cs 103 unit 7 slides - usc bitsbits.usc.edu/files/cs103/slides/unit7_fileio.pdf · cs 103 unit 7...

1

CS103Unit7Slides

FileI/O

MarkRedekopp

2

GettheFilesforToday• Gotoyourcs103/examplesdirectory

– $wget http://ee.usc.edu/~redekopp/cs103/redir_pipe.tar– $tarxvf redir_pipe.tar– $wget http://ee.usc.edu/~redekopp/cs103/cinfail.cpp– $wget http://ee.usc.edu/~redekopp/cs103/file_io_ex.tar– $tarxvf file_io_ex.tar

3

I/OStreams• I/Oisplacedintemporarybuffers/streamsbytheOS&C++libraries• cin pullsdatafromaninputstreamknownas'stdin'(standardinput)

– Itisusuallythestreamcomingfromthekeyboard

• cout putsdatainanoutputstreamknownas'stdout'(standardoutput)– Itisusuallydirectedtothemonitor

7 5 y ...

input stream [stdin] (user types all at once):

#include<iostream>using namespace std;int main(int argc, char *argv[]){int dummy, x; cin >> dummy >> x;

}

I t w a s t h e

output stream (stdout) in OS:

#include<iostream>

using namespace std;

int main(int argc, char *argv[]){cout << “It was the” << endl;cout << 4;

}

y ...stdin

4\n

It was the

6 1

stdout

4

I/OStreams• '>>'operatorusedtoreaddatafromaninputstream

– Alwaysstopsatwhitespace

• '<<'operatorusedtowritedatatoanoutputstream– 'endl'forcesaflush…FlushforcestheOStomovedatafromtheinternalOSstreamto

theactualoutputdevice(likethemonitor)

7 5 y ...

input stream (user types all at once):

#include<iostream>using namespace std;int main(int argc, char *argv[]){int dummy, x; cin >> dummy >> x;

}

I t w a s t h e

output stream in OS:

#include<iostream>

using namespace std;

int main(int argc, char *argv[]){cout << “It was the” << endl;cout << 4;

}

y ...input stream:4

output stream after flush:

4\n

It was the

6 1

5

FileI/OIntro

• Whatmethodsdoesauserhavetoprovideaprograminput:– cin– Commandline(argc,argv)

• Nowanewmethod:FileI/O(accessingdatainfiles)• Primarymethodforaprogramtoread/writefiles:

– Filestreams[Mainsubjectofourlecture]

• OS-basedtoolstoread/writefiledata– I/ORedirectionviatheOS(useof'<'and'>'atcommandline)– PipesviatheOS(useof|atcommandline)

6

Redirection&Pipes• TheOS(LinuxorWindowsorMac)providesthefollowing

abilitiesatthecommandline• '<'redirectcontentsofafileasinput(stdin)toprogram

– ./hailstats <input.txt– OSplacescontentsofinput.txtinto'stdin'inputstreamwhichbrokecan

accessvia'cin'

• '>'redirectprogramoutputtoafile– ./hailstats <input.txt>results.txt– OStakesoutputfrom'stdout'producedbycout andwritesthemintoa

newoutputfileontheharddrive:results.txt

• 'l'pipeoutputoffirstprogramtosecond– stdout offirstprogramisthenusedasstdin ofnextprogram

7

Redirection&PipeExamples• $./lab5_sol<input.txt

– Redirectscontentsofinput.txttostdin(i.e.cin)inlab5program

• Getthedemofiles:– Gotoyourcs103/examplesdirectory– $wget http://ee.usc.edu/~redekopp/cs103/redir_pipe.tar– $tarxvf redir_pipe.tar– $makerandgen– $makeaverage

• Runthemwithoutusingredirectionandpipes– $./randgen 2010

• Notice20valuesbetween1-10areoutputonstdout/cout

– $./average• NowtypeinalistofnumbersfollowedbytypingCtrl-D

0 10 10 100 500 200 220 20 301 80 180 25 251 180 50 30 602

input.txt

8

Redirection&PipeExamples

• OutputRedirection:>– $./randgen 2010>out.txt– Nowinspectout.txtcontents– Whatwouldhavedisplayedonthescreenisnowinout.txt

