I have the following code
#include<iostream>
#include<fstream>
#include<string>
using namespace std;
int main(void) {
fstream ofile;
ofile.open("test.txt", ios::in | ios::out | ios::app);
for(string line; getline(ofile, line) ; ) {
cout << line << endl;
}
ofile << "stackexchnange" << endl;
ofile.close();
return 0;
}
test.txt contains
hello world!
stackoverflow
Above code outputs
hello world!
stackoverflow
And after running the code stackexchange is not appending at the end of test.txt. How to read and then write in file?
Nawaz' comment is correct. Your read loop iterates until the fstream::operator bool (of ofile) returns false. Therefore, after the loop, either failbit or badbit must have been set. failbit is set when loop tries to read for the final time but only EOF is left to read. This is completely OK, but you must reset the error state flag before trying to use the stream again.
// ...
ofile.clear();
ofile << "stackexchnange" << endl;
fstream has two positions : input and output.
In your case they both are set to the beginning of the file when you open it.
So you have 4 methods:
tellp // returns the output position indicator
seekp // sets the output position indicator
tellg // returns the input position indicator
seekg // sets the input position indicator
in your case you can use the following line to set output position to the end of the file
ofile.seekp(0, std::ios_base::end);
PS
i missed ios::app flag. mea culpa. comment of #Nawaz gives the right answer: after reading the whole file it is necessary to call
ofile.clear(); //cleanup error and eof flags
Related
The example is taken from:
[http://www-h.eng.cam.ac.uk/help/tpl/languages/C++/1AComputing/Mich/index.php?reply=extraReadingfromfiles#extraReadingfromfilesanchor][1]
I wrote the code without the while loop to read file, the example used getline(stream, strgvar), but this is not admited by the editor
#include <iostream>
#include <fstream>
using namespace std;
int main()
{
string message;
ifstream fin; // variable to store information about a file
fin.open("s.txt"); // trying to open file for reading
// next line would try to check if file has been opened succesfully
if (not fin.good())
{
cout << "\n\t Couldn't open the s file." << endl;
cout << "\n\t It needs to be in the same folder as your program."
<<endl;
return 1; // In the main function this line quits from the
whole program.
}
// we have menaged to open the file. Now we'll read a line from the file into the string
while (message!="works!")
{
fin >> message;
cout << message << " ";
}
//getline(fin,message);
}
My questions is why the line now commented is rejected ?
fin>>message;
The stream extraction operator '>>' is used when you want to read a single word from file.
Find complete explanation at : https://www.google.com/amp/s/www.geeksforgeeks.org/cpp-program-read-file-word-word/amp/
While
getline(fin,message);
In this, a full line from the file will be read in message variable. It will continue reading and assigning file contents till a '\n' (Line Deliminator) character does not appear. And thats why you getline() statement is rejected.
For complete explanation visit : http://www.cplusplus.com/forum/windows/48212/
Your program is expected to read a word at a time. And to accomplish this, fin>>mesage is used. Basically stream extraction operator read the contents till a space appears, and hence it is used to read single word.
And if you still want to use getline (), then add a third parameter to your function call as space character ' '.
Like
getline(fin,message,' '); // and done
Basically the third parameter of getline function is Deliminator, by default it is '\n', but if you want to define your own Deliminator, you can do so by providing third parameter. It will read the contents of file till the Deliminator does not occurs while reading.
To use std::getline() include <string> in the header.
https://learn.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2010/2whx1zkx(v=vs.100)
istream also has a getline. More details here
https://learn.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-6.0/aa277361(v=vs.60)
Here is the code .
It is giving the EXACT output as shown below means it is reading the file.
But it is also showing fails as you can see it means fin.fails() is true.
I want to know why this is true although i am successful in reading the file.
#include<fstream>
#include<iostream>
using namespace std;
int main()
{ ifstream fin;
fin.open("pro.txt");
char ch;
int data;
while(!fin.eof())//!(fin >> ch).eof()
{
fin.get(ch);
cout<<ch;
if(fin.fail()) {
cout<<"fails";
break;
}
}
fin.clear();
fin.seekg(0);
int pos=(int)fin.tellg();
cout<<"\n this is :"<<pos;
fin.close();
return 0;
}
Output is :
this is my name
fails
this is 0
Contents of pro.txt:
this is my name
Don't know why this is happening!
