Sorting elements from a list of tuples into two separate lists

Question

As the title says I'm in the process of sorting tagged elements into two lists. My current code for this is:

lst = [("foo", "good"), ("bar", "bad")...("x", "n")]

def sort(items):
    good = []
    bad = []
    for i in range(len(items)):
        if items[i][1] == 'good':
            good += items[i][0]
        else:
            bad += items[i][0]
    return good, bad


alpha, beta = sort(lst)

My output is simply f . I've been trying to work this out for a while but the solution has been eluding me. Any suggestions?

Thank you all.

Answer 1

You can fix your code:

def sort(items):
    good = []
    bad = []
    for x, y in items:
        if y == 'good':
            good.append(x)
        else:
            bad.append(x)
    return good, bad

Note that += corresponds to list.extend which adds iterables element-wise. For strings that means char by char.

Answer 2

To create new lists with mapping or filtering, it's often shorter and easier to read to use list comprehensions rather than calling .append in a loop:

data = [("foo", "good"), ("bar", "bad"), ("head", "good"), ("tails", "bad"), ("x", "n")]

good_lst = [x for x,quality in data if quality == 'good']
bad_lst = [x for x,quality in data if quality != 'good']

print(good_lst)
# ['foo', 'head']
print(bad_lst)
# ['bar', 'tails', 'x']

Answer 3

Your code is fine, but you need to change one thing:

items[i][0] -> [items[i][0]]

When you want to use + you need to have values as list:

>>> [2,3] + [4]
[2,3,4]

Your code with one change:

lst = [("foo", "good"), ("bar", "bad"), ("head", "good"), ("tails", "bad")]

def sort(items):
    good = []
    bad = []
    for item in items:
        if item[1] == 'good':
            good += [item[0]]
        else:
            bad += [item[0]]
    return good, bad


alpha, beta = sort(lst)
print (alpha, beta)

Output:

['foo', 'head'] ['bar', 'tails']

Answer 4

You can try to solve this in a cleaner way using list comprenhension to compute both lists:

def sort(items):
    good = [items[idx][0] for idx in range(0,len(items)) if items[idx][1] == 'good']
    bad = [items[idx][0] for idx in range(0,len(items)) if items[idx][1] == 'bad']

    return good, bad

If performance is not an issue this could be better due to it is a declarative approach instead of an imperative one so it could be easier to read, but takes into account that it iterates the list twice.

Hopes it helps you!

Answer 5

+= is used when you try to add LHS to RHS with same datatype. When you do good += items[i][0] it's like adding foo as a list ( ['f', 'o', 'o'] ) to the list good . That's the reason you see the behavior. Hence use list.append() to add element to a list. Try out this:

lst = [("foo", "good"), ("bar", "bad"), ("head", "good"), ("tails", "bad")]

def sort(items):
    good = []
    bad = []
    for item in items:
        #print (item, type(item))
        if item[1] == 'good':
            good.append(item[0])
        else:
            bad.append(item[0])
    return good, bad


alpha, beta = sort(lst)
print (alpha, beta)

Output:

['foo', 'head'] ['bar', 'tails']

Answer 6

I just changed good += items[i][0] to good.append(i[0])

lst = [("foo", "good"),("bar", "bad"),("x", "n")]

def sort(items):
    good = []
    bad = []
    for i in items:
        if i[1] == 'good':
            good.append(i[0])
        else:
            bad.append(i[0])
    return good, bad


alpha, beta = sort(lst)
print(alpha)
print(beta)

Output:

['foo']
['bar', 'x']