Adding Base-n's through strings (between base-2 and base-36)

[gdlx]

I just wanted to add strings in any base form (example 1101+100 = 10001 in base-2 but it could be added using any base-form like in base-3 to base-36) and I'm having a big trouble with my code because it gave me incorrect results.

addition(char st[], char st2[], int base){
int i, j, carry = 0, ans, len, o=0, z=1, l=0;
char final[50];

if(strlen(st)>= strlen(st2))
len = strlen(st);
else
len = strlen(st2);

for(i=len; i>=0; i--){
if(isdigit(st[i])&&isdigit(st 2[j])){ //if both addends are digits
ans = (st[i] - '0') + (st2[i] - '0') + carry;
}
else if(isdigit(st[i])&&isalpha(st 2[j])){ //if first addend is a digit and second addend is an alphabet
ans = (st[i] - '0') + (st2[j] - 'A'+10) + carry;
}
else if(isalpha(st[i])&&isdigit(st 2[j])){ //if first addend is an alphabet and second addend is a digit
ans = (st[i] - 'A'+10) + (st2[j] - '0') + carry;
}
else if(isalpha(st[i])&&isalpha(st 2[j])){ //if both addends are alphabet
ans = (st[i] - 'A'+10) + (st2[j] - 'A'+10) + carry;
}

if(ans > base){
ans = ans - base;
carry = ans - base;
}
else if(ans == base){
ans = ans - base;
carry = 1;
}

if(ans >= 0 && ans <= 9)
final[i] = ans + 49;
else if(ans >= 65 && ans <= 90)
final[i] = ans + 65;

printf("%c", final[i]);
}
}

I really need help about this code. Thanks :-)

[weaknessforcats]

I would write functions that convert the string to a base 10 number. Add the numbers for each string. Then call a function to convert the base 10 number to a string in the correct base:

char str1[6] = "11101";
char str2[7] = "000111";
char str3[10];

int X = ConvertToInteger(str1, 2);
int Y = ConvertToInteger(str2, 2);

int Sum = X + Y;

ConvertToString(Sum,str3,2);

This approach will break your logic into chunks as opposed to a "strem of consciousness" if-else-if code.