C++ Tutorial/File Stream/file pointer
Содержание
- 1 Change a file at specific position
- 2 change file at given position
- 3 Check ofstream current marker position
- 4 Detecting EOF
- 5 get file content at given position
- 6 move to position 2 in the stream and output a 0 in position 2
- 7 Seek 0 to reset the file pointer
- 8 seek and Skip
- 9 seekg( ) moves the pointer offset number of characters from the specified origin
- 10 Tell the current position
- 11 Test File Position
- 12 uses both seekp( ) and seekg( ) to reverse the first <num> characters in a file.
Change a file at specific position
<source lang="cpp">#include <iostream>
- include <fstream>
- include <stdlib.h>
using namespace std; int main(int argc, char *argv[]){
if(argc!=4){
cout << "Usage: change <filename> <byte> <character>" << endl;
exit (1);
}
fstream out(argv[1], ios::in | ios::out | ios::binary);
if(!out)
{
cout << "Cannot open file!";
exit (1);
}
out.seekp(atoi(argv[2]), ios::beg);
out.put(*argv[3]);
out.close();
}</source>
change file at given position
<source lang="cpp">#include <iostream>
- include <fstream>
- include <stdlib.h>
using namespace std; main(int argc, char *argv[]) {
if(argc!=3) {
cout << "Usage: CHANGE <filename> <byte>\n";
return 1;
}
fstream out(argv[1], ios::in|ios::out);
if(!out) {
cout << "Cannot open file";
return 1;
}
out.seekp(atoi(argv[2]), ios::beg);
out.put("X");
out.close();
return 0;
}</source>
Check ofstream current marker position
<source lang="cpp">#include <iostream>
- include <fstream>
using namespace std; int main(int argc, char** argv) {
ofstream fout("test.out");
fout << "12345";
long curPos = fout.tellp();
if (curPos == 5) {
cout << "Test passed: Currently at position 5" << endl;
} else {
cout << "Test failed: Not at position 5" << endl;
}
fout.seekp(2, ios_base::beg);
fout << 0;
fout.flush();
ifstream fin("test.out");
int testVal;
fin >> testVal;
if (testVal == 12045) {
cout << "Test passed: Value is 12045" << endl;
} else {
cout << "Test failed: Value is not 12045 (it was " << testVal << ")" << endl;
}
}</source>
Detecting EOF
<source lang="cpp">#include <iostream>
- include <fstream>
- include <cctype>
- include <iomanip>
using namespace std;
int main(int argc, char *argv[]) {
if(argc!=2) {
cout << "Usage: Display <filename>\n";
return 1;
}
ifstream in(argv[1], ios::in | ios::binary);
if(!in) {
cout << "Cannot open input file.\n";
return 1;
}
register int i, j;
int count = 0;
char c[16];
cout.setf(ios::uppercase);
while(!in.eof()) {
for(i=0; i<16 && !in.eof(); i++) {
in.get(c[i]);
}
if(i<16) i--; // get rid of eof
for(j=0; j<i; j++)
cout << setw(3) << hex << (int) c[j];
for(; j<16; j++) cout << " ";
cout << "\t";
for(j=0; j<i; j++)
if(isprint(c[j])) cout << c[j];
else cout << ".";
cout << endl;
count++;
if(count==16) {
count = 0;
cout << "Press ENTER to continue: ";
cin.get();
cout << endl;
}
}
in.close();
return 0;
}</source>
get file content at given position
<source lang="cpp">#include <iostream>
- include <fstream>
- include <stdlib.h>
using namespace std; main(int argc, char *argv[]) {
char ch;
if(argc!=3) {
cout << "Usage: NAME <filename> <starting location>\n";
return 1;
}
ifstream in(argv[1]);
if(!in) {
cout << "Cannot open file";
return 1;
}
in.seekg(atoi(argv[2]), ios::beg);
while(in.get(ch))
cout << ch;
return 0;
}</source>
move to position 2 in the stream and output a 0 in position 2
<source lang="cpp">#include <iostream>
- include <fstream>
using namespace std; int main(int argc, char** argv) {
ofstream fout("test.out");
fout << "12345";
long curPos = fout.tellp();
if (curPos == 5) {
