How to read file in C++?

HelloWorld · #1 (**permalink**) 08-03-2007, 12:19 PM

here's the code that I have for now:

PHP Code:

  #include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main() {
ifstream infile, infile2;
infile.open("pdf1.txt");
infile2.open("pdf2.txt");
if (!infile) {
cout << "Error in reading pdf1.txt" << endl;
exit(1);
}
if (!infile2) {
cout << "Error in reading pdf2.txt" << endl;
exit(1);
}
string r = infile.read();
string r2 = infile2.read();
while (infile.read() != null) {
if (r != r2) {
cout << "Different at " << r << endl;
}
infile.read();
}
return 0;
}

I want it to read it line by line, but also have the ability to tokenize (in java lol..) something, please help me where should I look for this information and suggestion to what do I need to do to read a file and compare it to other file.

I want to compare one file with the other to see if they're the same because there's going to be a server backup at my workplace, so we have to make sure if we still have the same data from the previous server.

rpgfan3233 · #2 (**permalink**) 08-03-2007, 01:00 PM

I'm not sure about tokenization (not sure what that actually is), but one thing that your code is doing is checking whether a file is equal to false or 0 after you open it. The method you should be using is if (!infile.is_open()), for example. Another, perhaps better, way you could check is to check the failbit using if (infile.fail()) since the failbit is set if the requested file is not opened or the variable already represents an open file (calling open() on the same variable without closing it properly makes the call to open() fail, setting the failbit).

As for file comparison, perhaps you would prefer binary read/write instead of text. That way, you could compare the bytes rather than just text, where text may not actually be text. Also, if you wanted to, you could shorten your code a bit:

Code:

//just remove the "| ifstream::binary" if you want to use text
ifstream infile("pdf1.txt", ifstream::in | ifstream::binary);
ifstream infile2("pdf2.txt", ifstream::in | ifstream::binary);

You could simply have them on the same line like you do in your code, but I split them up for slightly better readability.

HelloWorld · #3 (**permalink**) 08-03-2007, 04:12 PM

Is there a way for me to read it line by line in C++?
I'm trying to make something like:

Code:

// Sorry, this is Java code in C++ forum
String line = br.readLine();

Lee · #4 (**permalink**) 08-03-2007, 04:36 PM

Just found this bit of code:

Quote:

string str;
getline(infile,str); // Get the frist line from the file, if any.

while (infile) { // Continue if the line was sucessfully read.

// do somethin with str here

getline(infile,str); // Try to get another line.
}

I edited it to put 1 example of your code in, i think if you was to test this you would find its what you want, at least i hope.

Hope it helps somewhat,
Lee.

Bench · #5 (**permalink**) 08-03-2007, 05:33 PM

When reading lines from files in a loop, the best place to put the getline is inside the while condition itself - this is the simplest way to make sure the file doesn't read over the end, and that the final iteration of the loop doesn't cause undefined behaviour

Code:

string str;
while( getline( infile, str ) )
{
    // ... str was read from infile OK
}

Sometimes its necessary to put the getline elsewhere, maybe if the sequence of file reads is a more complicated process, you want to put checks at every point along the way make sure each step is successful

Code:

string str;
int i;
while( true )
{
    infile >> i;
    if ( !infile )
        break;

    getline( infile, str );
    if ( ! infile )
        break;
    // read was successful - can do something with str and 'i'
}

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The Following User Says Thank You to rpgfan3233 For This Useful Post:
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