Differencing Order to Constant

Problem

Repeatedly difference a series until all remaining values are equal, and return the number of differencing steps used. A single remaining value counts as constant and stops the process.

At most five differences are attempted; return -1 if the series is still not constant after those five steps.

Hint

A degree-d polynomial trend needs d differences to become constant.

Test cases

case 1

in: [[1,2,3,4]]

expected: 1

case 2

in: [[1,4,9,16]]

expected: 2

case 3

in: [[2,2,2]]

expected: 0

case 4

in: [[1,2,4,8,16,32,64]]

expected: -1

case 5

in: [[7]]

expected: 0

Starter codePython

def differencing_order_to_constant(series):
    # Your code here
    pass

5 test cases · run your code against them in the editor