C程序设计语言(第二版)课后答案

“TheCProgrammingLanguage”,2ndedition,

KernighanandRitchie

《c程序设计语言》英文的配套答案，所列页码均为英文版的。

1.011.021.031.041.051.061.071,081.091.101.111.12

1.131.141.151.161.171.181.191.201.211.221.231.24

2.012.022.032.042.052.062.072.082.092.103.013.02

3.033.043.053.064.014.024.034.044.054.064.074.08

4.124.134.145.015.025.035.045.055.065.075.085.09

5.105.115.135.146.016.036.046.057.017.027.037.06

7.087.098.018.038.048.06

AnswertoExercise1-1

Runthe"hello,world"programonyoursystem.Experimentwithleaving

outpartsoftheprogram,toseewhaterrormessagesyouget.

Murphy'sLawdictatesthatthereisnosinglecorrectanswertothevery

firstexerciseinthebook.Ohwell.Here，sa〃helloworld"program:

^include<stdio.h>

intmain(void)

{

printf(，zhello,world\n，/);

return0;

Asyoucansee,Vveaddedareturnstatement,becausemainalwaysreturns

int,andit'sgoodstyletoshowthisexplicitly.

AnswertoExercise1-2

Experimenttofindoutwhathappenswhenprintf'sargumentstringcontains

\c,wherecissomecharacternotlistedabove.

By'above',thequestionisreferringto:

\n(newline)

\t(tab)

\b(backspace)

\〃(doublequote)

\\(backslash)Wehavetotreadcarefullyhere,becauseusinga

non-specifiedescapesequenceinvokesundefinedbehaviour.Thefollowing

programattemptstodemonstrateallthelegalescapesequences,not

includingtheonesalreadyshown(except\n,whichIactuallyneedin

theprogram),andnotincludinghexadecimalandoctalescapesequences.

^include<stdio.h>

intmain(void)

{

printf(z/Audibleorvisualalert.\a\n〃)；

printf(z，Formfeed.\f\n〃)；

printfC'Thisescape,\r,movestheactivepositiontotheinitial

positionofthecurrentline.\n〃)；

printf(""Verticaltab\vistricky,asitsbehaviourisunspecifiedunder

certainconditions.\n〃)；

return0;

AnswertoExercise1-3

Modifythetemperatureconversionprogramtoprintaheadingabovethe

table.

^include<stdio.h>

intmain(void)

floatfahr,Celsius;

intlower,upper,step;

lower=0;

upper=300;

step=20;

printfC?FC\n\n〃)；

fahr=lower;

while(fahr<=upper)

(

Celsius=(5.0/9.0)*(fahr-32.0);

printf(z，%3.Of%6.fahr,Celsius);

fahr=fahr+step;

)

return0;

AnswertoExercise1-4

WriteaprogramtoprintthecorrespondingCelsiustoFahrenheittable.

^include<stdio.h>

intmain(void)

(

floatfahr,Celsius;

intlower,upper,step;

lower=0;

upper=300;

step=20;

printf(ZZCF\n\n〃)；

Celsius=lower;

while(celsius<=upper)

(

fahr=(9.0/5.0)*celsius+32.0;

printf(，z%3.Of%6.lf\nz，,celsius,fahr);

celsius=celsius+step;

return0;

AnswertoExercise1-5

Modifythetemperatureconversionprogramtoprintthetableinreverse

order,thatis,from300degreesto0,

Thisversionusesawhileloop:

^include<stdio.h>

intmain(void)

(

floatfahr,Celsius;

intlower,upper,step;

lower=0;

upper=300;

step=20;

printf(Z，CF\n\n〃)；

Celsius=upper;

while(celsius>=lower)

(

fahr=(9.0/5.0)*Celsius+32.0;

printf(，z%3.Of%6.lf\n〃，celsius,fahr);

celsius=celsius-step;

)

return0;

Thisversionusesaforloop:

#include<stdio.h>

intmain(void)

