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

I forgot to add the indent under the if statement for the python code. Fixed that with an edit, my mistake. — the_martian
– the_martian, Commented Aug 11, 2016 at 19:12
note that in your python code you could do alist[i], alist[i+1] = alist[i+1], alist[i] — Wayne Werner
– Wayne Werner, Commented Aug 11, 2016 at 19:26
@Suever, I ran the python code and it works, but my original MATLAB code posted above does not run. — the_martian
– the_martian, Commented Aug 11, 2016 at 19:33

Suever · Accepted Answer · 2016-08-11 19:29:35Z

A few issues here:

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
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
```
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)
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

Daniel · Accepted Answer · 2016-08-11 19:23:44Z

1

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

answered Aug 11, 2016 at 19:23

Daniel

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1 Comment

the_martian Over a year ago

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

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