Incorrect result when adding binary strings - c

Question

I am getting an incorrect result when adding two binary strings "11" + "11". I am expecting "110" but the answer I am getting is "010". I did add a special case if the carry is still 1.
char *addBinary(char *str1, char *str2)
{
int len1 = strlen(str1); //assume both string lengths are same
char *res = (char *)malloc((len1 + 1) * sizeof(char));
int i, a, b;
int carry = 0;
char result;
for (i = len1 - 1; i >= 0; i--)
{
a = str1[i] - '0';
b = str2[i] - '0';
carry = a + b + carry;
printf("%d %d %d \n", a, b, carry % 2);
result = carry % 2 + '0'; //convert to character
carry = carry / 2;
str1[i] = result; //use the existing string for the result
}
if (carry == 0)
{ //if there is no carry just use the existing string and return
printf("Final without carry %s \n", str1);
return str1;
}
else
{ //there was a carry, so put 1 in the 0th location and copy the string
res[0] = 1;
memcpy(res + 1, str1, sizeof(char) * len1);
printf("Final %s %s %d\n", res, str1,strlen(res));
return res;
}
}
int main()
{
char bin_str1[] = "11";
char bin_str2[] = "11";
printf("%s \n",addBinary(bin_str1, bin_str2));
return 0;
}

Answer 1

You have two main problems:
First when you allocate space for res:
char *res = (char *)malloc((len1 + 1) * sizeof(char));
You need to allocate space for len1+2 (carry and NUL termination), otherwise your memcpy will write out of bounds (you should copy len1+1 elements, otherwise it is not guaranteed that res is NUL terminated). Also note that in the case that you return str1 the memory is leaked.
Second problem occurs when you set the first char of res you do net set it to a printable character:
res[0] = 1;
You should set it to '1' instead.

Answer 2

One more thing to think of is, that if you have something like this:
char bin_str1[] = "10";
char bin_str2[] = "11";
Your addBinary() Function does not returns res, but str1.
Now here is the point, if you assign a pointer to your function so that you can release the memory back to the System, you will have an invalid free().
One approach could be like this:
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <string.h>
char *addBinary(char *str1, char *str2);
int main( void )
{
char bin_str1[] = "11";
char bin_str2[] = "11";
char *ptr = addBinary( bin_str1, bin_str2 );
if ( ptr )
{
printf("%s \n",ptr );
free( ptr );
}
}
char *addBinary(char *str1, char *str2)
{
char *res = calloc( strlen( str1 ) + strlen( str2 ), sizeof( *res ) );
int a, b, carry = 0;
size_t i = strlen( str1 ) - 1;
while ( i > 0 )
{
a = str1[i] - '0';
b = str2[i] - '0';
carry += a + b;
printf("%d %d %d \n", a, b,( carry % 2));
str1[i] = (char)( carry % 2 + '0'); ///convert to character
carry = carry / 2;
i--;
}
if ( carry == 0 )
{ ///if there is no carry RETURN NULL
printf("Final without carry %s \n", str1);
free( res );
return NULL;
}
else
{ //there was a carry, so put 1 in the 0th location and copy the string
res[0] = '1';
strcat( res, str1 );
printf("Final %s %s %zu\n", res, str1,strlen( res ));
return res;
}
}
Or, you can drop malloc and use it like this:
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <string.h>
int addBinary( char *dest, char *const str1, const char *const str2);
int main( void )
{
char bin_str1[] = "11";
char bin_str2[] = "11";
char dest[256] = { 0 };
if ( addBinary( dest, bin_str1, bin_str2 ) )
{
printf("Dest = %s\n", dest );
}
}
int addBinary( char *dest, char *const str1, const char *const str2)
{
int a, b, carry = 0;
size_t i = strlen( str1 ) - 1;
while ( i > 0 )
{
a = str1[i] - '0';
b = str2[i] - '0';
carry += a + b;
str1[i] = (char)( carry % 2 + '0'); ///convert to character
carry = carry / 2;
i--;
}
if ( carry )
{
dest[0] = '1';
strcat( dest, str1 );
return 1;
}
return 0;
}
I made some changes to your code to reduce it for better reading.