i am trying to open a file with
27 string tline;
28 ifstream finp; // input file
29 ifstream dinp; // data files
30
31 finp.open(argv[1]);
32
33
34 cout << "finp.good() = " << finp.good() << endl;
35 cout << "finp.bad() = " << finp.bad() << endl;
36 cout << "finp.fail() = " << finp.fail() << endl;
and i end up with output
finp.good() = 0
finp.bad() = 0
finp.fail() = 1
now, i cannot find any good documentation on what would cause this other than that it is an internal logic problem. what am i supposed to do to correct this?
if it helps, i am running on linux where i need to include both <cstring> and <cstdlib> while i do not have to do this when running on OSX. could this be the problem? if so, how do i correct it?
fail() will return 1 when you attempt a conversion and it fails. For example, if the next character in the file is something other than a digit, and you attempt to read an int, then the failbit will be set, and fail() will return 1. Any conversion attempted when you're already at the end of the file will also set the failbit.
fail() will also return 1 when/if the badbit is set. This is set to signal a serious problem with the file itself, not just an inability to read some particular piece of data from the file.
forgive me, i made a simple mistake. when i ported my source files over to the linux system, i had ported over a script as well. i was confusing the executable with the script and the script had a filename hardcoded into it which was not in the directory.
basically, i was accidentally trying to read a file which was not there!
Related
I am using the code with ifstream that I used ~1 year ago, but now it does not work correctly. Here, I have the following file (so, just a line of integers):
2 4 2 3
I read it while constructing a graph from this file:
graph g = graph("file.txt");
where graph constructor starts with:
#include <iostream>
#include <fstream>
#include <sstream>
using namespace std;
graph::graph(const char *file_name) {
ifstream infile(file_name);
string line;
getline(infile, line);
cout << line << endl; // first output
istringstream iss;
iss.str(line);
iss >> R >> C >> P >> K;
iss.clear();
cout << R << " " << C << " " << P << " " << K; // second output
}
The second output (marked in code), instead of giving me 2 4 2 3, returns random(?) values -1003857504 32689 0 0. If I add the first output to check the contents of line after getline, it is just an empty string "".
All the files (main.cpp where a graph is instantiated, 'graph.cpp' where the graph is implemented and 'file.txt') are located in the same folder.
As I mentioned, this is my old code that worked before, so probably I do not see some obvious mistake which broke it. Thanks for any help.
These two locations:
where your program's original source code is located
where your program's input data is located
are completely unrelated.
Since "file.txt" is a relative path, your program looks for input data in the current working directory during execution. Sometimes that is the same as where the executable is. Sometimes it is not. (Only you can tell what it is, since it depends on how you execute your program.) There is never a connection to the location of the original source file, except possibly by chance.
When the two do not match, you get this problem, because you perform no I/O error checking in your program.
If you checked whether infile is open, I bet you'll find that it is not.
This is particularly evident since the program stopped working after a period of time without any changes to its logic; chances are, the only thing that could have changed is the location of various elements of your solution.
I'm working on a project that requires me to get a touchscreen working on Scientific Linux 6.4 (Linux kernel 2.6.32). Although the kernel does not support the touchscreen fully, I am able to see multi-touch events being generated in the /dev/input/eventX location for the touchscreen when I touch the screen.
I'm trying to write a simple C++ program to read the data from the /dev/input/eventX file and parse it so I can manually deal with the multi-touch events, since that seems the only way I'll get this working.
So I wrote the following program:
std::ifstream input("/dev/input/event10");
if(input.is_open()) {
while(input.good()) {
int header;
input >> header;
cout << std::hex << header << " ";
int data[16] = {};
for(int i = 0; i < 16; i++) {
input >> data[i];
cout << std::hex << data[i] << " ";
}
cout << endl;
}
input.close();
} else cout << "Unable to open event handler for input polling..." << endl;
Now, I don't exactly know if my method of reading and parsing the input itself is correct, but when I use the following command in bash:
sudo cat /dev/input/event10 | hexdump -C
I get the input data in the form of a number of lines beginning with an 8-digit hex value followed by 16 2-digit hex values (bytes).
The problem I'm having though is that I always get the message "Unable to open event handler for input polling..." suggesting an issue with opening the file. At first, I thought maybe that because nothing is in that file until an event is generated, it might not be able to be opened as an ifstream. I also tried running the program as sudo just in case it was a permissions issue and got the same message, so I believe it has to do with how I'm opening the file.
Does anyone know the proper way to open and read from these files?
EDIT: My question is regarding why the file is unable to be opened, not necessarily just how to parse the data. The suggested "duplicate" questions don't provide any helpful information in this regard.
Nevermind... there was a trailing space in my filename (which was detected rather than hard-coded). I added a trim function and now it opens just fine.
I'm working through some work with some code already provided. There is an if statement that includes 2 fstream.fails(). The code is continually returning true and I'm not sure if it's because of a problem with the provided code or if I am missing something.
I've googled around to try and better understand what could be going on, but due to me being new to C++, I'm finding it hard to find an answer that helps me understand what might be going on.
The provided code where I think might be a problem.
bool Navigation::BuildNetwork(const string &fileNamePlaces, const string &fileNameLinks)
{
fstream finPlaces(fileNamePlaces);
fstream finLinks(fileNameLinks);
if (finPlaces.fail() || finLinks.fail()) return false;
ifstream();
// Add your code here
}
Where the "Build network" function is called
ACW_Wrapper wrapper("log.txt");
// Build Navigation
wrapper.startTimer();
Navigation nav;
if (nav.BuildNetwork("Places.csv", "Links.csv")) {
const auto elapsed = wrapper.stopTimer();
std::cout << std::fixed << std::setprecision(1) << "BuildNetwork - " << elapsed << " microseconds" << std::endl;
}
else {
std::cout << "\n*** Error *** BuildNetwork" << std::endl;
}
I would expect it to return false since this is provided code, but I'm unsure if I need to add something that would give me the expected outcome.
