So i basically need my program to open a file and do something. When the program asks for the user to input the file name, and the user inputs the name of file correctly the first time, the operation works. But if the user typed the name wrong, the program says "invalid name try again" but then it is never able to open the file even if the user types the name correctly. Here's the code:
ifstream read_file(file.c_str());
while(true)
{
if(!(read_file.fail()))
{
...
}
else
{
cout << "Either the file doesn't exist or the formatting of the file is incorrect. Try again.\n>";
}
cin >> file;
ifstream read_file(file.c_str());
}
What is the problem, any thoughts? Thank you
You are redeclaring read_file inside the loop, but the code at the top of the loop always use the read_file outside the loop.
This is what you want instead:
ifstream read_file(file.c_str());
while(true)
{
if(!(read_file.fail()))
{
...
}
else
{
cout << "Either the file doesn't exist or the formatting of the file is incorrect. Try again.\n>";
}
cin >> file;
read_file.open(file.c_str()); /// <<< this has changed
}
Related
I'm attempting to create a repeating menu that will allow a user to re-enter a file name if the program is unable to open the file.
Right now it works correctly if I enter the name of an existing file, but if the file doesn't exist it prints the "File not found" then executes the rest of the program. I'm new to file streams and most of the code here was found through references. I'm a bit lost on what exactly is going on and what the best way to handle the situation is. Any guidance would be appreciated.
typedef istream_iterator<char> istream_iterator;
string fileName;
ifstream file;
do {
cout << "Please enter the name of the input file:" << endl;
cin >> fileName;
ifstream file(fileName.c_str());
if (!file) {
cout << "File not found" << endl;
}
} while (!file);
std::copy(istream_iterator(file), istream_iterator(), back_inserter(codeInput));
After constructing the object file will always exist, so your loop condition always fails. Change the condition to whether the file didn't open properly.
do {
...
}
while (!file.is_open())
this code will work.
do {
std::cout << "Please enter the name of the input file:" << std::endl;
std::cin >> fileName;
file = std::ifstream(fileName.c_str());
if (!file) {
std::cout << "File not found" << std::endl;
}
} while (!file);
your error was that you have 2 definition of the file variable.
the variable in while (!file) that is used is the one defined outside the do-while loop, and it is valid state is set to true by default.
In addition to #acraig5075 answer:
Writing a type then a variable name (ifstream file) is to create a new variable. Obviously you know this, but if you use the same name again in, for example, a loop, it makes a new and distinct variable.
ifstream file; // a unique variable
...
do {
...
ifstream file(fileName.c_str()); // another unique variable
...so change the usage inside the loop to:
file.open(fileName.c_str());
//Prompts user for a file name and stores it
string fileName;
cout << "Enter the file name: ";
cin >> fileName;
ifstream inFile (fileName);
inFile.open(fileName);
//Prompt the user until they give the name of a file that can be opened
bool validFileName = false;
while(validFileName == false)
{
if(inFile.is_open())
{
validFileName = true;
}
else
{
cout << "Please enter a valid file name: ";
cin >> fileName;
ifstream inFile;
inFile.open(fileName);
}
}
//this block prints to the terminal, so it's opening
if(inFile.is_open())
{ cout << "It works! \n"; }
I am trying to create a program that will work with a file, but there needs to be a section that checks to see if the file that the user types in is an actual file that the program can open. I've tried a few different ways to write the while loop, because it needs to keep asking until it receives a valid file. I have the valid file name "input.txt", but even when I type that into the terminal it continues to print the error message. I have tried to type the file name with and without quotes, so I'm not sure what it is caught up on. I know it is opening the file, because I added a second check afterward and it appears that it's opening, so I think it's an issue with how I have the error check statement written?
The problem is that you are using
ifstream inFile;
inFile.open(fileName);
in the loop. The variable in the loop hides the variable of the same name outside the loop. Remove the first of those lines.
FWIW, you can simplify your code to:
ifstream inFile (fileName);
while(!inFile)
{
// Prompt the user until they give the name of a file that can be opened
cout << "Please enter a valid file name: ";
cin >> fileName;
inFile.open(fileName);
}
if(inFile)
{
cout << "It works! \n";
}
In:
ifstream inFile (fileName);
inFile.open(fileName);
The file is opened on the first line.
Reopening it is redundant.
Also in the loop you declare a temporary variable inFile which goes out of stop at the end of the else statement. Make sure to declare it only once at the outmost scope you would like to use it.
I am writing a program that I allow the user to specify an input file to open and when I test with incorrect file names, the program is behaving very weird and it seems to have something to do with the input buffer, but I don't know where to begin other than using getline() instead of cin >> but I have already tried that.
here is the code that I think may be the problem:
bool openfile(ifstream&);
string userInput();
int main()
{
// ...
while (!openfile(inputFile))
openfile(inputFile);
string input = userInput();
// ...
