Please help me with a code to read the value of a byte in a file.

koxson · 05-07-2008, 08:45 AM

Hello

I'm new to c++ and to all forms of programming and am trying to get the source code for a program that prints out the hex value of any given byte in a file.

I've gone through; this site, google, yahoo, cplusplus.com, C++ for dummies 5th Edition and, unbelievably, more. The answer just seems to elude me. No one says how to do this. I can only get the size of a file or read a txt file, but not the hex, or any other value of any given byte in a file. Thats the problem. Can ANYONE show me the source of a simple program that does this?

I'm going to show you the closest I could get to this a majority may lose your lunch when you see it but you'll understand what a state I'm in with which I can only get the character of in a txt file and not the value of a byte of anyfile. I know I'm supposed to make the char unsigned but that stops me from compiling at all and give me errors.

[tag]

Code:

#include <iostream>
#include <fstream>
using namespace std;
int x, y;
int main () {
char * buffer;
  Start:
  cout << "Enter the number of the byte you are looking for: ";
  cin >> x;
  if (x<1) {
  cout << "We begin counting at 0. Please try again.\n"; goto Start;}
  ifstream is;
  is.open ("Yo.bin", ios::in|ios::binary);
  is.seekg (x, ios::beg);
  is.read (buffer,1);
  is.close();
  cout << "\nThe hex value of byte number"<< x <<" is ";
  cout.write (buffer,1);
  cout << "\n\nDo you want to return to the start of this program?\n";
  cout << "Press 1 for YES or any other key for NO: ";
  cin >> y;
  if (y == 1) {goto Start;}
  if (y == !1) {goto End;}
  End:
  cout << "Goodbye!\n";
  delete[] buffer;
        return 0;}
[/tag]

Thanks for reading.

anon · 05-07-2008, 09:14 AM

Whether something is displayed as a hex value or a decimal value or a character is just a matter of output representation. The byte itself is still the same.
To change the representation you can use I/O format flags. For example:

Code:

#include <iostream>
int main()
{
    char c = 'A';
    std::cout << std::showbase << std::hex << int(c);
}

In your code, buffer doesn't seem to be allocated, and since you are going to put just one character into it, may-be it could be declared as

Code:

char buffer[1] = {0};

(May-be you could also read into a single char, but that would probably require ugly casts.)

rags_to_riches · 05-07-2008, 09:18 AM

You never allocate memory for buffer (and yet you delete it).

Code:

char *buffer = new char[2];

Or, seeing as you're only reading a single char anyway,

Code:

char buffer;

and get rid of the delete call.

Gotos? In *my* C++ program?

Code:

int y = 0;
while (!y)
{
    // Do your thing
}

matsp · 05-07-2008, 09:19 AM

Is this a "find five problems"?

You don't need that much code to make me loose my lunch, surely this can easily be written without goto's:

Code:

  if (x<1) {
  cout << "We begin counting at 0. Please try again.\n"; goto Start;}

Note that I'm not religiously against goto's, just think they should be avoided when there is a better, simpler alternative.
Also, your error message doesn't seem to match exactly with the if-statement.

Second problem:

Code:

char * buffer;

This is a pointer to memory, but it's never assigned to any particular memory location, so it's going to crash, I suspect. Use a char variable, and pass the address to read.

Third problem:

Code:

cout.write (buffer,1);

This will display the ascii character (or whatever the local font representation of the data is), not the hex-value. To show a hex value, you need something like

Code:

cout << hex << buffer;

Fourth problem:

Code:

  cout << "Press 1 for YES or any other key for NO: ";
  cin >> y;

This would fail if the user types in "a" for an answer.

Problem five:

Code:

  if (y == !1) {goto End;}
  End:

So, not only do you use goto for ABSOLUTELY no reason, your if-statement is also not doing what you expect - not that you'd notice, because you end up at exactly the same place whether the if-statement is true or not, as there is no code between the goto and the lable end, but I challenge you to find a way to NOT print the message in this code:

Code:

  if (y == !1) {goto End;}
     cout << "Got here" << endl;
  End:

--
Mats

**Salem** · 05-07-2008, 09:21 AM

> delete[] buffer;
You didn't allocate it, so you can't use it and you can't delete it.