(

floatfahr,celsius;

intlower,upper,step;

lower=0;

upper=300;

step=20;

printfCCF\n\n/Z);

for(celsius=upper;Celsius>=lower;Celsius=Celsius-step)

(

fahr=(9.0/5.0)*celsius+32.0;

printf(，?%3.Of%6.lf\n〃，celsius,fahr);

)

return0;

ChrisSidinotesthatSection1.3HasashortForstatementexample,and

“Basedonthatexample,Ithinkthesolutionto1.5:

a)shoulddofahrtocelsiusconversion(whereasthesolutionsonyour

pagedocelsiustofahr)

b)shouldbesimilartotheexampleandassmall.z，Heoffersthissolution:

^include<stdio.h>

/*printFahrenheit-Celsiustable*/

int

mainO

(

intfahr;

for(fahr=300;fahr>=0;fahr=fahr-20)

printf(z，%3d%6.lf\n〃,fahr,(5.0/9.0)*(fahr-32));

return0;

}

AnswertoExercise1-6

VerifythattheexpressiongetcharO!=EOFis0or1.

/*Thisprogrampromptsforinput,andthencapturesacharacter

*fromthekeyboard.IfEOFissignalled(typicallythrougha

*control-Dorcontrol-Zcharacter,thoughnotnecessarily),

*theprogramprints0.Otherwise,itprints1.

*

*Ifyourinputstreamisbuffered(anditprobablyis),then

*youwillneedtopresstheENTERkeybeforetheprogramwill

*respond.

*/

^include<stdio.h>

intmain(void)

(

printf(z，Pressakey.ENTERwouldbenice：-)\n\n〃)；

printf(Z/Theexpressiongetchar0!=EOFevaluatesto%d\n〃,

getchar()!=EOF);

return0;

AnswertoExercise1-7

WriteaprogramtoprintthevalueofEOF.

^include<stdio.h>

intmain(void)

(

printf(z，ThevalueofEOFis%d\n\n〃，EOF);

return0;

)

Exercise1-8

Writeaprogramtocountblanks,tabs,andnewlines.

^include<stdio.h>

intmain(void)

(

intblanks,tabs,newlines;

intc;

intdone=0;

intlastchar=0;

blanks=0;

tabs=0;

newlines=0;

while(done==0)

(

c=getchar();

if(c==，，)

++blanks;

if(c='\tD

++tabs;

if(c=='\n,)

++newlines;

if(c==EOF)

(

if(lastchar!='\n')

(

++newlines;/*thisisabitofasemanticstretch,butitcopes

*withimplementationswhereatextfilemightnot

*endwithanewline.ThankstoJimStadforpointing

*thisout.

*/

)

done=1;

)

lastchar=c;

)

printf(z，Blanks:%d\nTabs:%d\nLines:%d\n〃，blanks,tabs,newlines);

return0;

Exercise1-9

Writeaprogramtocopyitsinputtoitsoutput,replacingeachstring

ofoneormoreblanksbyasingleblank.

^include<stdio.h>

intmain(void)

(

intc;

intinspace;

inspace=0;

while((c=getchar())!=EOF)

(

if(c=，')

(

if(inspace==0)

{

inspace=1;

putchar(c);

)

)

/*Wehaven，tmet'else'yet,sowehavetobealittleclumsy*/

if(c!='')

{

inspace=0;

putchar(c);

)

)

return0;

ChrisSidiwrites:"insteadofhavingan"inspace"boolean,youcankeep

trackofthepreviouscharacterandseeifboththecurrentcharacterand

previouscharacterarespaces:“

^include<stdio.h>

/*countlinesininput*/

int

main()

intc,pc;/*c二character,pc=previouscharacter*/

/*setpctoavaluethatwouldn，tmatchanycharacter,incase

thisprogramisevermodifiedtogetridofmultiplesofother

characters*/

pc=EOF;

while((c=getchar())!=EOF){

if(c='')

if(pc!='')Aorif(pc!=c)*/

putchar(c);

/*Wehaven，tmet'else，yet,sowehavetobealittle

clumsy*/

if(c!='')

putchar(c);

pc=c;

return0;

Stigwrites:〃Iamhidingbehindthefactthatbreakismentionedinthe

introduction”!

