Adding two really large numbers in C - c

Question

I am trying to add two really large numbers (say 30 digit long) in C. Here is my code:
#include <stdio.h>
#include <string.h>
int main()
{
char a[30] =
{
'1','1','1','1','1','1','1','1','1','1','1','1','1','1','1',
'1','1','1','1','1','1','1','1','1','1','1','1','1','1','1'
. };
b[30] =
{
'8','8','8','8','8','8','8','8','8','8','8','8','8','8','8',
'8','8','8','8','8','8','8','8','8','8','8','8','8','8','8'
};
int i, j, k, carry = 0, sum[1001];
if(strlen(a) >= strlen(b))
{
k = strlen(a);
}
else
{
k = strlen(b);
}
for(i = strlen(a); i > 0; i--)
{
for(j = strlen(b); j > 0; j--)
{
sum[k] = (a[i]-'0') + (b[j]-'0') + carry;
carry = sum[k]/10;
k--;
}
}
for(i = 0; i < k; i++)
{
printf("%d", sum[i]);
}
printf("\n");
return 0;
}
but it gives me no answer at all. What is wrong with it?

Answer 1

Small change to the code above to take care of the last trailing carry value if it > 0:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(){
int temp = 0;
char *fp ;
char *sp ;
char bigger = '\n';
char a[10] =
{
'1','2','3','4','5','6','7','8','9','1'
};
char b[10] =
{
'9','8','9','8','9','8','9','8','9','8'
};
int i, c, j, k, carry = 0, sum[1001] = {0};
if(sizeof(a) > sizeof(b))
{
k = sizeof(a);
j = sizeof(b);
bigger = 'a';
}
else
{
k = sizeof(b);
j = sizeof(a);
bigger = 'b';
}
c = k;
fp = malloc(k);
sp = malloc(j);
if (bigger == 'a')
{
strncpy(fp, a, k);
strncpy(sp, b, j);
}
else
{
strncpy(fp, b, k);
strncpy(sp, a, j);
}
for(i = strlen(fp); i > 0; i--,j--)
{
if(j>0 )
{
temp = *(fp+i-1)-'0' + *(sp+j-1)-'0' + carry;
if(temp < 10)
{
sum[k] += temp;
carry = 0;
}
else
{
sum[k] += temp%10;
carry = temp/10;
}
}
else
{
temp = *(fp+i-1)-'0' + carry;
if(temp < 10)
{
sum[k] += temp;
carry = 0;
}
else
{
sum[k] += temp%10;
carry = temp/10;
}
}
k--;
}
if(carry > 0)
printf("%d", carry);
for(i = 1; i <= c; i++)
{
printf("%d", sum[i]);
}
printf("\n");
return 0;
}

Answer 2

This should work. I changed the way your loops should work and also added some checks to ensure alignment and consistency in case the length of the large numbers change.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
int temp = 0;
char *fp ;
char *sp ;
char bigger = '\n';
char a[30] =
{'1','1','1','1','1','1',
'1','1','1','1','1','1',
'1','1','1','1','1','1','1','1','1','1','1','1','1',
'1','1','1','1','1'
};
char b[30] =
{
'8','8','8','8','8','8','8','8','8','8','8','8','8','8',
'8','8','8','8','8','8','8','8','8','8','8','8','8','8',
'8','8'
};
int i, c, j, k, carry = 0, sum[1001] = {0};
if(sizeof(a) > sizeof(b))
{
k = sizeof(a);
j = sizeof(b);
bigger = 'a';
}
else
{
k = sizeof(b);
j = sizeof(a);
bigger = 'b';
}
c = k;
fp = malloc(k);
sp = malloc(j);
if (bigger == 'a')
{
strncpy(fp, a, k);
strncpy(sp, b, j);
}
else
{
strncpy(fp, b, k);
strncpy(sp, a, j);
}
for(i = strlen(fp); i > 0; i--,j--)
{
if(j>0 )
{
temp = *(fp+i-1)-'0' + *(sp+j-1)-'0' + carry;
if(temp < 10)
{
sum[k] += temp;
carry = 0;
}
else
{
sum[k] += temp%10;
carry = temp/10;
}
}
else
{
temp = *(fp+i-1)-'0' + carry;
if(temp < 10)
{
sum[k] += temp;
carry = 0;
}
else
{
sum[k] += temp%10;
carry = temp/10;
}
}
k--;
}
if(carry > 0)
printf("%d", carry);
for(i = 1; i <= c; i++)
{
printf("%d", sum[i]);
}
printf("\n");
return 0;
}