• Inputredirection:<– $./average<out.txt– Theoutputcapturedfromrandgen isnowusedasinputtoaverage

• Pipes:|– $./randgen 2010|./average– Theoutputofrandgen isfedasinputtoaverage

9

MOREABOUTSTREAMSOthercapabilitiesyoucanuseforstreams

10

Input&OutputStreams• Thereareothertypesofinputandoutputstreamsotherthancin andcout

• Filestreamsgivesthesamecapabilitiesofcin andcout exceptdataisread/writtenfrom/toafileontheharddrive– Everythingyoudowithcin usingthe'>>'operatoryoucannowusetoaccessdatafromafileratherthanthekeyboard

– Everythingyoudowithcout usingthe'<<'operatoryoucannowusetooutputdatatoafile

• Let'slearnmoreaboutstreams'>>'…we'llseeitinthecontextofcin andcout butrealizeitwillapplytootherstreamswe'lllearnaboutnext

11

Getline()andLinesofText• cin stopsreadingatwhitespace– Ifyouwanttoreadawholelineoftextusecin.getline()

• ItwillreadspacesandtabsbutSTOPat'\n'

– cin.getline(char*buffer,int max_chars)

• Readsmax_chars-1leavingspaceforthenullcharacter

#include <iostream> using namespace std;

int main () { char mytext[80];cout << "Enter your full name" << endl;cin.getline(mytext, 80);

int last=0;for(int i=0; i<80; i++){if(mytext[i] == ' '){

last = i;break;

} }cout << "Last name starts at index: ";cout << last << endl;return 0;}

12

SampleCode

• Getthesamplecode– $wget http://ee.usc.edu/~redekopp/cs103/cinfail.cpp

13

InputStreamErrorChecking• Wecancheckerrorswhencin

receivesunexpecteddatathatcan’tbeconvertedtothegiventype

• Usethefunctioncin.fail()whichreturnstrueifanythingwentwrongopeningorreadingdatainfromthefile(willcontinuetoreturntruefromthenonuntilyouperformcin.clear())

• Trythiscodeyourselfandseewhathappenswithandwithoutthecheckusingfail()


int main ()

{

int x;cout << "Enter an int: " << endl;cin >> x; // What if the user enters:

// “abc”

// Check if we successfully read an intif( cin.fail() ) {cout << "Error: I said enter an int!";cout << " Now I must exit!" << endl;return 1;

}

cout << "You did it! You entered an int";cout << " with value: " << x;

return 0;}

14

UnderstandingInputStreams

int x=0;

cout << “Enter X: “;

cin >> x;

int y = 0;

cout << “Enter Y: “;

cin >> y;

X =

cin =

X = cin =

X =

cin =

cin.fail() is false

Y = cin =

Y = cin =

Y = cin =

cin.fail() is false

● User enters value “512” at 1st prompt, enters “123” at 2nd prompt

0

0

512

5 1 2 \n

\n

0

0

123

\n 1 2 3 \n

\n

\n

15


int x=0;


cin >> x;

int y = 0;


cin >> y;

X =

cin =

X = cin =

X =

cin =

cin.fail() is false

Y = cin =

Y = cin =

Y = cin =

● User enters value “23abc” at 1st prompt, 2nd prompt fails

0

0

23

2 3 a b

0

0

xx

c \n

a b c \n

a b c \n

a b c \n

a b c \n

cin.fail() is true

16


int x=0;


cin >> x;

int y = 0;


cin >> y;

● User enters value “23 99” at 1st prompt, 2nd prompt skipped

X =

cin =

X = cin =

X =

cin =

cin.fail() is false

Y = cin =

Y = cin =

Y = cin =cin.fail() is false

0

0

23

2 3 9

0

0

99

9 \n

9 9 \n

9 9 \n

9 9 \n

\n

17


char x[80];


cin.getline(x, 80);

● User enters value “23 99” at 1st prompt, everything read as string

X =

cin =

X = cin =

X =

cin =

cin.fail() is false

NOTE: \n character is discarded!