cout << "Test passed: Currently at position 5" << endl;
} else {
cout << "Test failed: Not at position 5" << endl;
}
fout.seekp(2, ios_base::beg);
fout << 0;
fout.flush();
ifstream fin("test.out");
int testVal;
fin >> testVal;
if (testVal == 12045) {
cout << "Test passed: Value is 12045" << endl;
} else {
cout << "Test failed: Value is not 12045 (it was " << testVal << ")" << endl;
}
}</source>
Seek 0 to reset the file pointer
<source lang="cpp">#include <fstream>
- include <string>
- include <iostream>
using namespace std; int main() {
fstream inout( "invoice.dat", ios::in | ios::out );
inout << 1234 << " " << 56.78 << " " << "apples" << "\n";
cout<< inout.tellg()<< endl;
cout << inout.tellp() << endl;
inout.seekg( 0 );
cout << inout.tellg() << endl;
cout << inout.tellp() << endl;
int x = 0;
double y = 0.0;
string z = " ";
inout >> x;
inout.seekg( 8, ios::cur );
inout >> z;
cout << x << " " << y << " " << z << endl;
return 0;
}</source>
seek and Skip
<source lang="cpp">#include <fstream>
- include <iostream>
using namespace std; int main(){
ifstream in("datafile");
in.unsetf(ios::skipws);
char ch;
while (in >> ch) {
cout << ch;
}
cout << "\n";
in.clear();
in.seekg(ios::beg);
in.setf(ios::skipws);
while (in >> ch) {
cout << ch;
}
cout << "\n";
return 0;
}</source>
seekg( ) moves the pointer offset number of characters from the specified origin
<source lang="cpp">/* It must be one of these three values: ios::beg Beginning-of-file ios::cur Current location ios::end End-of-file
- /
- include <iostream>
- include <fstream>
- include <cstdlib>
using namespace std;
int main(int argc, char *argv[]) {
if(argc!=4) {
cout << "Usage: CHANGE <filename> <character> <char>\n";
return 1;
}
fstream out(argv[1], ios::in | ios::out | ios::binary);
if(!out) {
cout << "Cannot open file.";
return 1;
}
out.seekp(atoi(argv[2]), ios::beg);
out.put(*argv[3]);
out.close();
return 0;
}</source>
Tell the current position
<source lang="cpp">#include <iostream>
- include <fstream>
- include <stdlib.h>
using namespace std; int main(int argc, char *argv[]){
if(argc!=4){
cout << "Usage: change <filename> <byte> <character>" << endl;
exit(1);
}
fstream out(argv[1], ios::in | ios::out | ios::binary);
if(!out)
{
cout << "Cannot open file!";
exit(1);
}
out.seekp(atoi(argv[2]), ios::beg);
out.put(*argv[3]);
cout << "Current position is: " << out.tellp() << endl;
out.close();
}</source>
Test File Position
<source lang="cpp">#include <fstream>
- include <string>
- include <iostream>
using namespace std; int main() {
fstream inout( "invoice.dat", ios::in | ios::out ); inout << 1234 << " " << 56.78 << " " << "apples" <<"\n"; cout << inout.tellg() << endl; cout << inout.tellp() << endl; inout.seekg( ios::beg ); cout << inout.tellg() << endl; cout << inout.tellp() << endl; int x; double y; string z; inout>> x >> y; inout >> z; cout << x << " " << y << " " << z << endl; return 0;
}</source>
uses both seekp( ) and seekg( ) to reverse the first <num> characters in a file.
<source lang="cpp">#include <iostream>
- include <fstream>
- include <cstdlib>
using namespace std;
int main(int argc, char *argv[]) {
if(argc!=3) {
cout << "Usage: Reverse <filename> <num>\n";
return 1;
}
fstream inout(argv[1], ios::in | ios::out | ios::binary);
if(!inout) {
cout << "Cannot open input file.\n";
return 1;
}
long e, i, j;
char c1, c2;
e = atol(argv[2]);
for(i=0, j=e; i<j; i++, j--) {
inout.seekg(i, ios::beg);
inout.get(c1);
inout.seekg(j, ios::beg);
inout.get(c2);
inout.seekp(i, ios::beg);
inout.put(c2);
inout.seekp(j, ios::beg);
inout.put(c1);
}
inout.close();
return 0;
}</source>
