new to C & need help Hamming code error detection

[dsemel]

#include<stdio.h>
#include<stdlib.h>
#include<math.h>
/*global vars */
   int length;
   int parity;
   char *hamming_string=NULL;

    int enter_params(){
   /* declare local vars */
   /* prompt for length of hamming code, parity */
      printf("Enter the length of the hamming code: ");
      scanf("%d", &length);
      printf("Enter the parity (0=even, 1-odd): ");
      scanf("%d", &parity);
   /* alocate space for hamming code */
      hamming_string=(char*)malloc(length*sizeof(char));
      return;
   }

    int check(){
   /* declare local vars */
      int i;
      int j;
      int k;
 
      int parity_check;
      int error_bit;

      printf("Enter the Hamming code: ");
      scanf("%s", hamming_string);

      for(i=1; i<length; i = i*2){
         parity_check=parity;
         error_bit=0;
  
         for(j=i; j<length; j=j + 2*i){
            for(k=j; k<i+2*i&&k<length; k++){
               if(k != i){
		parity_check = parity_check ^ (hamming_string[length - k] - '0');}
					}
				}
					
	error_bit= error_bit + (parity_check ^ (hamming_string[length - i] - '0')*i);

			
      }
      if(error_bit > 0){
         if(hamming_string[length-error_bit]=='0'){
            printf("There is an error in bit: %d", hamming_string[length-error_bit]=='0');
            hamming_string[length-error_bit]='1';
            printf("\n");
         }
         else{
            hamming_string[length-error_bit]='0';
         }
         printf("The corrected Hamming code is: %s", hamming_string);
         printf("\n");
      }
      
      else{
         printf("There is no bit error");
         printf("\n");
      }
       
       
      return;
   }



    int quit(){
      if(hamming_string!=NULL){
         free(hamming_string);
      }
   }


	int main()
{
	int interface = 0;
	while(interface != 3)
	{
		{
			printf("Error detection/correction:\n");
			printf("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n");
			printf("1) Enter parameters\n");
			printf("2) Check Hamming code\n");
			printf("3) Exit\n");
			printf("\n Enter Selection: ");
			scanf("%d", &interface);
		}
	
		switch(interface)
		{
			case 1: enter_params(); break;
			case 2: check(); break;
			case 3: quit(); break;
			default: break;
		}
	}
		return 1;
}

Hi, I enter length 7 for the Hamming code and enter 0 for Parity.

When I choose selection 2, I enter 1000110 for the Hamming code and I get a message that there is no bit error. The answer I am supposed to get is there is an error in bit 6 and the corrected Hamming code is 11000110.

the funny thing is that I get the correct answer when I enter 1 for Parity and the same length (7) and Hamming code(1000110). The correct answer for that problem is error in bit 1 and the corrected Hamming code is 1000111

thank you for your help!

[Mr.777]

1. You declared variable parity as a global.
2. You are only initializing it in enter_Parameters() function. ( whatever name given by you for choice 1).
3. Suppose, user enters choice 2 in the very first time, parity variable will not be initialized and; from here it depends upon the machine you are using.
a). either your compiler assigns 0 to all non initialized variables.
b). either it leaves the non initialized variables to keep garbage...
Check this thing over your machine and do some necessary measures that could solve your problem.

[dsemel]

 int check(){
   /* declare local vars */
      int i;
      int j;
      int k;
 
      int parity_check;
      int error_bit;

      printf("Enter the Hamming code: ");
      scanf("%s", hamming_string);
		
		
      for(i=1; i<length; i = i*2){
         parity_check=parity;
	error_bit=0;

         
  
         for(j=i; j<length; j=j + 2*i){
            for(k=j; k<i+2*i&&k<length; k++){
               if(k != i){
		parity_check = parity_check ^ (hamming_string[length - k] - '0');}
					}
				}
					
	error_bit = error_bit + (parity_check ^ (hamming_string[length - i] - '0')*i);

			
      }
      if(error_bit > 0){
         if(hamming_string[length-error_bit]=='0'){
            printf("There is an error in bit: %d", hamming_string[length-error_bit]=='0');
            hamming_string[length-error_bit]='1';
            printf("\n");
         }
         else{
            hamming_string[length-error_bit]='0';
         }
         printf("The corrected Hamming code is: %s", hamming_string);
         printf("\n");
      }
      
      else{
	printf("The error bit is: %d", error_bit);
         printf("There is no bit error");
         printf("\n");
      }
       
       
      return;
   }

my error_bit does not seem to update. The error_bit variable seems to stay zero. Is there a problem with where the error_bit variable is declared and initialized to zero?

[Mr.777]

This time initialization and declarations are right but one thing that i couldn't understand is,
your variables parity and parity check are static single variables and why do you need to assign parity to parity_check every time inside the loop??

[Salem]

Your indentation makes it really hard to see what's going on.

int check(){
    /* declare local vars */
    int i;
    int j;
    int k;
    int parity_check;
    int error_bit;

    printf("Enter the Hamming code: ");
    scanf("%s", hamming_string);

    for(i=1; i<length; i = i*2){
        parity_check=parity;
        error_bit=0;

        for(j=i; j<length; j=j + 2*i){
            for(k=j; k<i+2*i&&k<length; k++){
                if(k != i){
                    parity_check = parity_check ^ (hamming_string[length - k] - '0');
                }
            }
        }

        error_bit = error_bit + (parity_check ^ (hamming_string[length - i] - '0')*i);
    }

    if(error_bit > 0){
        if(hamming_string[length-error_bit]=='0'){
            printf("There is an error in bit: %d", hamming_string[length-error_bit]=='0');
            hamming_string[length-error_bit]='1';
            printf("\n");
        }
        else{
            hamming_string[length-error_bit]='0';
        }
        printf("The corrected Hamming code is: %s", hamming_string);
        printf("\n");
    }
    else{
        printf("The error bit is: %d", error_bit);
        printf("There is no bit error");
        printf("\n");
    }

    return;
}

See SourceForge.net: Indentation - cpwiki

> for(i=1; i<length; i = i*2)
Two things,
a) why are you starting at 1, when arrays start at 0?
b) why *2? You have the sequence 1,2,4,8,16
You seem to be missing large chunks of data