23 99

2 3 9 9 \n

18

MoreonErrorChecking• Usethefail()functiontodetect

errorswhenattemptingtoreaddata

• Ifacalltofail()returnstruethensubsequentcallstofail()willcontinuetoreturntrueuntilyoucallclear()

• Useignore()tocleanoutanytextstillintheinputstream

• Trythiscodeyourselfandseewhathappenswithandwithoutthecheckusingfail()


int main ()

{

int x;cout << "Enter an int: " << endl;cin >> x; // What if the user enters:

// “abc”

// Check if we successfully read an intwhile( cin.fail() ) {cin.clear(); // turn off fail flagcin.ignore(256, '\n'); // clear inputscout << "I said enter an int: ";cin >> x;

}

cout << "You did it! You entered an int";cout << " with value: " << x;

return 1;}

19

FILESTREAMSHowyourprogramcandirectlyaccessdatainfiles

20

ComputerOrganization

• ProcessorcanonlytalkdirectlytoRAM– Itneeds“translation”toaccessdataontheharddriveorotherdisk

• AllcodeanddataresidesinRAM– Allvariablesaccessibleinyourprogram

• Howdoweaccessfiles– TheOSprovidesroutinestoperformthetranslation

Memory

…

…

…

Code

Stack (area for

data local to a function)

Globals

0

…

Heap

fffffffc

Data files:.ppt.txt

.docx

110010101001…

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StartingFileI/O

• JustlikeMicrosoftWordoranyotherapplicationthatusesfilesyouhavetwooptions…– Readinfofromthefile(like'Open'command)

• Usean'ifstream' objecttoopenthefile• Readdatafromthefile• Closeitwhenyou'redone

– Writeinfotothefile(like'SaveAs'command)• Usean'ofstream' object• Writethedatatoafile• Closeitwhenyou'redone

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ImportantFact

• Foryourprogramtooperateondatainafile…• …youmustreaditintoaCvariable• Everythingwewillseesubsequentlyissimplyhowtogetdataintoavariable– Afterthatwecanjustprocessitnormally– Ifwewanttoproduceanoutputfilewewilljustwritingthevariablevaluestothefileusingsomemoretechniques

– C/C++providesfunctionsthatdothereading/writingforyou

23

TwoKindsofFiles:BinaryandText

• Weconceiveoffilesas“streams”(lineararrays)ofdata• Filesarebrokenintotwotypesbasedonhowtheyrepresent

thegiveninformation:– Textfiles:FileisjustalargesequenceofASCIIcharacters(everypiece

ofdataisjustabyte)– Binaryfiles:Datainthefileisjustbitsthatcanberetrievedin

differentsizechunks(4-byteint,8-bytedouble,etc.)

• Example:Storethenumber172 inafile:– Text:Wouldstore3ASCIIchar’s‘1’,’7’,’2’(ASCII0x31,0x37,0x32)

requiring3bytes– Binary:If172wasina‘char’var.,wecouldstorea1-bytevalue

representing172inunsignedbinary(0xAC)orif172wasinan‘int’var.wecouldstore4-byteswithvalue0x000000AC

InthisclasswewillonlyfocusonTextfiles

24

TEXTFILEI/O

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Activity

• Getthetestfiles– $wget http://ee.usc.edu/~redekopp/cs103/file_io_ex.tar– $tarxvf file_io_ex.tar

• sum_from_file_exercise

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TextFileI/O• Useifstream object/variable

forreadingafile– Candoanything'cin'cando

• Useofstream object/variableforwritingafile– Candoanything'cout'cando

• Mustinclude<fstream>• Use'<<'and'>>'operatorson

thestreambutrealizeoperationsarehappeningondataformthefile

#include <iostream> #include <fstream> using namespace std;

int main ()

{

int x; double y;

ifstream ifile (“input.txt“);

if( ifile.fail() ){ // able to open file?cout << "Couldn't open file" << endl;return 1;

