Lag Operator Value

Problem

The lag operator L moves a series back in time: applying L^k to x[t] gives x[t-k]. Given the series, an index t, and a non-negative lag k, return the value x[t-k].

If the lagged index falls outside the series, return None.

Hint

Guard the shifted index before indexing to avoid Python's negative wraparound.

Test cases

case 1

in: [[10,20,30,40],3,1]

expected: 30

case 2

in: [[10,20,30],2,2]

expected: 10

case 3

in: [[1,2,3],0,1]

expected: null

case 4

in: [[5,6,7],1,0]

expected: 6

Starter codePython

def lag_value(series, t, k):
    # Your code here
    pass

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