I'm reading a file through a function like this:
#include <iostream>
#include <fstream>
#include <string>
...
void readfile(string name){
string line;
int p = 0;
ifstream f(name.c_str());
while(getline(f,line)){
p++;
}
f.seekg(0);
cout << p << endl;
getline(f,line);
cout << line << endl;
}
Mi file has 3 lines:
first
second
third
I expected the output:
3
first
Instead I get:
3
(nothing)
why is my seekg not working?
Because seekg() fails if the stream has reached the end of the file (eofbit is set), which occurs due to your getline looping. As sftrabbit implies, calling clear() will reset that bit and should allow you to seek properly. (Or you could just use C++11, in which seekg will clear eofbit itself.)
Use iterators for read from the file
std::fstream file( "myfile.txt", std::ios::out );
std::string data = std::string(
std::istreambuf_iterator<char>( file ),
std::istreambuf_iterator<char>() );
Related
I want to display all the text that is in the fille to the output,
I use by using the code below, the code I got up and results posts are just a little out
#include <fstream>
#include <iostream>
using namespace std;
int main()
{
char str[10];
//Creates an instance of ofstream, and opens example.txt
ofstream a_file ( "example.txt" );
// Outputs to example.txt through a_file
a_file<<"This text will now be inside of example.txt";
// Close the file stream explicitly
a_file.close();
//Opens for reading the file
ifstream b_file ( "example.txt" );
//Reads one string from the file
b_file>> str;
//Should output 'this'
cout<< str <<"\n";
cin.get(); // wait for a keypress
// b_file is closed implicitly here
}
The above code simply displays the words "This" does not come out all into output.yang I want is all text in the file appear in the console ..
The overloaded operator>> for char* will only read up to the first whitespace char (it's also extremely risky, if it tries to read a word longer then the buf length you'll end up with undefined behavior).
The following should do what you want in the most simple manner, as long as your compiler supports the rvalue stream overloads (if not you'll have to create a local ostream variable and then use the stream operator):
#include <fstream>
#include <iostream>
int main()
{
std::ofstream("example.txt") << "This text will now be inside of example.txt";
std::cout << std::ifstream("example.txt").rdbuf() << '\n';
}
try something like this
#include <fstream>
#include <iostream>
using namespace std;
int main(){
string line;
ofstream a_file ( "example.txt" );
ifstream myfile ("filename.txt");
if (myfile.is_open()) {
while ( getline (myfile,line) ) {
a_file << line << '\n';
}
myfile.close();
a_file.close();
} else
cout << "Unable to open file";
}
Hope that helps
This is not the best way to read from a file. You probably need to use getline and read line by line. Note that you are using a buffer of fixed size, and you might cause an overflow. Do not do that.
This is an example that is similar to what you wish to achieve, not the best way to do things.
#include <fstream>
#include <iostream>
using namespace std;
int main() {
string str;
ofstream a_file("example.txt");
a_file << "This text will now be inside of example.txt";
a_file.close();
ifstream b_file("example.txt");
getline(b_file, str);
b_file.close();
cout << str << endl;
return 0;
}
This is a duplicate question of:
reading a line from ifstream into a string variable
As you know from text input/output with C++, cin only reads up to a newline or a space. If you want to read a whole line, use std::getline(b_file, str)
#include <iostream>
#include <fstream>
#include <stdlib.h>
#include <process.h>
using namespace std;
int main(){
system("cls");
char mline[75];
int lc=0;
ofstream fout("out.txt",ios::out);
ifstream fin("data.txt",ios::in);
if(!fin){
cerr<<"Failed to open file !";
exit(1);
}
while(1){
fin.getline(mline,75,'.');
if(fin.eof()){break;}
lc++;
fout<<lc<<". "<<mline<<"\n";
}
fin.close();
fout.close();
cout<<"Output "<<lc<<" records"<<endl;
return 0;
}
The above code is supposed to read from the file "data.txt" the following text
"The default behaviour of ifstream type stream (upon opening files ) allows users
to read contents from the file. if the file mode is ios::in only then reading is
performed on a text file and if the file mode also includes ios::binary along with
ios::in then, reading is performed in binary mode. No transformation of characters
takes place in binary mode whereas specific transformations take place in text mode."
and create a file out.txt , in which the same text is stored using line numbers ( A line can have 75 characters or ends at '.' - whichever occurs earlier ).
Whenever I run the program, it just gets stuck at the console - which doesnt respond upon pressing any keys whatsoever.
Can someone tell me what's going on in here ?
If any one of the attempted reads in the file is longer than 74 characters, getline will set the failbit for fin, and you will never reach the end of the file. Change your code to the following:
for (; fin; ++lc) {
fin.getline(mline,75,'.');
if (!fin.eof() && !fin.bad())
fin.clear();
fout<<lc<<". "<<mline<<"\n";
}
This will break your loop if you reach the end of the file or if something catastrophic happens to the stream. You'll also need to think about handling the extra read that is performed if the file ends with a period.
Consider switching to std::string.
#include <iostream>
#include <fstream>
#include <string>
int main()
{
int lc = 0;
std::ofstream fout("out.txt");
std::ifstream fin("data.txt");
for (std::string line; getline(fin, line, '.'); )
fout << ++lc << ". " << line << "\n";
std::cout << "Output " << lc << " records\n";
}
I'm trying to read from a file, but C++ is not wanting to run getline().
