Question

I’m having trouble overcoming this error. I’m doing a routine similar to the merge in mergesort. Maybe I’m instantiating something wrong. Here’s the code:

def MergeAndCountSplitInv(arr, length):
    left = arr[0:length/2]
    right = arr[length/2+1: length]

    i = 0
    j = 0
    numSplitInv = 0
    newArray = [0] * length
    for k in range(len(arr)):
        if (left[i] < right[j]):
            #newArray.insert(k, left[i])
            newArray.append(left[i])
            i = i + 1
        else: #(right[j] < left[i]
            #newArray[k].insert(k, right[j])
            newArray.append(right[j])
            j = j + 1
            numSplitInv = numSplitInv + (length/2 - i)

    return ReturnValue(newArray, numSplitInv)

The error is given as:

File "InversionCount.py", line 31, in MergeAndCountSplitInv
if (left[i] < right[j]):
IndexError: list index out of range

Thank you for any help

Answer 1

One problem is your understanding of slice indexes. These two lines:

left = arr[0:length/2]
right = arr[length/2+1: length]

should be changed to:

left = arr[:length/2]
right = arr[length/2:]

The start index is included, and the end index is the first element not included in the slice, so to get a complete partition, you use the same index for the end of the left as for the start of the right.

But the real problem is that you increment i and j without checking whether you’ve walked off the end of left or right, so you eventually get an index error.