Hamming Code (7,4) - C++ Implementation Failure

Question

#include <iostream>
#include <string>

using namespace std;

int get_bin_representation(char x){

  if(x == '1'){
    return 1;
  }
  else if(x == '0'){
    return 0;
  }
}
int gen_hamming_code(string token){

  int bits[4];
  int temp(0);

  for(int k=0; k<4; k++){
    bits[k] = get_bin_representation(token.at(k));
  }

  int ham_code[7];

  ham_code[0] = bits[0] + bits[1] + bits[3];
  ham_code[1] = bits[0] + bits[2] + bits[3];
  ham_code[2] = bits[0];
  ham_code[3] = bits[1] + bits[2] + bits[3];
  ham_code[4] = bits[1];
  ham_code[5] = bits[2];
  ham_code[6] = bits[3];

  for(int h=0; h<7; h++){
    temp = ham_code[h];
    ham_code[h] = temp%2;
    temp = 0;
  }

  for(int e=0; e<7; e++){
    cout << ham_code[e];
  }
  cout << endl;

  return 0;
}
int main(){

  string usr_input;
  string msg;
  int index(0);

  cout << "Hamming Code Program" << endl;

  while(true){

    cout << endl << ": ";
    getline(cin, usr_input);

    if(usr_input.find("gen") != std::string::npos){
        for(int i=0; i<usr_input.length(); i++){
            if(usr_input.at(i) == ' '){
                index = i;
            }
        }

        for(int j=index; j<usr_input.length(); j++){
            msg+=usr_input.at(j);
        }

        cout << "Hamming code (7,4): ";
        gen_hamming_code(msg);
    }
  }
}

I used the linear algebra definition supplied by Wikipedia ('Hamming Code (7,4)'). At several points in the program, I printed the variable contents, but fixing one problem lead to another. To verify if the output was correct, I compared it to the example available on Wikipedia, and the result produced by an online calculator.

UPDATED: Question resolved. I used an adaptation of the algorithm provided here (without AMP).

Are you doing this with bites or strings of 0 and 1? The reason I ask is you are using getline which is for reading strings in ASCII format.You may want to read fread from stdin. Also, if you are using bits you may want to have a look at std::bitset<T> for abetter way to handle your bits. — Freddy
The input is interpreted as a string, and then each character is converted to an integer, either zero or one - this is done by the get_bin_representation() function. — user2062542

Ben Voigt Ben Voigt · Accepted Answer · 2013-02-11T20:37:56

Well, this is wrong:

ham_code[0] = bits[0] + bits[1] + bits[3];

Hamming codes are defined using GF(2) arithmetic. Addition in GF(2) is the C++ xor operator (^). Use the right operator, and you can do away with the later %2 loop.

You've also intermingled the parity bits with the plaintext, which is something never done when I learned about it. As well, the online simulator is using the simple order (plaintext, parity) without interleaving.

Hamming Code (7,4) - C++ Implementation Failure

1 Answers