Python to Matlab Conversion?

Question

I have this python code here below (for bubble sort). Below that is my attempt at converting it to MATLAB code. I'm new to MATLAB, and I'm doing the conversion for practice. I would appreciate feedback on how accurate/incorrect my conversion is.

The python version:

def bubble_sort(alist):
 return bubble_sort_helper(alist, len(alist))
def bubble_sort_helper(alist, n):
 if n < 2:
 return alist
 for i in range(len(alist)-1):
 if alist[i] > alist[i+1]:
 temp = alist[i]
 alist[i] = alist[i+1]
 alist[i+1] = temp
 return bubble_sort_helper(alist, n-1)

My attempt at a MATLAB conversion:

function a = bubble_sort(alist)
 a = bubble_sort_helper(alist, size(alist))
end
function b = bubble_sort_helper(alist, n)
 if n < 2
 b = alist
 end
 for ii = size(alist)
 if alist(1) > alist (ii+1)
 temp = alist(ii)
 alist(ii) = alist(ii+1)
 alist(ii+1) = temp
 end
 end
 b = bubble_sort_helper(alistn n-1)
end

• the_martian: I forgot to add the indent under the if statement for the python code. Fixed that with an edit, my mistake.
• Suever: Did you test to see if it sorted your input?
• OneCricketeer: 1) Does it work as expected? 2) Why is it recursive?
• Wayne Werner: note that in your python code you could do alist[i], alist[i+1] = alist[i+1], alist[i]
• the_martian: @Suever, I ran the python code and it works, but my original MATLAB code posted above does not run.

Answer 1

A few issues here:

1. You need to use numel rather than size to get the number of elements in an array. size will give you a vector of sizes of each dimension and numel will give you the total number of elements

2. You need to actually create an array of values for your for loop to loop through. To do this, use the colon to create an array from 2 to n.

   for ii = 2:n
   end
   

3. You use ii as your looping variable but try to use i inside of the loop. Pick one and stick to it (preferably not i)

4. To flip the values you can simply do your assignment like this:

   alist([i-1, i]) = alist([i, i-1]);
   

Taken together, this will give you something like this:

function a = bubble_sort(alist)
 a = bubble_sort_helper(alist, numel(alist))
end
function b = bubble_sort_helper(alist, n)
 if n < 2
 b = alist;
 else
 for k = 2:n
 if alist(k-1) > alist(k)
 alist([k-1, k]) = alist([k, k-1]);
 end
 end
 b = bubble_sort_helper(alist, n-1);
 end
end

Answer 2

Your python version is inefficient:

def bubble_sort(alist):
 return bubble_sort_helper(alist, len(alist))
def bubble_sort_helper(alist, n):
 if n < 2:
 return alist
 for i in range(n-1):
 if alist[i] > alist[i+1]:
 alist[i], alist[i+1] = alist[i+1], alist[i]
 return bubble_sort_helper(alist, n-1)

and your matlab code is wrong:

function a = bubble_sort(alist)
 a = bubble_sort_helper(alist, size(alist))
end
function b = bubble_sort_helper(alist, n)
 if n < 2
 b = alist;
 else
 for i = 2:n
 if alist(i-1) > alist(i)
 temp = alist(i-1);
 alist(i-1) = alist(i);
 alist(i) = temp;
 end
 end
 b = bubble_sort_helper(alist, n-1);
 end
end

• the_martian: Ohh, thanks for the tip about my inefficiency. I'm trying to learn all I can about sorting algorithms so every bit helps.