#include<stdio.h>

intmain(void)

(

intc;

while((c=getchar())!=EOF){

if(c=…){

putchar(c);

while((c=getchar())=二''&&c!=EOF)

)

if(c==EOF)

break;/*thebreakkeywordismentioned

*intheintroduction...

**/

putchar(c);

)

return0;

Exercise1-10

Writeaprogramtocopyitsinputtoitsoutput,replacingeachtabby

\t,eachbackspaceby\b,andeachbackslashby\\.Thismakestabsand

backspacesvisibleinanunambiguousway.

Category0

GregoryPietschpointedoutthatmysolutionwasactuallyCategory1.He

wasquiteright.Betterstill,hewaskindenoughtosubmitaCategory

0solutionhimself.Hereitis:

/*GregoryPietsch<gkpl@>*/

/*

*Here，smyattemptataCategory0versionofl-10.

*

*GregoryPietsch

*/

^include<stdio.h>

intmain()

(

intc,d;

while((c=getchar())!=EOF){

d=0;

if(c=='\\D{

putchar(J\\');

putchar('\\');

d=1;

)

if(c=='\t'){

putchar(?\\');

putchart');

d=1;

)

if(c=='\b，){

putchar\\');

putchar(,b');

d=1;

)

if(d==0)

putchar(c);

)

return0;

Category1

Thissolution,whichIwrotemyself,isthesadlydiscreditedCat0answer

whichhasfoundanewleaseoflifeinCategory1.

ttinclude<stdio.h>

^defineESC_CHAR'\\'

intmain(void)

{

intc;

while((c=getchar())!=EOF)

switch(c)

case'\b':

/*TheOSonwhichItestedthis(NT)intercepts\bcharacters.

*/

putchar(ESC_CHAR);

putchar(*b');

break;

case'\t':

putchar(ESCCHAR);

putchar('t');

break;

caseESCCHAR:

putchar(ESC_CHAR);

putchar(ESCCHAR);

break;

default:

putchar(c);

break;

)

)

return0;

Exercise1-11

Howwouldyoutestthewordcountprogram?Whatkindsofinputaremost

likelytouncoverbugsifthereareany?

Itsoundsliketheyarereallytryingtogettheprogrammerstolearnhow

todoaunittest.Iwouldsubmitthefollowing:

0.inputfilecontainszerowords

1.inputfilecontains1enormouswordwithoutanynewlines

2.inputfilecontainsallwhitespacewithoutnewlines

3.inputfilecontains66000newlines

4.inputfilecontainsword/{hugesequenceofwhitespaceofdifferent

kinds}/word

5.inputfilecontains66000singleletterwords,66totheline

6.inputfilecontains66000wordswithoutanynewlines

7.inputfileis/usr/dictcontents(orequivalent)

8.inputfileisfullcollectionofmobywords

9.inputfileisbinary(e.g.itsownexecutable)

10.inputfileis/dev/nul(orequivalent)

66000ischosentocheckforintegraloverflowonsmallintegermachines.

Dannsuggestsafollowupexercisel-lla:writeaprogramtogenerate

inputs(0,1,2,3,4,5,6)

IguessitwasinevitablethatI'dreceiveasolutionforthisfollowup

exercise!HereisGregoryPietsch，sprogramtogenerateDann，ssuggested

inputs:

^include<assert.h>

#include<stdio.h>

intmain(void)

FILE*f;

unsignedlongi;

staticchar*ws="\f\t\v〃;

staticchar*al=^abcdefghijklmnopqrstuvwxyz/z

staticchar*i5=，，abcdefghijklm，/

〃nopqrstuvwxyz〃

，zabcdefghijklmz，

z，nopqrstuvwxyz/z

，，abcdefghijklm,，

〃、〃

n\n;

