Thread: 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?