How to read a byte and save ASCII value of byte in

问题:

I have a simple question that is confusing me.

Goal: I want to read a given byte from a file (say the first byte) and make int x with the ASCII value of that byte. So, for example, if the byte/character is 'a', I want x to be 97 (= 61 in hex). I have the following reading the first byte of the file example.txt:

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main(){
  unsigned int x;
  unsigned char b;
  ifstream myFile ("example.txt", ios::out | ios::binary);
  myFile.seekg (0, ios::beg);
  myFile >> b;
  x = (unsigned int)b;
  cout << hex << x;
  return b;
}

Problem: If the first byte is represented by 08, then indeed I get an output of 8. But if the byte is represented by 09, then I get 0. I have noticed that I seem to get the following byte unless that byte is also 09. I don't know if my problem is only when the byte is represented in ASCII by 09.

Question: So how to I read say the first (or third or whatever) byte from a file and make an int with the ASCII value of that byte?

(I am on Windows XP)

回答1:

This should fix it.

 myFile >> noskipws >> b;

回答2:

Couple of suggestions:

• Check if the file has actually been opened. If it wasn't, ensure the file is in the current dir, or supply the full path.
• You probably want ios::in (not ios::out).
• Use noskipws unless you actually want to skip whitespaces.
• What's the purpose of returning the character? The convention is to interpret a non-zero exit code as a failure (though admittedly, there is no strict standard on this).

The following program reads the 4th character and prints its HEX value just fine for me:

#include <iostream>
#include <fstream>
#include <stdlib.h>

using namespace std;

int main() {

    ifstream myFile("<path>\\example.txt", ios::in | ios::binary);

    if (myFile) {

        unsigned char b;
        myFile.seekg(3) >> noskipws >> b;

        if (myFile) { // File was long enough?
            unsigned int x = b;
            cout << hex << x;
            return EXIT_SUCCESS;
        }

    }

    return EXIT_FAILURE;

}

(Replace <path> with actual path.)

回答3:

Try reading using ifstream::read instead of operator>>. This has worked for me:

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main(){
  unsigned int x;
  unsigned char b;
  ifstream myFile ("example.txt", ios::out | ios::binary);
  myFile.seekg (0, ios::beg);
  myFile.read(reinterpret_cast<char*>(&b), sizeof(b));
  x = (unsigned int)b;
  cout << hex << x;
  return b;
}