Computer Science Canada

Problems opening a user defined .txt file
Author:  AnubisProgrammer [ Mon Mar 20, 2006 3:13 pm ]
Post subject:  Problems opening a user defined .txt file
I'm guessing that it's obviously possible, I'm just not sure of the syntax to get ifstream to open a file that a user has created.


string filename;
cout<<"Please enter the name of your text file (including extension): ";
getline(cin, filename);
ifstream infile;
infile.open("filename");


so, it's asking for the user to type the file name including extension. (not sure if I need to do that or not) then, when it hits infile.open("filename") how do I type ("filename") so it will call the variable "filename" and not try to open a non-existant file?
Author:  Dan [ Mon Mar 20, 2006 3:17 pm ]
Post subject: 
By removing the "s in infile.open("filename");

P.S. it just could be that i am realy used to c rather then c++ but your code seems odd to me....are u using the standert librays?
Author:  Tony [ Mon Mar 20, 2006 3:24 pm ]
Post subject: 
as Dan points out, filename is your variable, while "filename" is a string that has a literal value of "filename".
Author:  md [ Mon Mar 20, 2006 4:25 pm ]
Post subject: 
Sicne your using C++ and streams why are you using getline when you could do the much more correct std::cin >> filename; ?
Author:  AnubisProgrammer [ Mon Mar 20, 2006 4:45 pm ]
Post subject: 
well, first of all, I changed

infile.open("filename")
to
infile.open(filename)
and it's still not working. I get this error:

error C2664: 'void __thiscall std::basic_ifstream<char,struct std::char_traits<char> >::open(const char *,int)' : cannot convert parameter 1 from 'class std::basic_string<ch
ar,struct std::char_traits<char>,class std::allocator<char> >' to 'const char *'
No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called

"filename" is declared as a string at the moment...should it be?

and I don't use the proper std::cin style because my teacher's a moron and never taught us that. He actually didn't even teach us getline, I learned it myself. I'm a grade 12 in a high school programming class, they don't teach us too much. I honestly don't even understand std strings. Except that I include using namespace std; after my libraries. Will std::cin>>filename take an entire string, including spaces?
Author:  md [ Mon Mar 20, 2006 4:49 pm ]
Post subject: 
No, getline will get the entire line with white spaces cin >> filename will stop at hte first space. Either way works fine though.

As for the string error it's because yes; the function is expecting a character array and you are passing it a class. use the c_str() function to get a character array representing the string. Normally I'd recoment using std::strng simply becuse it keeps you from having to worry about things like memory management when all you want is a short string. Then just use c_str() when you need a character array.
Author:  AnubisProgrammer [ Mon Mar 20, 2006 4:52 pm ]
Post subject: 
yes....a character array. Like I said, I'm trying to learn a lot of this myself, so could you possibly show an example of how to start a character array or how to use the c_str() function...because I have no idea...sorry.
Author:  md [ Mon Mar 20, 2006 4:59 pm ]
Post subject: 
Example fo converting to a character string:
c++:

        std::string test = "Blarg";
        const char *character_string;

        character_string = test.c_str();


If you don't know what a character string is, and even if you can't pass varaibles properly then really I dunno that you should be messing around with file IO.
Author:  AnubisProgrammer [ Mon Mar 20, 2006 5:19 pm ]
Post subject: 
I agree, without learning more I probably shouldn't be messing with file I/O. However, my class has gotten an assignment that deals with I/O, even though the teacher hasn't taught I/O at all...as you can tell, I don't like my teacher. He's the reason I ask so many questions on here, you guys actually answer them. Anywho, I tried that conversion and hit a little problem. Last question (I hope):

code:
string file;
cin>>file;
const char *filename;
filename=file.c_str();
infile.open(filename);


with the proper ifstream infile and such, should that work?
Author:  md [ Mon Mar 20, 2006 5:24 pm ]
Post subject: 
Whoops! I totally forgot about that constant declaration... if you took that out it would work. Teh code I gave was not to show how to use c_str() in your code; just a very general example of how to call it.

You could much more easily use
c++:

string file;
cin>>file;
infile.open(file.c_str());

:


You can syndicate this boards posts using the file backend.php or view the topic map using sitemap.php.

Terms of Use | Privacy Policy