Add Two Integer Arrays into One Integer Array

Question

I am looking to take two arrays of Integers and add them together, but I am having some difficulty with the carrying bit.

public static int [] add(int [] a, int [] b) 
{
    int col = 0;
    int carry = 0;           
    int[] totalArray = new int[MAX_DIGITS + 1];           
    for(int i = MAX_DIGITS - 1; i > 0; i--) {
        col = a[i] + b[i] + carry; 
        if(col >= 10) {
            carry = col / 10; 
            totalArray[i] %= 10; 
        } else {
            totalArray[i] = col;
            carry = 0; 
        }
    }
    totalArray[0] = carry; 
    return totalArray;      
}

The above code adds 204 + 435 correctly but fails to add 22 + 3122 + 12 correctly (ouputs 65). I have written other methods to make sure all of the digits are properly indexed within the array and have tested those methods. Seeing as it works for 204 + 435 but not the other, I can't see what's tripping me up. I do not want to use the BigInteger class - before anyone suggests I should.

Given int[]a [0000000000000000000000999] and int[]b [0000000000000000000000483] currently add(a,b) returns [0000000000000000000001000]

Answer 1

This works just fine. Just make certain that all your arrays only occupy locations 1 thru MAX_DIGITS-1 with actual values. This allows for carries into totalArray[0].

static int MAX_DIGITS = 6;

int[] a = {0,0,0,0,2,2};
int [] b = {0,0,3,1,2,2};
int [] c = {0,0,0,0,1,2};

int[] part1 = add(a,b);
int [] total = add(part1, c);
System.out.println(Arrays.toString(total));

Prints

[0, 0, 3, 1, 5, 6]

• All I did was reduce the size of the totalArray array to MAX_DIGITS
• and change the i > 0 to i >= 0 in the loop.
• ensure that index 0 was 0 in all the arrays.
• the rest was your code.

public static int[] add(int[] a, int[] b) {
    int col = 0;
    int carry = 0;
    int[] totalArray = new int[MAX_DIGITS];
    for (int i = MAX_DIGITS-1; i >= 0; i--) {
        col = a[i] + b[i] + carry;
        if (col >= 10) {
            carry = col / 10;
            totalArray[i] %= 10;
        } else {
            totalArray[i] = col;
            carry = 0;
        }
    }
    totalArray[0] = carry;
    return totalArray;
}