I get this error:
C:\main.cpp:18: error: no matching function for call to 'getline(std::ofstream&, std::string&)'
std::getline (file,line);
^
This is the code:
#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <fstream>
#include <string>
using namespace std;
int main(){
string line;
std::ofstream file;
file.open("test.txt");
if (file.is_open())
{
while ( file.good() )
{
getline (file,line);
cout << line << endl;
}
file.close();
}
}
std::getline is designed for use with input stream classes (std::basic_istream) so you should be using the std::ifstream class:
std::ifstream file("test.txt");
Moreover, using while (file.good()) as a condition for input in a loop is generally bad practice. Try this instead:
while ( std::getline(file, line) )
{
std::cout << line << std::endl;
}
std::getline reads characters from an input stream and places them into a string. In your case your 1st argument to getline is of type ofstream. You must use ifstream
std::ifstream file;
In C++, is there a function in the fstream library (or any library) that allows me to read a line to a delimiter of '\n' without extracting?
I know the peek() function allows the program to 'peek' at the next character its reading in without extracting but I need a peek() like function that does that but for a whole line.
You can do this with a combination of getline, tellg and seekg.
#include <fstream>
#include <iostream>
#include <ios>
int main () {
std::fstream fs(__FILE__);
std::string line;
// Get current position
int len = fs.tellg();
// Read line
getline(fs, line);
// Print first line in file
std::cout << "First line: " << line << std::endl;
// Return to position before "Read line".
fs.seekg(len ,std::ios_base::beg);
// Print whole file
while (getline(fs ,line)) std::cout << line << std::endl;
}
How can I make my std::fstream object start reading a text file from the second line?
Use getline() to read the first line, then begin reading the rest of the stream.
ifstream stream("filename.txt");
string dummyLine;
getline(stream, dummyLine);
// Begin reading your stream here
while (stream)
...
(Changed to std::getline (thanks dalle.myopenid.com))
You could use the ignore feature of the stream:
ifstream stream("filename.txt");
// Get and drop a line
stream.ignore ( std::numeric_limits<std::streamsize>::max(), '\n' );
// Get and store a line for processing.
// std::getline() has a third parameter the defaults to '\n' as the line
// delimiter.
std::string line;
std::getline(stream,line);
std::string word;
stream >> word; // Reads one space separated word from the stream.
A common mistake for reading a file:
while( someStream.good() ) // !someStream.eof()
{
getline( someStream, line );
cout << line << endl;
}
This fails because: When reading the last line it does not read the EOF marker. So the stream is still good, but there is no more data left in the stream to read. So the loop is re-entered. std::getline() then attempts to read another line from someStream and fails, but still write a line to std::cout.
Simple solution:
while( someStream ) // Same as someStream.good()
{
getline( someStream, line );
if (someStream) // streams when used in a boolean context are converted to a type that is usable in that context. If the stream is in a good state the object returned can be used as true
{
// Only write to cout if the getline did not fail.
cout << line << endl;
}
}
Correct Solution:
while(getline( someStream, line ))
{
// Loop only entered if reading a line from somestream is OK.
// Note: getline() returns a stream reference. This is automatically cast
// to boolean for the test. streams have a cast to bool operator that checks
// good()
cout << line << endl;
}
The more efficient way is ignoring strings with std::istream::ignore
for (int currLineNumber = 0; currLineNumber < startLineNumber; ++currLineNumber){
if (addressesFile.ignore(numeric_limits<streamsize>::max(), addressesFile.widen('\n'))){
//just skipping the line
} else
return HandleReadingLineError(addressesFile, currLineNumber);
}
HandleReadingLineError is not standart but hand-made, of course.
The first parameter is maximum number of characters to extract. If this is exactly numeric_limits::max(), there is no limit:
Link at cplusplus.com: std::istream::ignore
If you are going to skip a lot of lines you definitely should use it instead of getline: when i needed to skip 100000 lines in my file it took about a second in opposite to 22 seconds with getline.
Call getline() once to throw away the first line
There are other methods, but the problem is this, you don't know how long the first line will be do you? So you can't skip it till you know where that first '\n' is. If however you did know how long the first line was going to be, you could simply seek past it, then begin reading, this would be faster.
So to do it the first way would look something like:
#include <fstream>
#include <iostream>
using namespace std;
int main ()
{
// Open your file
ifstream someStream( "textFile.txt" );
// Set up a place to store our data read from the file
string line;
// Read and throw away the first line simply by doing
// nothing with it and reading again
getline( someStream, line );
// Now begin your useful code
while( !someStream.eof() ) {
// This will just over write the first line read
getline( someStream, line );
cout << line << endl;
}
return 0;
}
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
string textString;
string anotherString;
ifstream textFile;
textFile.open("TextFile.txt");
if (textFile.is_open()) {
while (getline(textFile, textString)){
anotherString = anotherString + textString;
}
}
std::cout << anotherString;
textFile.close();
return 0;
}
this code can read file from your specified line from file but you have to make file in file explorer before hand my file name is "temp" code is given below
https://i.stack.imgur.com/OTrsj.png
hope this can help
You can use ignore function as follow:
fstream dataFile("file.txt");
dataFile.ignore(1, '\n'); // ignore one line
#include <fstream>
#include <iostream>
using namespace std;
int main ()
{
char buffer[256];
ifstream myfile ("test.txt");
// first line
myfile.getline (buffer,100);
// the rest
while (! myfile.eof() )
{
myfile.getline (buffer,100);
cout << buffer << endl;
}
return 0;
}