Also, use loop constructs in place of those goto statements.

Also, there is no point in x and y being global.

Edit: Meh, beaten *3

matsp · 05-07-2008, 09:25 AM

Quote:

Originally Posted by Salem

> delete[] buffer;
You didn't allocate it, so you can't use it and you can't delete it.

Also, use loop constructs in place of those goto statements.

Also, there is no point in x and y being global.

Edit: Meh, beaten *3

And I just realized it wasn't find five faults, but find eight faults...

--
Mats

m37h0d · 05-07-2008, 09:37 AM

you had the right idea, but a few details were off. consider the following:

Code:

        int x;
        bool go=true;
        while(go)
        {
                cout << "Enter the number of the byte you are looking for: ";
                cin >> x;
                if (x<0)
                {
                        cout << "We begin counting at 0. Please try again.\n";
                }
                else
                {
                        unsigned int y;
                        char buffer;
                        ifstream is;
                        is.open ("Yo.bin", ios::binary);
                        is.seekg (x, ios::beg);
                        is.read (&buffer,1);
                        is.close();
                        cout << "\nThe hex value of byte number "<< x <<" is " << hex << (int)buffer << endl;
                        cout << "\n\nDo you want to return to the start of this program?\n";
                        cout << "Press 0 for NO or any other key for YES: ";
                        cin >> y;
                        if(!y)
                        {
                                go = false;
                        }
                }
        }
        cout << "Goodbye!\n";
        return 0;

good luck.

matsp · 05-07-2008, 09:41 AM

Code:

cout << "\nThe hex value of byte number "<< x <<" is " << hex << (int)(unsigned char)buffer << endl;

if you want to avoid having it sign-converted.

--
Mats

m37h0d · 05-07-2008, 09:46 AM

could you not just cast it once to unsigned int?

matsp · 05-07-2008, 09:48 AM

Quote:

Originally Posted by m37h0d

could you not just cast it once to unsigned int?

Perhaps, but I feel that it will still sign-extend to int first, then "unsign" that.

--
Mats

koxson · 05-29-2008, 10:20 AM

I wanted to come back to thank everyone on this thread for their help, especially m37h0d for putting it in terms the layman to the world of c++ can understand.

Thanks m37h0d!

Elysia · 05-29-2008, 10:56 AM

http://cpwiki.sourceforge.net/Common...llocating_them

m37h0d · 05-29-2008, 11:36 AM

Quote:

Originally Posted by koxson

I wanted to come back to thank everyone on this thread for their help, especially m37h0d for putting it in terms the layman to the world of c++ can understand.

Thanks m37h0d!

i did?

yay me!

making the code as clean and simple as possible should always be a goal imo. i didn't write that in any fashion that was supposed to be "for the layman"; i just tried to make it as clear and simple as possible, which as far as i'm concerned is always good programming!

05-07-2008, 08:45 AM	#1
koxson Registered User Join Date: May 2008 Posts: 22	Please help me with a code to read the value of a byte in a file. Hello I'm new to c++ and to all forms of programming and am trying to get the source code for a program that prints out the hex value of any given byte in a file. I've gone through; this site, google, yahoo, cplusplus.com, C++ for dummies 5th Edition and, unbelievably, more. The answer just seems to elude me. No one says how to do this. I can only get the size of a file or read a txt file, but not the hex, or any other value of any given byte in a file. Thats the problem. Can ANYONE show me the source of a simple program that does this? I'm going to show you the closest I could get to this a majority may lose your lunch when you see it but you'll understand what a state I'm in with which I can only get the character of in a txt file and not the value of a byte of anyfile. I know I'm supposed to make the char unsigned but that stops me from compiling at all and give me errors. [tag] Code: #include <iostream> #include <fstream> using namespace std; int x, y; int main () { char * buffer; Start: cout << "Enter the number of the byte you are looking for: "; cin >> x; if (x<1) { cout << "We begin counting at 0. Please try again.\n"; goto Start;} ifstream is; is.open ("Yo.bin", ios::in\|ios::binary); is.seekg (x, ios::beg); is.read (buffer,1); is.close(); cout << "\nThe hex value of byte number"<< x <<" is "; cout.write (buffer,1); cout << "\n\nDo you want to return to the start of this program?\n"; cout << "Press 1 for YES or any other key for NO: "; cin >> y; if (y == 1) {goto Start;} if (y == !1) {goto End;} End: cout << "Goodbye!\n"; delete[] buffer; return 0;} [/tag] Thanks for reading. Last edited by koxson; 05-07-2008 at 08:47 AM.
koxson is offline