/*Generatethefollowing:*/

/*0.inputfilecontainszerowords*/

f=fopen(z，test0/z,〃w〃)；

assert(f!=NULL);

fclose(f);

/*1.inputfilecontains1enormouswordwithoutanynewlines*/

f=fopen("test1〃，〃w〃)；

assert(f!=NULL);

for(i=0;i<((66OOO11I/26)+1);i++)

fputs(al,f);

fclose(f);

/*2.inputfilecontainsallwhitespacewithoutnewlines*/

f=fopen(〃test2〃，〃w〃)；

assert(f!=NULL);

for(i=0;i<((66OOO11I/4)+1);i++)

fputs(ws,f);

fclose(f);

/*3.inputfilecontains66000newlines*/

f=fopen(z，test3z，,〃w〃)；

assert(f!=NULL);

for(i=0;i<66000;i++)

fputc('\n',f);

fclose(f);

/*4.inputfilecontainsword/

*{hugesequenceofwhitespaceofdifferentkinds}

*/word

*/

f=fopen(，ztest4，z,〃w〃)；

assert(f!=NULL);

fputs("word”,f);

for(i=0;i<((66OOO11I/26)+1);i++)

fputs(ws,f);

fputs("word",f);

fclose(f);

/*5.inputfilecontains66000singleletterwords,

*66totheline

*/

f=fopen(，，test5，/,〃w〃)；

assert(f!=NULL);

for(i=0;i<1000;i++)

fputs(15,f);

fclose(f);

/*6.inputfilecontains66000wordswithoutanynewlines*/

f=fopen(〃test6〃，〃w〃)；

assert(f!=NULL);

for(i=0;i<66000;i++)

fputs("word〃，f);

fclose(f);

return0;

}

Exercise1-12

Writeaprogramthatprintsitsinputonewordperline.

^include<stdio.h>

intmain(void)

(

intc;

intinspace;

inspace=0;

while((c=getchar())!=EOF)

{

if(c==''||c=='\t'||c=='\n')

(

if(inspace==0)

(

inspace=1;

putchar('\n);

)

/*else,don，tprintanything*/

)

else

(

inspace=0;

putchar(c);

}

)

return0;

}

Exercise1-13

Writeaprogramtoprintahistogramofthelengthsofwordsinitsinput.

Itiseasytodrawthehistogramwiththebarshorizontal;avertical

orientationismorechallenging.

/*Thisprogramwasthesubjectofathreadincomp.lang.c,becauseof

thewayithandledEOF.

*Thecomplaintwasthat,intheeventofatextfile'slastlinenot

endingwithanewline,

*thisprogramwouldnotcountthelastword.Iobjectedsomewhattothis

complaint,onthe

*groundsthat〃ifithasn，tgotanewlineattheendofeachline,it

isn，tatextfile”.

*

*Thesegroundsturnedouttobeincorrect.Whethersuchafileisatext

fileturnsoutto

*beimplementation-defined.I'dhadagoatcheckingmyfacts,andhad

-asitturnsout-

*checkedthewrongfacts!(sigh)

*

*Itcostmeanextravariable.Itturnedoutthattheleastdisturbing

waytomodifythe

*program(Ialwayslookfortheleastdisturbingway)wastoreplace

thetraditional

*while((c=getchar())!=EOF)withanEOFtestactuallyinsidetheloop

body.Thismeant

*addinganextravariable,butisundoubtedlyworththecost,because

itmeanstheprogram

*cannowhandleotherpeople5stextfilesaswellasmyown.AsBenPfaff

saidatthe

*time,〃Beliberalinwhatyouaccept,strictinwhatyouproduce”.Sound

advice.

*

*Thenewversionhas,ofcourse,beentested,anddoesnowaccepttext

filesnotendingin

*newlines.

*

*Ihave,ofcourse,regeneratedthesampleoutputfromthisprogram.

