Converting a binary string to hex

This is a discussion on Converting a binary string to hex within the C++ Programming forums, part of the General Programming Boards category; I have a string in binary and I need to convert it to hex. (Obvious right?) What I'm trying to ...

  1. #1
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    Converting a binary string to hex

    I have a string in binary and I need to convert it to hex. (Obvious right?)

    What I'm trying to do is this:
    Code:
    for (z = 0; z < 20; z++)
    	sprintf(string, "%02x", out[z]);
    Both out and string are defined as unsigned char[20]

    Now, I know that hex characters take two bytes, so I also tried
    Code:
    for (z = 0; z < 20; z++)
    	y += sprintf(string+y, "%02x", out[z]);
    Where y is initialized to 0. I keep getting damage after normal block errors. I thought it was becuase I was overwriting memory so I increased the size of string to 200, which should be MORE than enough room, but I get the same error. Any suggestions?

  2. #2
    Gawking at stupidity
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    You need to look at 4 binary digits to make 1 hex digit. Try:
    Code:
    itsme@itsme:~/C$ cat bintohex.c
    #include <stdio.h>
    #include <string.h>
    
    int main(void)
    {
      unsigned char binstr[] = "1001110001100111";
      unsigned char hexstr[5];
      unsigned int hexnum;
      unsigned char four[4];
      int i, j;
    
      for(i = 0;i < 4;++i)
      {
        hexnum = 0;
    
        strncpy(four, binstr + (i * 4), 4);
        for(j = 0;j < 4;++j)
          hexnum += (four[j] - '0') << (3 - j);
    
        sprintf(hexstr + i, "%X", hexnum);
      }
    
      puts(hexstr);
    
      return 0;
    }
    Code:
    itsme@itsme:~/C$ ./bintohex
    9C67
    itsme@itsme:~/C$
    If you understand what you're doing, you're not learning anything.

  3. #3
    Hardware Engineer
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    There is a "secret" standard library function called strtol() [String To Long]. It can convert a string representation in (almost) any base to a type long.

    Once you have a "regular" integer value, you can display it in decimal, octal, or hex with cout, or you can use sprintf() if you want a string representation.

    code fragment...
    Code:
       int Base, x;
       char InputString[40];
       char *pEnd = NULL;          // Required for strtol()
    
       cout << "Base? (2-36, 0 to exit) " ;
       cin >> Base;
    
       if(!Base)
                return 0;
    
        cout << "Number? ";
        cin >> InputString;
        x = (int)strtol(InputString, &pEnd, Base);     // String to long
    
        cout << "\t" << dec << x << "\t\t Decimal" << endl;
        cout << "\t" << oct << x <<  "\t\t Octal" << endl;
        cout << "\t" << hex << x << "\t\t Hex" << endl;
    Last edited by DougDbug; 07-25-2006 at 11:35 AM.

  4. #4
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    If I wanted to print x to a string using sprintf, what flag would I use within the sprintf? %x?

  5. #5
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    It occurs to me that I was unclear, what I'm trying to convert is not a string of ASCII characters representing binary, it's pure, raw binary data (Specifically from a SHA-1 message digest).

  6. #6
    Gawking at stupidity
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    So you basically have an array of 20 unsigned 1-byte numbers? Are you just wanting to convert those 20 numbers into a string that represents them in hex?

    Try changing the code you had:
    Code:
    for (z = 0; z < 20; z++)
    	y += sprintf(string+y, "%02x", out[z]);
    to...
    Code:
    for (z = 0; z < 20; z++)
      sprintf(string+(z*2), "%02x", out[z]);
    You were close, but y is not needed. Your string only needs to be exactly 41 chars big to fit the hex string (2 chars for each of the 20 numbers plus 1 for the terminating zero).
    Last edited by itsme86; 07-25-2006 at 12:28 PM.
    If you understand what you're doing, you're not learning anything.

  7. #7
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    Thanks much itsme86, that worked.

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