how to get start index for maximum sum sub array

Question

I am using the following program to find the maximum sum and indices of the sum.I am able to get the right index but having trouble finding the right index.

def max_sum_new(a):
max_current = max_global = a[0]
r_index = 0
for i in range(1, len(a)):
    max_current = max(a[i], max_current+a[i])

    if max_current > max_global:
        max_global = max_current
        r_index = i

return (max_global, r_index)

#Driver Code:
my_arr = [-2, 3, 2, -1]
print(max_sum_new(my_arr))

I am getting

(5, 2)

which is expected but i also want to get the starting index which in this case should be 1

so i expect

(5, 1, 2)

is there any way to get a start index here? How should i put the variable to record start index?

Answer 1

You just need to apply the same logic as you did with r_index. Whenever you change the max_current, that's when the start of the maximum sub-array changes. In the end it looks like this

max_current = max_global = a[0]
r_index = 0
left_index = 0
for i in range(1, len(a)):
    if a[i] > max_current + a[i]:
        # Here is where your start index changes
        left_index = i
        max_current = a[i]
    else:
        max_current = a[i] + max_current

    if max_current > max_global:
        max_global = max_current
        r_index = i

print(max_global, left_index, r_index)