}

ifile >> x >> y; if ( ifile.fail() ){cout << "Didn't enter an int and double";return 1;

}

ofstream ofile(“output.txt“);

ofile << “Int from file is “ << x << endl;ofile << “Double from file is “ << y << endl;

ifile.close(); ofile.close();

return 0;}5 -3.5

input.txtInt from file is 5Double from file is -3.5

output.txt

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GettingLinesofText• Usingthe>>operatortogetaninput

stringoftext(char*orchar[]variablepassedtocin)implicitlystopsatthefirstwhitespace

• Howcanwegetawholelineoftext(includingspaces)– cin.getline(char*buf,int bufsize);– ifile.getline(char*buf,int bufsize);– Readsmaxofbufsize-1characters

(includingnewline)• Thisprogramreadsallthelinesoftext

fromafile

#include <iostream> #include <fstream> using namespace std;

int main ()

{

char myline[100]; int i = 1;

ifstream ifile (“input.txt“);

if( ifile.fail() ){ // can't open?return 1;

}

ifile.getline(myline, 100);while ( ! ifile.fail()) {cout << i++ << “: “ << myline << endl;ifile.getline(myline, 100);}

ifile.close(); return 0;

}

The fox jumped over the log.\n

The bear ate some honey.\n

The CS student solved a hard problem.\n

1: The fox jumped over the log.

2: The bear ate some honey.

3: The CS student solved a hard problem.

input.txt

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Activity

• reverse_it exercise• search_and_count exercise

29

FILELOCATION/POINTERS&INPUTOPERATORS

30

FileAccess• Yourifstream object(ifile)

implicitlykeepstrackofwhereyouareinthefile

• EOF (end-of-file)orothererrormeansnomoredatacanberead.Usethefail()functiontoensurethefileisokayforreading/writing

I t w a s t h e b e s t o f

...

fp

char c; ifile >> c;

a t f e r . T h e E n d ! EOF

fp


fp

...

...

...

char c;while( ! ifile.fail() ) {ifile >> c;if(! ifile.fail()){ // process c }

}

Hard Drive

Hard Drive

31

>>Operator• Recallthatwithcin the>>operatorstops

gettingavaluewhenitencounterswhitespaceandalsoskipswhitespacetogettothenextvalue

– Sodoifstream objects

• Intheexampleonthisslide,thespaceswillNOTbereadin

– Theywillbeskippedbythe>>operator

• Togetrawdatafromthefile(includingwhitespaces)usetheget()function

I t 6 7 8

fp

char s[40]; ifile >> s;// returns “It” and stops at space

...

ifstream ifile("data.txt");

File text

I t 6 7 8

fp

...

File text

char x; ifile >> x;// skips space & gives x='6'

I t 6 7 8

fp

...

File text

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>>Operator• Togetrawdatafromthefile(including

whitespaces)usetheifstream::get()function

– Returnsthecharacteratthe‘fp’andmoves‘fp’onbyone

• Toseewhatthenextcharacteriswithoutmovingthe‘fp’pointerontothenextcharacter,useifstream::peek()function

– Returnsthecharacteratthe‘fp’butdoesNOTmove‘fp’on

I 1 6 7 8

fp

char c = ifile.get(); // returns ‘I’C = ifile.get(); // returns ‘ ‘

...

ifstream ifile("data.txt");

File text

I 1 6 7 8 ...

char c; ifile >> c; // returns ‘I’Ifile >> c; // skips space and

// returns ‘1’

fp

File text

33

ChangingFilePointerLocation(ifstream)

• Ratherthanreadsequentiallyinafileweoftenneedtojumparoundtoparticularbytelocations

• ifstream.seekg()– Gotoaparticularbytelocation– Passanoffset relativefrom

currentpositionorabsolutebytefromstartorendoffile

• ifstream.tellg()– Returnthecurrentlocation’s

byte-offsetfromthebeginningofthefile

int main(int argc, char *argv[]){int size; char c;ifstream is(“stuff.txt”);

is.seekg(0,ios_base::end);size = is.tellg();cout << “File size (bytes)=“ << size << endl;

is.seekg(1, ios_base::beg);cout << “2nd byte in file is “;is >> c;cout << c << endl;is.seekg(-2, ios_base::cur);cout << “1st byte in file is “;is >> c;cout << c << endl;is.close();return 0;