Actually,there，sno

*〃ofcourse〃aboutit-Inearlyforgot.

*/

^include<stdio.h>

ttdefineMAXWORDLEN10

intmain(void)

(

intc;

intinspace=0;

longlengtharr[MAXWORDLEN+1];

intwordlen=0;

intfirstletter=1;

longthisval=0;

longmaxval=0;

intthisidx=0;

intdone=0;

for(thisidx=0;thisidx<=MAXWORDLEN;thisidx++)

(

lengtharr[thisidx]=0;

)

while(done==0)

(

c=getchar();

if(c==||c=='\t'||c='\n'||c==EOF)

(

if(inspace==0)

(

firstletter=0;

inspace=1;

if(wordlen<=MAXWORDLEN)

(

if(wordlen>0)

(

thisval=++1engtharr[word1en-1];

if(thisval>maxval)

(

maxval=thisval;

)

)

)

else

(

thisval=++lengtharr[MAXWORDLEN];

if(thisval>maxval)

(

maxval=thisval;

)

if(c==EOF)

(

done=1;

else

(

if(inspace==1|firstletter==1)

(

wordlen=0;

firstletter=0;

inspace=0;

)

++wordlen;

)

for(thisval=maxval;thisval>0;thisval一)

{

printf(z/%4d|thisval);

for(thisidx=0;thisidx<=MAXWORDLEN;thisidx++)

|

if(lengtharr[thisidx]>=thisval)

(

printfC*");

)

else

(

printf("");

)

)

printf("\n");

)

printf(z/+”);

for(thisidx=0;thisidx<=MAXWORDLEN;thisidx++)

(

printf("—；

)

printf('\n");

for(thisidx=0;thisidx<MAXWORDLEN;thisidx++)

{

printf(z，%2d”,thisidx+1);

)

printfO%d\nz/,MAXWORDLEN);

return0;

Here,stheoutputoftheprogramwhengivenitsownsourceasinput:

113|*

112|*

111I*

110|*

109I*

108|*

107I*

106|*

105I*

104|*

103I*

102|*

101|*

100I*

99|*

98I*

97|*

96I*

95|*

94|**

93|**

92|**

91|**

90|**

89|**

88|**

87|**

86|**

85|**

84|**

83|**

82|**

81|**

80|**

79|**

78|**

77|**

76|**

75|**

74|**

73|**

72|**

71|**

70|**

69|**

68|**

67|**

66|**

65|**

64|**

63|***

62|***

61|***

60|***

59|***

58|***

57|***

56|***

55|***

54|***

53|***

52|****

51|****

50|****

49|****

48|****

47|****

46|****

45|****

44|****

43|*****

42|*****

41|*****

40|*****

39|*****

38|*****

37|*****

361*****

351*****

341*****

331*****

321*****

311*****

301******

291******

281*******

271*******

261*******

251********

241********

231********

221*********

211*********

201*********

191*********

181*********

171*********

161*********

151*********

141**********

131**********

121**********

111**********

101**********

91***********

81***********

71***********

61***********

51***********

41***********

31***********

21***********

11***********

12345678910>10

Exercise1-14

Writeaprogramtoprintahistogramofthefrequenciesofdifferent

charactersinitsinput.

Naturally,I'vegoneforaverticalorientationtomatchexercise13.I

hadsomedifficultyensuringthattheprintingoftheX-axisdidn，t

involvecheating.Iwantedtodisplayeachcharacterifpossible,butthat

wouldhavemeantusingisprint(),whichwehaven，tyetmet.SoIdecided

todisplaythevalueofthecharacterinstead.(Theresultsbelowshow

theoutputonanASCIIsystem-naturally,arunonanEBCDICmachinewould

givedifferentnumbers.)Ihadtojumpthroughafewhoopstoavoidusing

the%operatorwhich,again,wehaven，tyetmetatthispointinthetext.

