Lect08 Filddde

7/23/2019 Lect08 Filddde

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File I/O

COMP1117 ComputerProgramming


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Content

File Output StreamsFile Input Streams

Character I/O

String Streams


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Streams

In C++, a streamis a sequence of data

input stream

sourcefrom hich dataare readone !" onecin# ta$e input from $e"!oard, e%g% cin >> x >> y

output stream

destinationto hich dataare rittenone !" one

cout# send output to the screen, e%g% cout


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File IO &ia streams

cin and coutOsmall/short interactions ith users

'large num!ers of inputs or outputs

'inputs and outputs that need permanent storage

e%g% "our homeor$

File streams can help ith these

(ead from or rite to )les

*eha&e li$e cin/cout


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File streams

)le stream is associated ith a )leinput, output

o create a file stream

-eclarea variableith the )le stream t"peConnect the variableto a )le

Input/output "our datafrom/to the stream

same as using cin/cout

-isconnect the variablefrom the)lein the end

filestream

user


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File output stream

o create a file output stream-eclarea variableith the t"peofstream

Connect the variableto a )le

Output "our datato the stream.same as using

cout)

-isconnect the variablefrom the)lein the end

ofstreamis de)ned in the li!rar"

file

user

ofstream


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#include #include using namespace std;int main(){ ofstream target; int n; target.open("myresult.txt"); cin >> n;

for(int i = 0; i < n; i){ for(int ! = 0; ! < n; !) target n; for(int i = 0; i < n; i){ for(int ! = 0; ! < n; !) target


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File output stream

If output )le does not e2ist in the currentdirector"

a ne )le ill !e created

3lse

the contents of the e2isting )le ill !e erased .!e

careful4

See output$file.ccas an e2ample


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File input stream

o create a file input stream-eclarea variableith the t"peifstream

Connect the variableto a )le

(ead datafrom the streamas using cin

-isconnect the variablefrom the)lein the end%

ifstreamis de)ned in the li!rar"

file user

ifstream


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File input stream

read a series of num!ers from a )le and calculate thesum

#include #include using namespace std;int main(){ ifstream myfile;

myfile.open("myfile.dat"); ...

#include #include using namespace std;int main(){

ifstream myfile;

myfile.open("myfile.dat"); ...

Include theli!rar"-eclare the )le

output stream&aria!le

Connect the&aria!le to a)le


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File input stream

errorill occur if open a)le thatdoes note2ist

myfile.fail()

return trueif the )le cannot !e opened, otherise

false

int main(){ ifstream my%ar; my%ar.open("myfile.dat");

if(my%ar.fail()){ cout


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File input stream

5o to readthe data from input )le6e can use a while loop, same as using cin

ifstream myinfile;int x; int total =0;myinfile.open("myfile.dat");*ile(+++){ myinfile >> x;

total = x; ...

ifstream myinfile;int x; int total =0;

myinfile.open("myfile.dat");*ile(+++){ myinfile >> x;

total = x; ...

hat is the condition6

5o to chec$ if it is the end of a

)le6


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myinfile >> x;myinfile >> x;

File input stream

implicit mar$er maintained !" the s"stemto indicate the starting positionof the ne2te2traction%

Once the )le is opened, the mar$erpoints to the

)rst !"te

8n1 9 : 1 ; ;< = 8n = 8t 3OF%%%

fter e2tracting the )rst integer, the mar$ermo&esto the character folloing the integer%

8n1 9 : 1 ; ;< = 8n = 8t 3OF%%%


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File input stream

fter e2tracting the second integer>

myinfile >> x;myinfile >> x;

8n1 9 : 1 ; ;< = 8t = 8n 3OF%%%

If e $eep e2tracting inputs from the )le, themar$ere&entuall" reaches the end of the )le%

8n1 9 : 1 ; ;< = 8t = 8n 3OF%%%

5o to chec$ the end of the )le6


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File input stream

he end of )lecan !e found out !" chec$ingthe &alue of the folloing e2pression

myinfile >> xmyinfile >> x

ction of the e2pression

reada &alue from the )le, and

assignthe &alue to variable x

?alue of the e2pression .upon completion4

reading is successful, its &alue is m"in)le,otherise @

m"in)le AA 2 AA "


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File input stream

ifstream myinput;int x; int total = 0;

myinput.open("myfile.dat");*ile(myinput >> x) total = x;cout


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File input stream

nother a" to chec$ the end of )le#mem!er functionmyinput.eof()

ifstream myinput;int x; int total = 0;

myinput.open("myfile.dat");*ile(true){ myinput >> x; if(myinput.eof())

rea; total = x;cout


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e npu s reaminitiali0ation

If e ant to read the )le namefrom user, eneed to ta$e the c-style string&ersion of the%ariale%

ifstream myfile;string name;cin >> name;

myfile.open(name.cstr());

ifstream myfile;string name;cin >> name;

myfile.open(name.cstr());

e can com!ine thedeclarationand the )leopening statementsinto one statement%

ifstream myfile;myfile.open("myfile.txt")

;

ifstream myfile;myfile.open("myfile.txt")

;

ifstreammyfile("myfile.txt");

ifstreammyfile("myfile.txt");


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t"pe%cpp

ifstream input.Bfoo%t2t4Dinput AA 2 AA "D

.input AA 24 AA "D

If there is more items left in input

input AA 2 ill ha&e &alue input

input AA "

3lse

input AA 2 ill ha&e &alue @

@ AA "


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t an" moment, a )le can onl" !e connectedto one file stream.either inputor output4

)le must !e )rst disconnected.closed4 to astream !efore it is reEconnected

hen "ou open a )le for riting, if the )lee2ists, the s"stem ill )rst erase its contents%

ou can open a )le for appending, i%e% add ne

data at the end of an e2isting )le, !" openingit as#

ofstream myfile;myfile.open("myfile.txt"1 ios--app);

ofstream myfile;myfile.open("myfile.txt"1 ios--app);


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String stream

reat strings as streams

similar to cin/cout and )les

Gseful to do man" con&ersion operations

!eteen string and other data t"pes, such asint, dou!le, etc%

o use stringstream, "ou ha&e to

Hinclude sstreamA


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String output stream

Gse ostringstreamD see outputEtoEstring%cc

he contents of a string stream can !e

e2tracted using the str.4 mem!er function%

ostringstream des; // create an output string streamdes


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32ample

Open a )le test222%t2t, here 222 isdetermined !" an integer &aria!le .see

more)le%cc4ofstream outfile; int i=2;

ostringstream fname;fname.str(34); //set stringstream to emptyfname


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Input string stream

istringstream

Mem!er function B%eof.4 indicates end of string

// an input string stream wit initial contentsstring data!src = tmcan 1" 4# $ % "& &0';istringstream src(data!src);

string login;int a1 a" a3 a4 a# a$;src login a1 a" a3 a4 a# a$;

// an input string stream wit initial contentsstring data!src = tmcan 1" 4# $ % "& &0';istringstreamsrc(data!src);string login;

int a1 a" a3 a4 a# a$;src login a1 a" a3 a4 a# a$;

Lect08 Filddde

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