Question

I have here a code for sorting lists using merge sort.. I just got it somewhere in the net.. But honestly, I can’t follow the flow of the code… I mean, I can’t get how it is implemented. I can understand some part especially the first one where it divides the entire list in two and then sort each side of the list. And then what?? Will you please tell what’s happening here? Thank you. 🙂

def merge(badlist):
    if len(badlist) == 1:  
    return badlist  
    m = len(badlist)/2  
    l = merge(badlist[:m])  
    r = merge(badlist[m:])  
    if not len(l) or not len(r):  
        return l or r  
    result = []  
    i = j = 0  
    while (len(result) < len(r) + len(l)):  
        if l[i] < r[j]:  
            result.append(l[i])  
            i += 1  
        else:  
            result.append(r[j])  
            j += 1  
        if i == len(l) or j == len(r):  
            result.extend(l[i:] or r[j:]) 
            break  
    return result  
print merge(badlist)  

Answer 1

Going line by line,

if not len(l) or not len(r):  
        return l or r

This checks whether either of the lists is empty. If it is, then the other list is returned.

result = []  
i = j = 0  

Here, result and i,j are initialized

while (len(result) < len(r) + len(l)):  

Runs the while loop till all elements of both lists have been copied to the result list.

    if l[i] < r[j]:  
            result.append(l[i])  
            i += 1  
    else:  
            result.append(r[j])  
            j += 1  

This block checks which of l[i] or r[j] is lesser and then appends it to result, and advances the respective counter. This goes on till either of the lists gets completely used up, which is checked for in the following code block :

    if i == len(l) or j == len(r):  
            result.extend(l[i:] or r[j:]) 
            break 

Now, whichever list still contains elements, is appended to the end of result list as is.

return result

After which result is returned.

As mentioned, the indentation is incorrect in your sample, which I have corrected here.