如何在 C++ 中获取文件？

Question

So the teacher has posed this assignment:

You have been hired by the United Network Command for Law Enforcement, and you have been given files containing null cyphers you have to decrypt.

So for the first file given (as an example), every other letter is correct (ie: 'hielqlpo' is hello (assuming you start with the first letter). My first question is, how do I read in a file? The document is on my desktop in a folder and the file is named document01.cry. I'm not sure the command I need to put that file into the program.

I'm also not overly sure how to grab a letter and skip a letter, but honestly I want to tinker with that before I post that question. So for now..:my question is as stated in the title? How do you grab a file for reading in C++?

If it makes a difference (As I'm sure it does), I'm using Visual C++ 2008 Express Edition (because it's free and I like it, I've also attached what I have so far. please keep in mind it's -very- basic...and I added the getchar(); at the end so when it does run properly, the window stays open so I can see it (as Visual Express tends to close the window as soon as it's done running.)

The code so far:

#include<iostream>

using namespace std;

int main()
{
    while (! cin.eof())
    {
        int c = cin.get() ;
        cout.put(c) ;
    }
    getchar();
}

PS: I realize that this code grabs and puts out every character. For now that's fine, once I can read in the file I think I can tinker with it from there. I'm also poking at a book or two I have on C++ to see it anything pops up and screams "Pick me!" Thanks again!

EDIT:: Also curious, is there a way to input the file you want? (Ie:

char filename;
cout << "Please make sure the document is in the same file as the program, thank you!" << endl << "Please input document name: " ;
cin >> filename;
cout << endl;
ifstream infile(filename, ios::in);

This code doesn't work. It shoots back an error saying the char can't be converted to a const char *. How can this problem be fixed?

EDIT 2: Never mind about said part 2, I found it out! Thanks again for the assistance!

Answer 1

To do file operations, you need the correct include:

#include <fstream>

Then, in your main function, you can open a file stream:

ifstream inFile( "filename.txt", ios::in );

or, for output:

ofstream outFile( "filename.txt", ios::out );

You can then use inFile as you would use cin, and outFile as you would use cout. To close the file when you are done:

inFile.close();
outFile.close();

Answer 2

C++: Input/Output with files

Answer 3

[EDIT] include support for command line arguments [EDIT] Fixed possible memory leak [EDIT] Fixed a missing reference

#include <fstream>
#include <iostream>

using namespace std;

int main(int argc, char *argv){
    ifstream infh;  // our file stream
    char *buffer;

    for(int c = 1; c < argc; c++){
        infh.open(argv[c]);

        //Error out if the file is not open
        if(!infh){
            cerr << "Could not open file: "<< argv[c] << endl;
            continue;
        }

        //Get the length of the file 
        infh.seekg(0, ios::end);
        int length = infh.tellg();

        //reset the file pointer to the beginning
        is.seekg(0, ios::beg);

        //Create our buffer
        buffer = new char[length];

        // Read the entire file into the buffer
        infd.read(buffer, length);

        //Cycle through the buffer, outputting every other char
        for(int i=0; i < length; i+= 2){
            cout << buffer[i]; 
        }
        infh.close();
    }
    //Clean up
    delete[] buffer;
    return 0;
}

Should do the trick. If the file is extremely large, you probably shouldn't load the entire file into the buffer.

Answer 4

I figured it out, To be honest no one answer helped, it was a combination of the three. plus the comments from them, Thank you all very much. I've attached the code I used, as well as a copy of the document. The program reads in every character: then spits out the deciphered text. (IE. 1h.e0l/lqo is hello) The next step (extra credit) is to set it up so the user inputs how many characters to skip before reading in the next character to input.

This step I intend to do on my own, but again, thank you very much for all the assistance everyone, Proving once again how awesome this site is, one line of code at a time!

EDIT:: Code adjusted to accept user input, as well as allow for multiple uses without recompiling (I realize it looks like a huge sloppy mess, but that's why it's EXTREMELY commented...because in my mind it looks nice and neat)

#include<iostream>
#include<fstream>   //used for reading/writing to files, ifstream could have been used, but used fstream for that 'just in case' feeling.
#include<string>    //needed for the filename.
#include<stdio.h>   //for goto statement

using namespace std;

int main()
{
    program:
    char choice;  //lets user choose whether to do another document or not.
    char letter;  //used to track each character in the document.
    int x = 1;    //first counter for tracking correct letter.
    int y = 1;    //second counter (1 is used instead of 0 for ease of reading, 1 being the "first character").
    int z;        //third counter (used as a check to see if the first two counters are equal).
    string filename;    //allows for user to input the filename they wish to use.
    cout << "Please make sure the document is in the same file as the program, thank you!" << endl << "Please input document name: " ;
    cin >> filename; //getline(cin, filename);
    cout << endl;
    cout << "'Every nth character is good', what number is n?: ";
    cin >> z;   //user inputs the number at which the character is good. IE: every 5th character is good, they would input 5.
    cout << endl;
    z = z - 1;  //by subtracting 1, you now have the number of characters you will be skipping, the one after those is the letter you want.
    ifstream infile(filename.c_str()); //gets the filename provided, see below for incorrect input.
    if(infile.is_open()) //checks to see if the file is opened.
    {
        while(!infile.eof())    //continues looping until the end of the file.
        {   
                infile.get(letter);  //gets the letters in the order that that they are in the file.
                if (x == y)          //checks to see if the counters match...
                {
                    x++;             //...if they do, adds 1 to the x counter.
                }
                else
                {
                    if((x - y) == z)            //for every nth character that is good, x - y = nth - 1.
                    {
                        cout << letter;         //...if they don't, that means that character is one you want, so it prints that character.
                        y = x;                  //sets both counters equal to restart the process of counting.
                    }
                    else                        //only used when more than every other letter is garbage, continues adding 1 to the first
                    {                           //counter until the first and second counters are equal.
                        x++;
                    }
                }
        }
        cout << endl << "Decryption complete...if unreadable, please check to see if your input key was correct then try again." << endl;
        infile.close();
        cout << "Do you wish to try again? Please press y then enter if yes (case senstive).";
        cin >> choice;
        if(choice == 'y')
        {
            goto program;
        }
    }
    else  //this prints out and program is skipped in case an incorrect file name is used.
    {
        cout << "Unable to open file, please make sure the filename is correct and that you typed in the extension" << endl;
        cout << "IE:" << "     filename.txt" << endl;
        cout << "You input: " << filename << endl;
        cout << "Do you wish to try again? Please press y then enter if yes (case senstive)." ;
        cin >> choice;
        if(choice == 'y')
        {
            goto program;
        }
    }
    getchar();  //because I use visual C++ express.
}

EDIT:::

I tried inserting the text, but I couldn't get it to come out right, it kept treating some of the characters like coding (ie an apostrophe apparently is the equivalent of the bold command), but you could just try putting in "0h1e.l9lao" without the parenthesis into a.txt and it should give the same outcome.

Thanks again everyone for the help!

Answer 5

Although your queston has been answered, two little tips: 1) Instead of counting x and y to see if you are on an odd or even character you can do the following:

int count=0;
while(!infile.eof())
{       
    infile.get(letter);
    count++;
    if(count%2==0)
    {
        cout<<letter;
    }
}

% essentially means 'remainder when divided by,' so 11%3 is two. in the above it gives odd or even.

2) Assuming you only need to run it on windows:

system("pause");

will make the window stay open when its finished running so you don't need the last getchar() call (you may have to #include<Windows.h> for that to work).