Vfrdtyky

I'm currently new to python and got stuck at this question, can't seem to find the proper answer.

question:Given a list of words, return a list with the same words in order of length (longest to shortest), the second sort criteria should be alphabetical. Hint: you need think of two functions.

This is what I have so far:

def bylength(word1,word2):

    return len(word2)-len(word1)



def sortlist(a):

    a.sort(cmp=bylength)

    return a

it sorts by length but I don't know how to apply the second criteria to this sort, which is by alphabetical descending.

You can do it in two steps like this:

the_list.sort() # sorts normally by alphabetical order

the_list.sort(key=len, reverse=True) # sorts by descending length

Python's sort is stable, which means that sorting the list by length leaves the elements in alphabetical order when the length is equal.

You can also do it like this:

the_list.sort(key=lambda item: (-len(item), item))

Generally you never need cmp, it was even removed in Python3. key is much easier to use.

n = ['aaa', 'bbb', 'ccc', 'dddd', 'dddl', 'yyyyy']



for i in reversed(sorted(n, key=len)):

    print i

yyyyy dddl dddd ccc bbb aaa

for i in sorted(n, key=len, reverse=True):

     print i

yyyyy dddd dddl aaa bbb ccc

-Sort your list by alpha order, then by length.



See the following exmple:



>>> coursesList = ["chemistry","physics","mathematics","art"]

>>> sorted(coursesList,key=len)

['art', 'physics', 'chemistry', 'mathematics']

>>> coursesList.append("mopsosa")

>>> sorted(coursesList,key=len)

['art', 'physics', 'mopsosa', 'chemistry', 'mathematics']

>>> coursesList.sort()

>>> sorted(coursesList,key=len)

['art', 'mopsosa', 'physics', 'chemistry', 'mathematics']

Although Jochen Ritzel said you don't need cmp, this is actually a great use case for it! Using cmp you can sort by length and then alphabetically at the same time in half the time sorting twice would take!

def cmp_func(a, b):

    # sort by length and then alphabetically in lowercase

    if len(a) == len(b):

        return cmp(a, b)

    return cmp(len(a), len(b))



sorted_the_way_you_want = sorted(the_list, cmp=cmp_func)

Example:

>>> the_list = ['B', 'BB', 'AA', 'A', 'Z', 'C', 'D']

>>> sorted(the_list, cmp=cmp_func)

['A', 'B', 'C', 'D', 'Z', 'AA', 'BB']

Note, if your list is a mix of upper and lower case replace cmp(a, b) with cmp(a.lower(), b.lower()) as python sorts 'a' > 'Z'.

In python3 you'd need to be sorting objects with __lt__ style comparison functions defined or functools.cmp_to_key() which does that for you.