list index out of range through an array

Question

I'm not sure what's wrong. I assume I have surpassed an array limit perhaps.

 def lcs(arr):
 if len(arr) == 0: # arr of length 0
 return
 sumarr = []
 counter = 0
 sum = 0
 for i in arr:
 if arr[i] > 0:
 sum = sum + arr[i]
 if arr[i] < 0:
 sumarr[counter] = sum
 counter += 1
 print max(sumarr)

The error I get is:

 Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<stdin>", line 8, in lcs
 IndexError: list index out of range

#

I've modded the code a bit

 def lcs(arr):
 if len(arr) == 0: # arr of length 0
 return
 sumarr = []
 counter = 0
 sum = 0
 for i in arr:
 if i > 0:
 sum = sum + i
 if i < 0:
 sumarr[counter] = sum
 counter += 1
 print max(sumarr)

However I'm getting this error

 Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<stdin>", line 13, in lcs
 ValueError: max() arg is an empty sequence

I thought I was constantly updating my sumarr[]. How come the error is telling me that I am passing an empty list into max()?

Answer 1

When you do for i in arr, i gets the elements of the array (one at a time), not their indices. What you want in your example is to replace each arr[i] with i.

Answer 2

Use enumerate to go point to right position of the array:

 def lcs(arr):
 if len(arr) == 0: # arr of length 0
 return
 sumarr = []
 counter = 0
 sum = 0
 for count, i in enumerate(arr):
 if arr[count] > 0:
 sum = sum + arr[i]
 if arr[count] < 0:
 sumarr[counter] = sum
 counter += 1
 print max(sumarr)

• shx2: What's the point of using enumerate if the count variable is not used? (should have i instead of arr[count])