05-07-2008, 09:18 AM	#3
rags_to_riches and the Hat of Ass Join Date: Dec 2007 Posts: 731	You never allocate memory for buffer (and yet you delete it). Code: char buffer = new char[2]; Or, seeing as you're only reading a single char anyway, Code: char buffer; and get rid of the delete call. Gotos? In my* C++ program? Code: int y = 0; while (!y) { // Do your thing }
rags_to_riches is offline

05-07-2008, 09:19 AM	#4
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	Is this a "find five problems"? You don't need that much code to make me loose my lunch, surely this can easily be written without goto's: Code: if (x<1) { cout << "We begin counting at 0. Please try again.\n"; goto Start;} Note that I'm not religiously against goto's, just think they should be avoided when there is a better, simpler alternative. Also, your error message doesn't seem to match exactly with the if-statement. Second problem: Code: char * buffer; This is a pointer to memory, but it's never assigned to any particular memory location, so it's going to crash, I suspect. Use a char variable, and pass the address to read. Third problem: Code: cout.write (buffer,1); This will display the ascii character (or whatever the local font representation of the data is), not the hex-value. To show a hex value, you need something like Code: cout << hex << buffer; Fourth problem: Code: cout << "Press 1 for YES or any other key for NO: "; cin >> y; This would fail if the user types in "a" for an answer. Problem five: Code: if (y == !1) {goto End;} End: So, not only do you use goto for ABSOLUTELY no reason, your if-statement is also not doing what you expect - not that you'd notice, because you end up at exactly the same place whether the if-statement is true or not, as there is no code between the goto and the lable end, but I challenge you to find a way to NOT print the message in this code: Code: if (y == !1) {goto End;} cout << "Got here" << endl; End: -- Mats __________________ Compilers can produce warnings - make the compiler programmers happy: Use them! Please don't PM me for help - and no, I don't do help over instant messengers.
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05-07-2008, 09:21 AM	#5
Salem and the hat of vanishing Join Date: Aug 2001 Location: The edge of the known universe Posts: 21,214	> delete[] buffer; You didn't allocate it, so you can't use it and you can't delete it. Also, use loop constructs in place of those goto statements. Also, there is no point in x and y being global. Edit: Meh, beaten *3 __________________ If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut. Up to 8Mb PlusNet broadband from only £5.99 a month!
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05-07-2008, 09:41 AM	#8
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	Code: cout << "\nThe hex value of byte number "<< x <<" is " << hex << (int)(unsigned char)buffer << endl; if you want to avoid having it sign-converted. -- Mats __________________ Compilers can produce warnings - make the compiler programmers happy: Use them! Please don't PM me for help - and no, I don't do help over instant messengers.
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05-07-2008, 09:46 AM	#9
m37h0d 3735928559 Join Date: Mar 2008 Posts: 662	could you not just cast it once to unsigned int?
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05-29-2008, 10:20 AM	#11
koxson Registered User Join Date: May 2008 Posts: 22	I wanted to come back to thank everyone on this thread for their help, especially m37h0d for putting it in terms the layman to the world of c++ can understand. Thanks m37h0d!
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