Thread: Reading text file by line number

This assignment is driving me insane!

I have a text file that looks like this:

Code:

3
CODE TIME DESCRIPTION
CODE TIME DESCRIPTION
CODE TIME DESCRIPTION
4
CODE1 CODE2 TIME1 TIME2
CODE1 CODE2 TIME1 TIME2
CODE1 CODE2 TIME1 TIME2
CODE1 CODE2 TIME1 TIME2

All of the codes are a 3 letter code, the times are in the form 00:00 and the description is the full version of the code. The first number tells you how many lines follow it and the second tells how many lines follow that.

I'm trying to read the file one line at a time, by reading the first line, the next group of lines, the next number line and the last lines. But I'm stuck due to the line number having to be converted to a string. Is there any way of using an int for the line number?

Here's my code so far:

Code:

ifstream inFile;
     inFile.open(fileName.c_str(), ios::in);
     
     if(!inFile) {
         cerr << "Error: can't open input file" << fileName << endl;
         exit(1);
     }
     
     int noOfAirports;
     string aLine = "1";
     
     //Reads line 1 of file to find out how many airports there are
     while(getline(inFile, aLine)) {
         inFile >> noOfAirports;
     }
     
     int lineNo = 2 + noOfAirports;
     string bLine;
     stringstream out;
     out << lineNo;
     bLine = out.str();

     //code needs to go here to read next group of lines, something along the lines of for(int i=2; i <(lineNo); i++)
     
     int noOfFlights;
     //Reads file to find out how many flights there are
     while(getline(inFile, bLine)) {
         inFile >> noOfFlights;
     }

     //code here some sort of loop to read from noOfFlights + 1 to inFile.eof()
     
     inFile.close();

So, you can use strtol on the c_str() element of the string that you read in, I suppose. Or read it in as a stringstream and then write it from the stringstream into an int.

It may compile, but it probably doesn't do what you want it to. You're not reading from the file to your stringstream, but putting the number you want in the stringstream, and then reading from the file to cLine.

And how do you convert a string time1 that looks like "7:30" to int hour 7 and int min 30? (Your program will do the same, I would guess.)

Okay, so I have the wrong code then? I assumed that getline(inFile, cLine) would take line number cLine in file inFile and read that line. From what you've said, I guess that's not correct? How would I do that then?

I haven't converted any strings to ints at all, all I've done is the reverse. To do that, I did this:

Code:

         int hour;
         string hourNo;
         stringstream out;
         out << hour;
         hourNo = out.str();

         int min;
         string minNo;
         stringstream out;
         out << min;
         minNo = out.str();

         string time = hourNo + ":" + minNo;

If you're reversing stringstream, would you just be reversing the arrows? So maybe:

Code:

stringstream out;
string time = in.str();
out >> hour >> ":" >> min;
int hour;
int min;

I'm not sure I have it quite right. :/

Would it be a "."? Or maybe a " "? Or maybe it's another variable to store the ":"? I'm not really sure.

Nothing I try will compile. First I was getting that hour and min were undeclared, and then when I declared them I was getting an ambiguous overload.

I'm not sure this part is as important as the problem with getline(inFile, cLine), though. That's the part that's ruining the entire program right now, and I don't know what the problem is.

What leads you to believe there's something wrong with getline?

Oh, that was what I was led to believe from above:

It may compile, but it probably doesn't do what you want it to. You're not reading from the file to your stringstream, but putting the number you want in the stringstream, and then reading from the file to cLine.

Okay, so I have the wrong code then? I assumed that getline(inFile, cLine) would take line number cLine in file inFile and read that line. From what you've said, I guess that's not correct? How would I do that then?

So, C++ is not a write-only language. You should be able to read what you've written to see what it is going to do. For instance, seeing the code

Code:

for (int i=2; i < lineNo; i++) {
         string cLine;
         stringstream out;
         out << i;
         cLine = out.str();
         
         getline(inFile, cLine);
     }

you should be able to tell that out << i puts i into out and that getline reads from inFile into cLine. (If you think that getline cares about the previous value of cLine, then that's just being silly.)

Now, it looks like you've been programming by magic (i.e., typing commands/functions and hoping they do what you want). I would suggest looking up all those functions you're using, like getline, either in your reference book or online at say cplusplus.com, to see what they actually do, and then putting all your tools together. Some hints: once you've read that first number from the file, that number should go somewhere in your for loop. It is neither necessary, possible, nor desirable for you to read a line from a file by line number.