Still can't find why the fin.fails() is true because i think no one willing to
But i figured out to read white spaces and file without making fin.fails() =true .
just replace the while(!fin.eof()) with while(fin.get(ch)) and remove the fin.get(ch) present in while body -> this was the reason why my output was skipping one character for each read.. this i had mentioned in the comments.
The problem is the following: Reading the last character in your file with get will not set the eofbit, so the call to fin.eof() in your while condition still returns false after you read the last character of the file inside the loop body. Then during the iteration following the read of the last character you try to read another character inside the loop body, even though there is no character to read anymore. This will set the eofbit and the failbit as per specification of get.
TL;DR: Reading over the end of a file is supposed to set the failbit.
On your suggestion i have changed the code as you suggested but still problems are there when ios::out is replaced with ios::ate nothing is written in the file(Writing does not work). Is there any way to check that if the next bit is eof rather than reading it and then checking it? as suggested by you.And sometimes when i do file handling it shows the position of file pointer to be -1 what could that mean???
Code:
int main ()
{
char p[80];
fstream file("text1.txt",ios::out|ios::in); //if ios::ate is added here it results into infinite loop
cout<<"Starting position of the file is "<<file.tellg()<<endl;
getch();
if(file.is_open())
cout<<"file is open\n";
else
cout<<"file is not open\n";
getch();
file.seekp(0);
while(file>>p)
{
cout<<p<<endl;
}
file.clear();
cout<<"\nThe current position of the file pointer is "<<file.tellg()<<endl;
file.seekp(0);
if(file.eof())
cout<<"\n the eof\n";
while(file>>p)
{
cout<<p<<endl;
}
file.close();
return 0;
}
Output:
Starting position of the file is 0
file is open
Hello
man
how
are
you
The current position of the file pointer is 21
Hello
man
how
are
you
With this kind of reading from file reaching end-of-file causes setting both eof and failbit. Failbit is setted because creating your read loop with file.eof() condition doesn't indicate that next read will be the end of the stream. It just states that we didn't reach eof yet, so with:
while(file.eof())
{
file >> p;
}
It's possible that last read will be eof only, and we'll work with uninitialised data. IF this happens no characters will be extracted inside p and both eof and fail flags will be set.
When working with c++98 need to reset failbit to false by using:
file.clear();
To avoid bad readings situation you should extract characters from file inside while condition: while(file >> p). I recommend this or this questions on stack overflow.
So proper C++98 code should look like this:
while(file >> p)
{
std::count << p << std::endl;
}
file.clear();
file.seekp(0);
while(file >> p)
{
std::count << p << std::endl;
}
file.close();
I tested it couple of times on Visual Studio 2013 and it worked everytime.
Considering ios::ate mode:
ios::out, ios::in are modifiers that states how do we open file in question. If you want to read something from file you need to use ios::out flag, and for writing you need to use ios::in.
On the other hand ios::ate just tells compiler to open file and immediately go to the end of file. So if you substitute ios::out with ios::ate writing would be impossible, and program will rise failflag on file << "Hello...";.
And if you just want to append data, but read from the beginning of file you should use ios::app instead, because it tells to seek eof before each write.
I have an assignment where I am writing input on various things (in the form of structs) and then writing to a binary file. I have to be able to both read and write to the file while the program is open. One of the methods needs to print out all of the clients in the binary file. It seems to be working, except whenever I call that method, it seems to erase the contents of the file and prevent more from being written to it. Here's the applicable snippets:
fstream binaryFile;
binaryFile.open("HomeBuyer", ios::in | ios::app | ios::binary);
The same file is supposed to be usable between times you run the program, so I should open it with ios::app, correct?
