Toroidal Board for Game of Life

I'm trying to finish up this Game of Life project I have started. I've read multiple posts on creating a toroidal (wrap-around) grid and have attempted to do the same, but I'm not too sure I've got the correct solution.

It was recommended to me to use (i-1)%size for wrapping rows, but I don't really understand why, nor do I think I'm using it correctly. Could someone provide some insight into this, or possibly offer an alternative?

vector<string> nextState(long row, long col, vector<string> table,
vector<string> newTable, vector<long> birth, vector<long> survive) {

for (int i = 0; i < row; i++) {
        for (int j = 0; j < col; j++) {
            long Alivecell = 0;

            // Count live neighbors of particular cell
            // Wrap rows if index out of range
            if (table[(i-1) % table.size()][j-1] == '#') {
                Alivecell++;
            }
            if (table[(i-1) % table.size()][j] == '#') {
                Alivecell++;
            }
            if (table[(i-1) % table.size()][j+1] == '#') {
                Alivecell++;
            }
            if (table[i][j-1] == '#') {
                Alivecell++;
            }
            if (table[i][j+1] == '#') {
                Alivecell++;
            }
            if (table[i+1][j-1] == '#') {
                Alivecell++;
            }
            if (table[i+1][j+1] == '#') {
                Alivecell++;
            }
            if (table[i+1][j] == '#') {
                Alivecell++;
            }

            // Check rules for cell birth/survival/death
            newTable[i][j] = '.';

            if (table[i][j] == '#' && Alivecell <= 1) {

                newTable[i][j]='.'; // Death
            }
            else if (table[i][j] == '#' && Alivecell >= 4) {

                newTable[i][j] = '.'; // Death
            }
            else if (table[i][j] == '.' && Alivecell == 3) {

                newTable[i][j]= '#'; // Birth
            }
            else if (table[i][j] == '#' && 
                    (Alivecell == 2 || Alivecell == 3 )) {

                newTable[i][j] = '#'; // Survive
            }

        }
    }
return newTable;
}