Remove elements of one list from another list using “while”

Question

I've created a simple program which removes elements of one list from another list using in method at least two times:

def remove(l_list,s_list):
    """Removes items of s_list from l_list and returns the rest"""
    res=[]
    for item in l_list:
        if item not in s_list:
            res.append(item)
    return res

I want to replace if not in s_list operation by the loop "while" and compare complexity of these functions.

So, I've made the following code(doesn't work):

def remove2(l_list,s_list):
    res=[]
    for item in l_list:
        found=False
        i=0
        while len(s_list)>i and not found:
            if item==s_list[i]:
                found=True
                if not found:
                    res.append(item)
            i+=1
    return res

Examples:

>>> remove2([1,2,3],[1,2])
[3]
>>> remove2([1,2,3],[1])
[2,3]

What am I doing wrong? What is wrong with my logic?

Answer 1

You should append the item after the while loop:

def remove2(l_list,s_list):
    res=[]
    for item in l_list:
        found=False
        i=0
        while len(s_list)>i and not found:
            if item==s_list[i]:
                found=True
            i+=1
        if not found:
            res.append(item)

    return res

Instead of using the found variable you could simplify this if you use break :

def remove2(l_list,s_list):
    res=[]
    for item in l_list:
        i=0
        while len(s_list)>i:
            if item==s_list[i]:
                break
            i+=1
        else:  
            # this else belongs to the while loop and is executed if and only if
            # the loop wasn't terminated by "break".
            res.append(item)

    return res

Answer 2

There's certainly a logic flaw here:

        if item==s_list[i]:
            found=True
            if not found:
                res.append(item)
        i+=1

found can never be False at that inner if -- you just set it to True !

Also, do you need to go through this painstaking check? You can "easily" :-) do this with a one-line command. With just list comprehensions:

return [item for item in l_list if item not in s_list]

You could also turn them into sets, take the difference, and convert back to a list:

return list(set(l_list).difference(set(s_list)))