^include<stdio.h>

/*NUM_CHARSshouldreallybeCHARMAXbutK&Rhaven，tcoveredthatat

thisstageinthebook*/

ttdefineNUM_CHARS256

intmain(void)

{

intc;

longfreqarr[NUMCHARS+1];

longthisval=0;

longmaxval=0;

intthisidx=0;

for(thisidx=0;thisidx<=NUM_CHARS;thisidx++)

(

freqarr[thisidx]=0;

}

while((c=getchar())!=EOF)

(

if(c<NUM_CHARS)

{

thisval=++freqarr[c];

if(thisval>maxval)

maxval=thisval;

else

{

thisval=++freqarr[NUM_CHARS];

if(thisval>maxval)

(

maxval=thisval;

)

)

)

for(thisval=maxval;thisval>0;thisval一)

(

printf(z，%4d”,thisval);

for(thisidx=0;thisidx<=NUMCHARS;thisidx++)

(

if(freqarr[thisidx]>=thisval)

(

printf('*");

)

elseif(freqarr[thisidx]>0)

{

printf(/z");

)

)

printf("\n");

)

printf("+");

for(thisidx=0;thisidx<=NUMCHARS;thisidx++)

(

if(freqarr[thisidx]>0)

(

printf;

}

)

printf(/z\n");

for(thisidx=0;thisidx<NUMCHARS;thisidx++)

(

if(freqarr[thisidx]>0)

{

printf(级d”,thisidx/100);

}

)

printf('\n");

for(thisidx=0;thisidx<NUMCHARS;thisidx++)

(

if(freqarr[thisidx]>0)

(

printf(thisidx-(100*(thisidx/100)))/10);

)

)

printf("\n");

for(thisidx=0;thisidx<NUMCHARS;thisidx++)

(

if(freqarr[thisidx]>0)

(

printfthisidx-(10*(thisidx/10)));

)

)

if(freqarr[NUMCHARS]>0)

{

printf("〉％d\n”,NUM_CHARS);

)

printf("\n");

return0;

Here'stheoutputoftheprogramwhengivenitsownsourceasinput:

474|*

473|*

472I*

471|*

470I*

469|*

468I*

467|*

466I*

465|*

464I*

463|*

462I*

461I*

460I*

459I*

458I*

457I*

456I*

455I*

454I*

453I*

452I*

451I*

450I*

449I*

448I*

447I*

446I*

445I*

444I*

443I*

442I*

441I*

440I*

439I*

438I*

437I*

436I*

435I*

434I*

433I*

432I*

431I*

430I*

429I*

428I*

427I*

426I*

425I*

424I*

423I*

422I*

421I*

420I*

419I*

418I*

417I*

416I*

415I*

414I*

413I*

412I*

411I*

410I*

409I*

408I*

407I*

406I*

405I*

404I*

403I*

402I*

401I*

400I*

399I*

398I*

397I*

396I*

395I*

394I*

393I*

392I*

391I*

390I*

389I*

388I*

387I*

386I*

385I*

384I*

383I*

382I*

381I*

380I*

379I*

378I*

377I*

376I*

375I*

374I*

373I*

372I*

371I*

370I*

369I*

368I*

367I*

366I*

365I*

364I*

363I*

362I*

361I*

360I*

359I*

358I*

357I*

356I*

355I*

354I*

353I*

352I*

351I*

350I*

349I*

348I*

347I*

346I*

345I*

344I*

343I*

342I*

341I*

340I*

339I*

338I*

337I*

336I*

335I*

334I*

333I*

332I*

331I*

330I*

329I*

328I*

327I*

326I*

325I*

324I*

323I*

322I*

321I*

320I*

319I*

318I*

317I*

316I*

315I*

314I*

313I*

312I*

311I*

310I*

309I*

308I*

307I*

306I*

305I*

304I*

303I*

302I*

301I*

300I*

299I*

298I*

297I*

296I*

295I*

294I*

293I*

292I*

291I*

290I*

289I*

288I*

287I*

286I*

285I*

284I*

283I*

282I*

281I*

280I*

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