}
bool openfile(ifstream &inputFile)
{
string filename;
cout << "Please enter the name of the file or type quit to exit the program: ";
cin >> filename;
cout << endl;
if (filename == "quit")
exit(4);
else
inputFile.open(filename);
if (!inputFile)
{
cout << "The file \"" << filename << "\" could not be opened or does not exist.\n";
return false;
}
return true;
}
string userInput()
{
string englishSentence;
cout << "Please enter a sentence or type quit to exit the program: \n";
getline(cin, englishSentence);
if (englishSentence == "quit")
exit(4);
return englishSentence;
}
Those are the two functions that read any input. openfile() is called first as you can see. Any help is greatly appreciated. Let me know if you suspect something else in my code and I will paste it.
while (!openfile(inputFile))
openfile(inputFile);
What this does is attempt to open the file twice each iteration, as long as the first attempt fails. Also, you need to make sure that inputFile is closed before attempting to open it again since it appears that you're reusing the same file object repeatedly.
Certainly first try something like:
while (!openfile(inputFile))
;
You can just do:
while (!openfile(inputFile));
Since the way you have it it would request an input filename twice if it fails the first time.
Basically to outline the problem:
Begin Loop
Request Filename
Invalid Filename
Request Replacement Filename
Return to start of the loop (Checks again)
Some problems I see from your code:
int main(); { ... } does not define the main function. You need to drop the semicolon, or it won't even compile.
while (!openfile(inputFile)) openfile(inputFile); repeats openfile(inputFile) unnecessarily. If the first one (in the condition) fails and the second one (in the body) succeeds, a third call will be made (in the condition) to check if the loop should continue. What you probably want is just while (!openfile(inputFile)) { }.
You open a file in openfile and never use it in the subsequent userInput.
I am starting the first part of a school assignment and I must prompt the user to enter a filename, check for the existence of the file, and if it exists, open it for processing; otherwise I am to have the user enter another filename.
When I compile and run my program below, I get the error message "No file exists. Please enter another filename." When I type in names of files that don't exist it just runs the first part of my do while loop again. I'm a beginner at C++ but I've done this before and I feel as if it should be running properly. Any help would be appreciated.
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
struct customerData
{
int _customerID;
string _firstName, _lastName;
double _payment1, _payment2, _payment3;
};
void processFile();
int main()
{
processFile();
system ("pause");
return 0;
}
void processFile()
{
string filename;
ifstream recordFile;
do
{
cout << "Please enter a filename\n";
cin >> filename;
recordFile.open(filename);
if (recordFile.good())
// {
// enter code for if file exists here
// }
;
}
while(recordFile.fail());
{
cout << "No file by that name. Please enter another filename\n";
cin >> filename;
recordFile.open(filename);
}
}
To check whether a file was successfully opened you must use the std::fstream::is_open() function, like so:
void processfile ()
{
string filename;
cout << "Please enter filename: ";
if (! (cin >> filename))
return;
ifstream file(filename.c_str());
if (!file.is_open())
{
cerr << "Cannot open file: " << filename << endl;
return;
}
// do something with open file
}
The member functions .good() and .fail() check for something else not whether the file was opened successfully.
I'm not 100% sure what your intent is here, but do you understand that you've only got one loop here? After your do/while loop, you've got some code in braces, but that's not connected to any loop construct... it's simply a new scope (which doesn't serve a purpose here).
So, your program does this:
1) Ask for filename. Try to open it. If file stream can be read, do the "enter code here" part.
2) Check if filestream is "bad". if so, go back to step 1. Otherwise, continue.
3) Print out "no file by that name", prompt for a new file, try to open it
That's almost certainly not what you want.
You can use c code.
FILE *fp = fopen("file" "r");
if(fp){
//do stuff
}
else{
//it doesnt exist
}
on a side note, when using namespace std try to make it not global
you can put it inside of your functions instead when necessary
int main(){
using namespace std;
//other std stuff
}
The program is supposed to prompt the user for their username. Upon receiving the username it concatenates it with '.history' to create username.history. Then it opens that file (username.history) and reads the input from it. I am running into a segfault here though. Whenever it opens the file, which is empty because the file doesn't exist, it reads multiple lines and then throws the segfault. I think the problem might stem from how I'm trying to open the file, but I'm not sure. Here is the portion that is causing problems:
// File input and output
ifstream f_in;
ofstream f_out;
// Prompt user for their username.
char username[80];
cout << "Please input your username: " << endl;
cin >> username;
cout << endl;
cout << "Loading history file if it exists." << endl;
// Create file naem and initialize the file line counter to 0.
strcat(username, ".history");
int fcount = 0;
// Open file and read in lines if there are any.
// Place read lines into the command string for use later.
char tmp[50];
f_in.open(username);
while(!f_in.eof()){
f_in >> tmp;
cmd[fcount] = tmp;
fcount++;
}
f_in.close();
Other pertinent info:
cmd is declared as a global variable (char cmd[200][50])
Any help will be greatly appreaciated.
Not sure if it is the only issue, but cmd[fcount] = tmp is wrong. You should use strcpy().
while(f_in.good())
{
f_in >> tmp;
cmd[fcount] = tmp;
fcount++;
}