Representing a Large Integer with an Array

I'm a real beginner. I'm trying to create an array of integers to represent a very large number (three digits of the number per element of the array). I'll start out by reading two large numbers as strings from a file, and converting each of them to an integer array. Then I need to perform addition, subtraction, and multiplication on these two arrays, and store the results in a third array. Then, I'll convert the resulting integer array back into a string and output it.

The sizes of the arrays and the segment of code used to add the numbers (below) were given to me by my professor. He also gave us the basic idea for converting to/from a string/integer array, but I'm quite sure I haven't executed it correctly. Here's what I've got so far, just for the addition operation:

Code:

`void main()`

{

int i, ca, length1, length2, length3, t;

char s1[20], s2[20], s3[20]; //s1, s2 are read from file, s3 will be printed result

int a[16], b[16], c[16]; //used in performing arithmetic operations

FILE *ff;

ff = fopen("a.txt", "r");

fscanf(ff, "%s", s1); //1st large number

fscanf(ff, "%s", s2); //2nd large number

length1 = strlen(s1);

length2 = strlen(s2);

//CONVERTS LARGE NUMBER STRINGS TO INT ARRAYS

for(i = 0; i < 16; i++){

a[i] = (s1[length1 - (3 * i + 3)] - 48)*100 + (s1[length1 - (3 * i + 2)] - 48)*10 + (s1[length1 - (3 * i + 1)] - 48);

b[i] = (s2[length2 - (3 * i + 3)] - 48)*100 + (s2[length2 - (3 * i + 2)] - 48)*10 + (s2[length2 - (3 * i + 1)] - 48);

}

//TO ADD THE TWO ARRAYS

for(i = 0; i < 22; i++){ //adds

c[1] = a[i] + b[i];

}

for(i = 0; i < 21; i++){ //rectifies carry-over from addition

ca = c[i] / 10;

c[i] = c[i] % 10;

c[i + 1] = c[i + 1] + ca;

}

//CHANGE RESULTING INT ARRAY INTO A STRING

for(i = 0; i < 16; i++){

s3[i] = c[3 * i] % 10 + 48;

c[3 * i] = c[3 * i] / 10;

}

length3 = strlen(s3);

//REVERSES STRING

for(i = 0; i < length3 / 2; i++){

t = s3[i];

s3[i] = s3[length3 - i - 1];

s3[length3 - i - 1] = t;

}

//PRINTS RESULT FROM ADDITION

printf("The sum is %s \n", s3);

fclose(ff);

}

Does this look remotely close at all?