CPP - Maximum Subarray Sum 2 video doubt

#1

In the line 35 of the above code sir calculated the current sum for the subarray. Now when the first for loop initialises the int i=0 and second for loop initialises int j=0 (first subarray) the expression csum[i-1] seems invalid (csum[0-1]), please explain.

#2

You are right cumsum[i-1]will be invalid for i=0, it take 0 value or some garbage , usually it takes value 0 so its not affecting the answer but in practice try to avoid such thing by writing if condition
if (i!=0){
currentSum = cumSum[j] - cumSum[i-1];
}
else{
currentSum=cumSum[j];
}

Also better algo for this question is kadane algo which take O(n) time , this code takes O(n^2) time .