Python - Merge Sort Recursive Algorithm

Question

I was trying to implement this pseudocode for the recursive merge sort algorithm:

**procedure**  *mergesort*(L = a1, a2,…,an )
**if**  n  > 1 **then** 
     m := ⌊n/2⌋
     L1 := a1, a2,…,am 
     L2 := am+1, am+2,…,an
     L   := merge(mergesort(L1), mergesort(L2 ))
{L is now sorted into elements in increasing order}

**procedure**  *merge*(L1, L2 :sorted lists)
L := empty list
**while** L1  and L2  are both nonempty
 remove smaller of first elements of L1 and L2 from its list; 
         put at the right end of L
 **if** this removal makes one list empty 
     **then** remove all elements from the other list and append them to L
return L {L is the merged list with the elements in increasing order}

The purpose its to write it on python, so far I have coded all of it but it does not run properly, everytime I run it prints: function merge at 0x0000000002024730. Here is the python code:

#Recursive Merge Sort
import math
ent = [10000, 967, 87, 91, 117, 819, 403, 597, 1201, 12090]
def merge(L1, L2):

        while len(L1) and len(L2) != 0:
            L.append(L1[0])
            L1.remove(L1[0])
            L.append(L2[0])
            L2.remove(L2[0])
            if len(L1) == 0:
                L.append(L2)
            elif len(L2) == 0:
                L.append(L1)
        return L


def mergesort(ent):

if len(ent)>1:
    m=math.floor(len(ent)/2)
    L1 = []
    L2 = []
    L1=L1+ent[:m]
    L2=L2+ent[m+1:len(ent)]
    L=merge(mergesort(L1),mergesort(L2))




print(merge)

I have some doubts about how the functions are supossed to work recursively together, which is why I think I cant solve and code it right. Any help or suggestions?

Answer 1

You're not executing merge , but printing the function itself . Do this:

print(merge())

However, your logic is a bit messed up, you don't even have a recursive function there.

Take a look at this question

Also, i think what you need is to call mergesort:

def mergesort(ent):
    if len(ent)>1:
        m=math.floor(len(ent)/2)
        L1 = ent[:m]
        L2 = ent[m+1:len(ent)]
        L=merge(mergesort(L1),mergesort(L2))
return L

And execute it:

print(mergesort(ent))