Here's the method to add an entry:
void addClient(fstream &binaryFile) {
HomeBuyer newClient; //Struct the data is stored in
// -- Snip -- Just some input statements to get the client details //
binaryFile.seekp(0L, ios::end); //This should sent the write position to the
//end of the file, correct?
binaryFile.write(reinterpret_cast<char *>(&newClient), sizeof(newClient));
cout << "The records have been saved." << endl << endl;
}
And now the method to print all the entries:
void displayAllClients(fstream &binaryFile) {
HomeBuyer printAll;
binaryFile.seekg(0L, ios::beg);
binaryFile.read(reinterpret_cast<char *>(&printAll),sizeof(printAll));
while(!binaryFile.eof()) { //Print all the entries while not at end of file
if(!printAll.deleted) {
// -- Snip -- Just some code to output, this works fine //
}
//Read the next entry
binaryFile.read(reinterpret_cast<char *>(&printAll),sizeof(printAll));
}
cout << "That's all of them!" << endl << endl;
}
If I step through the program, I can input as many clients as I want, and it will output them all the first time I call displayAllClients(). But as soon as I call displayAllClients() once, it seems to clear out the binary file, and any further attempts at displaying clients gives me no results.
Am I using seekp and seekg incorrectly?
From what I understand, this should set my write position to the end of the file:
binaryFile.seekp(0L, ios::end);
And this should set my read position to the beginning:
binaryFile.seekg(0L, ios::beg);
Thanks!
Pasting comment in as this resolved the issue.
You need to call binaryFile.clear() before seekp() and seekg() if EOF is set, otherwise they won't work.
This is the documentation for ios::app
ios::app
All output operations are performed at the end of the file, appending the
content to the current content of the file. This flag can only be used in
streams open for output-only operations.
Since this is homework, I'll let you draw your own conclusions.
Im working on a text file decoder along with an encoder, they work off of two different text files. The decoder prints the decoded message underneath the encoded message but it also prints a bunch of other stuff as well. How do i fix this
#include <fstream>
#include <iostream>
#include <string>
using namespace std;
int main() {
ifstream fin; // input file
string line;
ofstream fout;
//open output file
fout.open("secret.txt", ios::app);
if (!fout.good()) throw "I/O error";
// open input file
fin.open("secret.txt");
if (!fin.good()) throw "I/O error";
// read input file, decode, display to console
while (fin.good()) {
getline(fin, line);
for (int i = 0; i < line.length(); i++) // for each char in the string...
line[i]--; // bump the ASCII code down by 1
fout << line << endl; // display on screen
}
// close file
fin.close();
return 0;
}
the text file from the encoder reads
Uftujoh234
Ifmmp!nz!obnf!jt!cpc
Dmptfe!
Uftujoh
which decodes to
Testing123
Hello my name is bob
Closed
Testing
this is all the extra stuff it also prints in the text file
Sdrshmf012
Gdkknlxm`ldhrana
Bknrdc
Sdrshmf
Rcqrgle/01
Fcjjmkwl_kcgq`m`
Ajmqcb
Rcqrgle
Qbpqfkd./0
Ebiiljvk^jbfp_l_
#ilpba
Qbpqfkd
Paopejc-./
Dahhkiuj]iaeo^k^
?hkoa`
Paopejc
O`nodib,-.
C`ggjhti\h`dn]j]
>gjn`_
O`nodib
N_mncha+,-
B_ffigsh[g_cm\i\
=fim_^
N_mncha
M^lmbg`*+,
A^eeh
The extra data you see is actually valid output from decoding the data in "secret.txt".
I'm not sure if this is what you want, but are you aware that you are reading and writing to the same file each time you run your application?
You'll append more and more "decoded" data to the file, and therefore you get the extra output you are referring to.
Also, there is an issue with your while-loop.
fin.good () will remain true until some of the error bits has been set inside of fin, though it will enter the loop one time too much since you should check to state of the stream immediately after your call to getline (fin, ...).
Currently the reading will fail but you will still process the "unread" data.
std::getline will return the stream object, and since a std::istream (as well as std::ostream) implicitly can be converted to a boolean to check it's current state you should use that as your loop-condition.
Change your loop into something as the below and see if that solves your problem.
while (getline (fin, line))
{
for (int i = 0; i < line.length(); i++) // for each char in the string...
line[i]--; // bump the ASCII code down by 1
fout << line << endl; // display on screen
}
The extra stuff isn't extra. You are writing data into the same file you are reading, so what you do is:
write line
read line
You are renencoding the data you already encoded.