Question

Im a newbie to python , and I came across this particular code snippet to calculate the greatest slice in an sequence, however I simply cant seem to understand the following code

best = A[0] 
for size in range(1,n+1): 
   cur = sum(A[:size]) 
   for i in range(n-size): 
      cur += A[i+size] -= A[i] # <- what happens here?
      best = max(best, cur) 

Any idea on how the code functions and with that line in particular would be helpful!

Answer 1

First off, the error in your code is an extra =. Working code:

n = len(A)
best = A[0] 
for size in range(1,n+1): 
   cur = sum(A[:size]) 
   for i in range(n-size): 
      cur += A[i+size] - A[i] # <- what happens here?
      best = max(best, cur)

The outer for loop creates larger and larger slices, starting with size set to 1. Then cur is set to the sum of the first size items of A.

In the inner loop, this slice is “moved” to the right by adding the value just to the right of the slice (A[i+size]) and subtracting the first value of the slice (A[i]).

Finally best is set to whichever is larger of the newly computed sum and the largest value found so far.

In the end, best contains the largest sum. Unless A contains negative values, the answer is trivial: sum(A).

EDIT: I just noticed that there’s a bug: The leftmost slice is not counted, except for slice size 1 (best = A[0]). So if A = [4, 3, 2] it outputs 5 instead of 9. Fix by adding a best = max(best, cur) line above the inner for loop.