From the ios::fail documentation:
true if badbit (Read/writing error on I/O operation) and/or failbit (Logical error on i/o operation) are set.
which suggests that at least one of the two files is not opened successfully.
The problem is not in the code, the problem is with your program trying to open files that are not probably found (or it doesn't have permissions to).
Copy the files to the same workspace (folder) with your executable, and try again.
well! I have a text file including some integer values and non-integers like character strings and white spaces so I want only to read integers values so I used a vector of integers but when I read the file the opining is ok but it seems the first input fails thus breaks the loop!!!
here is my main example:
ifstream in("file.txt");
if(in.fail())
cout << "opening failed!" << endl;
//opening is fine!
int value;
vector<int> v;
while(in >> value) // the problem here; it fails why?
{
cout << "ok"; // not printed
v.push_back(value);
}
cout << v.size() << endl; // 0??!!
this is the content of file.txt:
32 43 24 32
15 23
57
77 81
if I make a vector of chars it's ok but I want only to use one of integers
*** I already used a code like this and worked fine but now I don't know what happened??!!! it's really annoting
any help, comment, tip is welcome and appreciated
This line:
while(in >> value)
says while I can read integers...
But in the post this may not be true - you are not handling this case.
Either read stuff that is not integers and handle it. Or just read strings and then decide what to do.
In addition
cout << "ok"; // not printed
is because it is buffered.
Do this
cout << "ok" << flush; // printed
excuse me first for annoying you with nonsense question. finally I managed to discover the error:
in my main folder of project I unintentionally created a winrar file input.rar then I didn't remove it but rename it to input.txt it's ok I opened it manually and removed some unreadable characters. then I put inside it the content above of integers then my c++ application succeeds in opening it but can't read it.
*now I removed it input.txt which was input.rar and created a new document text input.txt and now everything is good!!!
thank you for your collaboration. and this post may help someone else.
* don't create rar file or other formats then rename them to be text files and try to read them via your c++ fstream because it'll fail in fact it'll produce an error-prone which looks impossible to solve
The following code works on bidirectional streams and finds the record id from file and then replaces contents for that record from the file. But before overwriting the content, it shifts the put pointer to the position of the get pointer. Through tellp()and tellg() it is found that they both were already at the same position before shifting. But on removing the seekp() line the code does not overwrite the data.
Contents in data.txt:
123 408-555-0394
124 415-555-3422
263 585-555-3490
100 650-555-3434
Code:
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main()
{
int inID = 263;
const string& inNewNumber = "777-666-3333";
fstream ioData("data.txt");
// Loop until the end of file
while (ioData.good()) {
int id;
string number;
// Read the next ID.
ioData >> id;
// Check to see if the current record is the one being changed.
if (id == inID) {
cout << "get pointer position " << ioData.tellg() << endl; //Displays 39
cout << "put pointer position " << ioData.tellp() << endl; //Displays 39
ioData.seekp(ioData.tellg()); //Commenting this line stops code from working
ioData << " " << inNewNumber;
break;
}
// Read the current number to advance the stream.
ioData >> number;
}
return 0;
}
Question:
What is the need of using seekp() to shift the position of the put pointer if it is already there, as the get and put pointers move together?
The question linked by #Revolver_Ocelot in the comments gives relevant information. The most important part is that you have to either flush or seek between read and write access. I therefore modified your code in the following way:
if (id == inID) {
cout << "get pointer position " << ioData.tellg() << endl; //Displays 39
cout << "put pointer position " << ioData.tellp() << endl; //Displays 39
ioData.flush();
cout << "get pointer position " << ioData.tellg() << endl;
cout << "put pointer position " << ioData.tellp() << endl;
ioData.seekp(ioData.tellg()); //Commenting this line stops code from working
ioData << " " << inNewNumber;
break;
}
This gives the following interesting output:
get pointer position 39
put pointer position 39
get pointer position 72
put pointer position 72
(Calling flush() doesn't actually resolve the problem. I just added it to your code in order to show you that it modifies the file pointer.)
My assumption on your original code is the following: If you write to your file after reading from it first, without calling seekp() in between, then the file pointer gets modified by the write command before the data is actually written to the file. I assume that the write command performs some kind of flushing and that this modifies the file pointer in a similar way as the flush() command that I added to your code.
When I ran the code above on my PC, the flush() command moved the file pointer to position 72. If we remove the seekp() command from your original code, I think that the write command will also move the file pointer to position 72 (or maybe another invalid position) before actually writing to the file. In this case writing fails, because position 72 is behind the end of the file.
Consequently, ioData.seekp(ioData.tellg()); is needed to ensure that the file pointer is set to the correct file position, because it can change when you switch between reading from and writing to your file without calling seekp().
The last paragraph of this answer gives some similar explanation.
It is because it's a rule of c++ bidirectional streams that if someone wants to shift from input operation to output operation. Then one must use seek() function to make such shift.
This functionality is borrowed from the core of c language as whenever someone uses a bidirectional stream then programmer may be working with two different buffers in which one buffer may be for input and another for output. Now synchronizing both the buffers would be a performance inefficient solution. As most of the time programmer may not need to use both the input and output functionality and program would be maintaining both the buffers for the programmer for no good reason.
So as an alternative to this, another solution was implemented to let programmer explicitly perform the flushing and other management by invoking seek() function.
Which means that seek() function that we often use does not simply repositions the file pointer but also updates the buffers and stream also.
See also
why fseek or fflush is always required between reading and writing in the read/write "+" modes