python - How to nicely handle [:-0] slicing? -

in implementing autocorrelation function have term like

for k in range(start,n):     c[k] = np.sum(f[:-k] * f[k:])/(n-k) 

now works fine if start = 1 i'd handle nicely start @ 0 case without conditional.

obviously doesn't work because f[:-0] == f[:0] , returns empty array, while i'd want full array in case.

don't use negative indices in case

f[:len(f)-k] 

for k == 0 returns whole array. other positive k it's equivalent f[:-k]