}

2nd arg. to seekg()ios_base::beg = pos. from beginning of fileios_base::cur = pos. relative to current locationios_base::end = pos. relative from end of file

(i.e. 0 or negative number)

34

ChangingFilePointerLocation(ofstream)

• Wecanseekandtellinanofstream• ofstream.seekp()

– Gotoaparticularbytelocation– Passanoffsetrelativefromcurrentpositionorabsolutebytefromstartorend

offile• ofstream.tellp()

– Returnthecurrentlocation’sbyte-offsetfromthebeginningofthefile

2nd arg. to seekg()ios_base::beg = pos. from beginning of fileios_base::cur = pos. relative to current locationios_base::end = pos. relative from end of file

(i.e. 0 or negative number)

35

FILEI/OLABOVERVIEW

36

Lab7Overview

• Modifythewordscramblegamedoneinclasstoallowforawordbank(choiceofwordstouse)tobereadinfromafile

37

Lab7

• Openupthefileandcheckifitsucceeds

3 computertrojan hello

wordbank.txtIfstream object

38

Lab7

• Openupthefileandcheckifitsucceeds• Readthenumberofwordsexpected,checkifitsucceeds,andifso,allocatethewordBank arrayofpointers

3computertrojan hello

wordbank.txt

wordBank[0]wordBank[1]wordBank[2]

Ifstream object

39

Lab7

• Inaloopreadineachwordintoabufferandthenallocatesomememorytoholdthatwordandcopyittothatmemory


wordbank.txt

wordBank[0] 0x100wordBank[1]wordBank[2]

Ifstream object

computer

buf[41]

computer0x100

40

Lab7



wordbank.txt

wordBank[0] 0x100wordBank[1] 0x240wordBank[2]

Ifstream object

trojan

buf[41]

computer0x100

trojan0x240

41

Lab7



wordbank.txt

hello

wordBank[0] 0x100wordBank[1] 0x240wordBank[2] 0x320

buf[41]

computer

trojan

hello

0x100

0x240

0x320

Ifstream object

42

BINARYFILEI/OYouarenotresponsibleforthismaterial

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BinaryFileI/O• read()– memberofifstream

– Passapointertowhereyouwantthedatareadfromthefiletobeplacedinmemory(e.g.&x ifxisasingleint ordata ifdataisanarray)…thispointershouldbecasttoachar*

– Pass#ofbytesyouwanttoread=number_of_elements *size_of_element

• write()– memberofofstream– Sameargument

schemeasread()

int main(int argc, char *argv[]){int x;double data[10];ifstream ifile;

ifile.open(“stuff.dat”,ios::binary);if ( ifile.fail()){cerr << “File doesn’t exist\n”;return 1;

}ifile.read(static_cast<char *>(&x), 1*sizeof(int));ifile.read(static_cast<char *>(data), 10*sizeof(double));

ifile.close();return 0;

}

44

stdin,stdout,stderr• MostOS’es mapconsoleI/O

(keyboardandmonitorI/O)to3predefinedFILEpointers:– stdin (inputfromkeyboard)=cin– stdout (outputtomonitor)=cout

• Normaloutput– stderr (outputtomonitor)=cerr

• Exception/errorinformation– Unix/Linuxcanallowyoutoredirect

stdout vs.stderr separately• >./prog >log_of_stdout.txt• >./prog >&log_of_stderr.txt

int main(int argc, char *argv[]){char first_char;char first_line[80];

// read char from keyboardcin << first_char;

// read entire line of text from// keyboardcin.getline(first_line, 80);

// echo line back to stdoutcout << first_line;

