Trouble with program terminating due to file.find_last_of command

[plbaldri]

I have written a very simple and basic program for an intro Comp Sci class and am having trouble with it crashing. The program needs to accept a filename on the command line at the prompt and has to verify that the filename has a .txt extension. The code below works great as long as a '.' is entered regardless of the extension, but if no '.' is entered into the file name the program crashes. I'm sure it has to do with the fact the program is looking for the '.' in order to get the extension.. but I'm at a loss as to how to keep the program from crashing when it does not find it. I'm sure it's a very simple solution I'm overlooking :) I underlined the part of the code I think is the problem. I'm compiling in Visual C++ Express and running it on Vista. The error I get when it crashes is that my application has requested the runtime to terminate in an unusual way.

#include<iostream>
#include<fstream>
#include<string>
#include<iomanip>
using namespace std;
 
int main(int argc,char *argv[])
{
string line;
string extension;
string fileName;
string file=argv[1];
    if(argc<2)
    {
        cout<<"No argument given \n"<<"Usage: eng2morse filename.txt \n";
        exit(1);
    }
int l=strlen(*argv);
    int ext = file.find_last_of(".");
    extension=file.substr(ext,5);
    fileName=file.substr(0,ext)+".mrs";
    if(extension==".txt"){}
    else
    {
        cout<<"Invalid file extension. \n"<<"Usage: eng2morse filename.txt \n";
        return(1);
    }
 
ifstream english;
ofstream morse;
english.open(argv[1],ios::in);
if ( english.fail() )
   {
      cout<<"Invalid file name. \n"<<"Usage: eng2morse filename.txt \n";
      exit(1);
   }
morse.open(fileName.c_str(),ios::app);
 

Many thanks!!!!

[Laharl]

To keep it from crashing, do an immediate check on ext after find_last_of. The find_last_of function returns string::npos if the character is not found, which is not usable as a parameter for substring since it is not a position in the string, by definition (I'd guess it's negative). Then, if the '.' is found, you can check if the extension actually is .txt.

[plbaldri]

To keep it from crashing, do an immediate check on ext after find_last_of. The find_last_of function returns string::npos if the character is not found, which is not usable as a parameter for substring since it is not a position in the string, by definition (I'd guess it's negative). Then, if the '.' is found, you can check if the extension actually is .txt.

Thanks for the reply! I actually did make a change and ran a check right after, but what I did was change from find_last_of to simply file.find (not sure why i didn't do that to start with) and I checked for ext>0.. and exited the program with error message if not >0... it seems to be working well so far. Could there be any unforseen complications with my method that I may be missing?

[Laharl]

You could simply check if ext == string::npos, which would guarantee accuracy, rather than the educated guess we have about the value of npos.