 1=credit,
2=debit, ");
printf("3=balance, 4=exit\n\n");
break;
}
}
58
The Null Statement
Astatementcanbenulldevoidof
symbolsexceptforthesemicolonatthe
end.
Thefollowinglinecontainsthree
statements:
i = 0; ; j = 1;
Thenullstatementisprimarilygoodfor
onething:writingloopswhosebodies
areempty.
59
The Null Statement
Considerthefollowingprimefindingloop:
for (d = 2; d < n; d++)
if (n % d == 0)
break;
Ifthen%d==0conditionismovedintothe
loopscontrollingexpression,thebodyofthe
loopbecomesempty:
for (d = 2; d < n && n % d != 0; d++)
/* empty loop body */ ;
Toavoidconfusion,Cprogrammers
customarilyputthenullstatementonalineby
itself.
60
The Null Statement
Accidentallyputtingasemicolonaftertheparenthesesin
anif,while,orforstatementcreatesanullstatement.
Example1:
if (d == 0);
/*** WRONG
***/
printf("Error: Division by zero\n");
Thecallofprintfisntinsidetheifstatement,soits
performedregardlessofwhetherdisequalto0.
Example2:
i = 10;
while (i > 0);
/*** WRONG
***/
{
printf("T minus %d and counting\n", i);
--i;
}
Theextrasemicoloncreatesaninfiniteloop.
61
The Null Statement
Example3:
i = 11;
while (--i > 0);
/*** WRONG ***/
printf("T minus %d and counting\n", i);
Theloopbodyisexecutedonlyonce;themessage
printedis:
T minus 0 and counting
Example4:
for (i = 10; i > 0; i--);
/*** WRONG ***/
printf("T minus %d and counting\n", i);
Again,theloopbodyisexecutedonlyonce,andthe
samemessageisprintedasinExample3.
62