// output to stderrcerr << “I had an error.” << endl;

return 0;}

45

UNUSEDFALL2013

46

CSTYLEI/OYouarenotresponsibleforthismaterial

47

FILE*variables• Toaccessfiles,C(withthehelpoftheOS)

hasadatatypecalled‘FILE’whichtracksallinformationandisusedtoaccessasinglefilefromyourprogram

• YoudeclareapointertothisFILEtype(FILE*)

• You“open”afileforaccessusingfopen()– Passitafilenamestring(char*)andastring

indicatingreadvs.write,textvs.binary– ReturnsaninitializedfilepointerorNULLifthere

wasanerroropeningfile

• You“close”afilewhenfinishedwithfclose()– Passthefilepointer

• Bothofthesefunctionsaredefinedinstdio.h

int main(int argc, char *argv[]){char first_char;char first_line[80];FILE *fp;

fp = fopen(“stuff.txt”,”r”);if (fp == NULL){printf(“File doesn’t exist\n”);exit(1)

}// read first char. of filefirst_char = fgetc(fp);// read thru first ‘\n’ of filefgets(first_line, 80 ,fp);

fclose(fp);return 0;

}

Second arg. to fopen()“r” / “rb” = read mode, text/bin file“w” / “wb” = write mode, text/bin file“a” / “ab” = append to end of text/bin file“r+” / “r+b” = read/write text/bin fileothers…

48

FileAccess• ManyfileI/Ofunctions

– Textfileaccess:• fprintf(),fscanf()• fputc(),fgetc(),fputs(),fgets()

– Binaryfileaccess:• fread(),fwrite()

• Yourfilepointer(FILE*var)implicitlykeepstrackofwhereyouareinthefile

• EOF constantisreturnedwhenyouhittheendofthefileoryoucanusefeof()whichwillreturntrueorfalse.


...

fp

c = fgetc(fp)

a t f e r . T h e E n d ! EOF

fp


fp

...

...

...

while( ! feof(fp) )// okay to access next// byte of file

if((c = fgetc(fp) != EOF)// process c

49

TextFileInput• fgetc()

– GetasingleASCIIcharacter

• fgets()– Getalineoftextoracertainnumberofcharacters(uptoand

including‘\n’)– StopsatEOF…IfEOFisfirstcharreadthenthefunctionreturnsNULL– WillappendtheNULLcharattheendofthecharactersread

• fscanf()– ReadASCIIchar’sandconverttoanothervariabletype– Returnsnumberofsuccessfulitemsreador‘EOF’ifthatisthefirst

characterread

50

TextFileOutput• fputc()

– WriteasingleASCIIcharactertothefile

• fputs()– Writeatextstringtothefile

• fprintf()– Writetheresultingtextstringtothefile

51

BinaryFileI/O• fread()

– Passapointertowhereyouwantthedatareadfromthefiletobeplacedinmemory(e.g.&x ifxisanint ordata ifdataisanarray)

– Passthenumberof‘elements’toreadthenpassthesizeofeach‘element’

– #ofbytesread=number_of_elements*size_of_element

– Passthefilepointer

• fwrite– Sameargumentschemeasfread()

int main(int argc, char *argv[]){int x;double data[10];FILE *fp;


}fread(&x, 1, sizeof(int), fp);fread(data, 10, sizeof(double),fp);


}

52

ChangingFilePointerLocation• Ratherthanread/writing

sequentiallyinafileweoftenneedtojumparoundtoparticularbytelocations

• fseek()– Gotoaparticularbytelocation– Canbespecifiedrelativefrom

currentpositionorabsolutebytefromstartorendoffile

• ftell()– Returnthecurrentlocation’sbyte-

offsetfromthebeginningofthefile

int main(int argc, char *argv[]){int size;FILE *fp;


}fseek(fp,0,SEEK_END);size = ftell(fp);

printf(“File is %d bytes\n”, size);


}

Third arg. to fseek()SEEK_SET = pos. from beginning of fileSEEK_CUR = pos. relative to current locationSEEK_END = pos. relative from end of file

(i.e. negative number)

cs 103 unit 7 slides - usc bitsbits.usc.edu/files/cs103/slides/unit7_fileio.pdf · cs 103 unit 7...

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