Merge sort for python

Question

Here are my merge and mergeSort functions. merge merges two separate arrays and mergesSort sorts and merges them with recursion:

def merge(arrL, arrR):
    arrNew = []
    d = len(arrL) + len(arrR)
    i = 0
    j = 0
    for k in range (0, d-1):
        if (arrL[i] < arrR[j]) :
            arrNew.append(arrL[i]) # appends to the end of the array
            i = i + 1
        else:
            arrNew.append(arrR[j])
            j = j + 1
    return arrNew

def mergeSort(arr, m, n):
    if (n - m == 1):
        return arr[m]
    else:
        p = (m + n) // 2
        arrL = mergeSort(arr, m, p)
        arrR = mergeSort(arr, p, n)
        arrNew = merge(arrL, arrR)
        return arrNew

I am getting an error from lines 32, 33 and 13:

d = len(arrL) + len(arrR)
TypeError: object of type 'int' has no len()

What is causing this error? merge is taking two arrays as inputs.

Answer 1

What is causing this error? merge is taking two arrays as inputs.

Except when it doesn't.

if(n-m == 1):
  return arr[m]

This output of mergeSort is not an array.

Answer 2

My guess is it's this line

if(n-m == 1):
    return arr[m]

which presumably is returning the content arr[m] of the array and not an array itself.

Since your code sorts arrays, when this naked element gets recursed on, it will generate the error you're seeing.

Answer 3

There are multiple problems in the code:

• in mergeSort , you should return arr instead of arr[m] when the length of the array is less than 2. The test if (n - m == 1) does not allow for empty arrays:

   if (n - m < 2): return arr

• in merge , the main loop should run d times, ie: instead of for k in range (0, d-1): you should write:

   for k in range (d):

• the test in the merge loop should also check if the index value in still in range. If the second slice is exhausted, the element arrL[i] should be selected:

   for k in range (d): if i < len(arrL) and (j >= len(arrR) or arrL[i] < arrR[j]): arrNew.append(arrL[i]) i = i + 1 else: arrNew.append(arrR[j]) j = j + 1

Here is a modified version:

def merge(arrL, arrR):
    arrNew = []
    i = 0
    j = 0
    for k in range(len(arrL) + len(arrR)):
        if i < len(arrL) and (j >= len(arrR) or arrL[i] < arrR[j]):
            arrNew.append(arrL[i])
            i = i + 1
        else:
            arrNew.append(arrR[j])
            j = j + 1
    return arrNew

def mergeSort(arr, m, n):
    if (n - m < 2):
        return arr
    else:
        p = (m + n) // 2
        arrL = mergeSort(arr, m, p)
        arrR = mergeSort(arr, p, n)
        return merge(arrL, arrR)