Access and store value by index in a pointer - c

Question

I've got an assignment where I have to sum whole numbers up to 100 digits.
They gave me this struct to represent big numbers (I think there are better ways to represent this, but I'm not allowed to modify it):
typedef struct {
char* string;
int lengthError;
} BigNumber;
Where string is the number itself and lengthError is the length of the number or an error that is a previously defined enum.
I've also have the implementation of the sum function
BigNumber *sum(BigNumber* num1, BigNumber* num2) {
BigNumber* result = malloc(sizeof(BigNumber));
int limit = getLength(num1->lengthError, num2->lengthError);
result->string = malloc(limit);
int digitResult;
int index = limit -1;
int carry = 0;
while(index != -1) {
int d1 = ((int)num1->string[index]) - ((int)'0');
int d2 = ((int)num2->string[index]) - ((int)'0');
digitResult = d1 + d2 + carry;
if (digitResult > 9) {
digitResult = digitResult - 10;
carry = 1;
} else {
carry = 0;
}
itoa(digitResult, &result->string[index], 10); //I think this is the problem
index--;
}
result->string[limit] = '\0';
printf("result: %s\n", result->string);
return result;
}
I haven't finished writing that function, I know there are a lot of flaws in it, but the problem is that I can't get to sum 12 + 12. The result I get is 2.
I thought approaching this problem by picking the lasts character of both numbers, transform them into an int and sum them having in mind the carry digit. After I got the result in digitResult I try to convert it to a char and store it in the corresponding position of the result->string pointer
Once it has finished the operation, I add an \0 at the last position of the result->string.
So the question is, how do I make this operation to work as desired? Debugging the code, I noticed that the first time it stores the first result in result->string, following the example above this would be a number 4, it stores trash in that position instead. In the second addition, I store a number 2 correctly and that's the final result I get in when I print the result.

Answer 1

Your use of the itoa function is a problem (though, as you have also suggested, maybe not the only one).
The itoa function converts its first argument into a null-terminated string - so, as well as writing the character representation of digitResult at the indicated place in the string, it also adds a '\0' character after it. Thus, your string will always be terminated immediately after the last digit you write, and 12 + 12, giving 24 will appear to be just the first character: 2.
What you can do instead is to convert the digit yourself (reversing the operation you used to get the d1 and d2 values), then just directly set the string element to the converted digit.
So, instead of:
itoa(digitResult, &result->string[index], 10);
use:
result->string[index] = (char)(digitResult + '0');