Thread: Help using argc and argv[]

Hi all, I'm getting a MS visual C++ Runtime Library error when I run this I just want it to print the error when no arguments are put forward or just the return key is pressed after calling the program on the cmd line. Have I misunderstood what cerr is? whats up with the Runtime error?

Many thanks in advance,
hewlettuser.

code:

Code:

#include <fstream>
#include <iostream>
using namespace std;


int main(int argc, char* argv[])
{
	char ch;		
	fstream file;		


	if(argc == 1 || argv[1] == "\n")
	{
		cerr << "stats: No filename specified"
		<<endl << endl;
			return 1;	//Return code
	}
return 0;

You can't use == to compare two C-style strings. For == to work, one of the strings must be a C++ string. To compare C-style strings, use strcmp (from <cstring>). Alternatively, make one of the strings a C++ string. Or, since you're really only comparing a single character, you could use == to compare the character.

Code:

// compare as C-strings (include <cstring>)
if (strcmp(argv[1], "\n") == 0)

// compare as C++-strings (include <string>)
if (argv[1] == string("\n"))
//or
if (string(argv[1]) == "\n")
//or
if (string(argv[1]) == string("\n"))

// compare a single character
if (argv[1][0] == '\n')
// this is not exactly the same since it's possible that the next character
// is not '\0', so to be exactly the same it would need to be
if (argv[1][0] == '\n' && argv[1][1] == '\0')

However, it's unusual (though not impossible) that a command-line string will consist solely of a newline character.

It's best to just assume it's a good filename if the parameter is present, then try opening the file and check for errors there.

Code:

#include <iostream>
#include <fstream>
using namespace std;

int main(int argc, char* argv[])
{
    if (argc != 2)
    {
        cerr << "Usage: stats filename\n";
        return 1;
    }

    fstream f(argv[1]);
    if (!f) {
        cerr << "stats: Can't open file " << argv[1] << '\n';
        return 1;
    }

    return 0;
}

The last character is being printed twice because you're not checking for the end of the file properly. The end-of-file condition is detected by the get method not the eof method, i.e., the eof method does not "look ahead" to see the end of the file; instead it can only report that the end of the file has already been hit by an input method like get. In your code when get hits the end of the file you still print out ch, which contains the character that was read during the previous iteration.

It's best not to use the eof method and instead control the loop with the function you use to read from the file:

Code:

    while (file.get(ch))
    {
        cout << ch;
    }

Thank-you for all your amazing feedback, its helped me a great deal, cheers.