python how to unzip elements in a list, zip(*) doesn't work for me

Question

I wrote this program, where I need to make a list of names, with last names alphabetized, and first names in front of it. This is what I did so far:

firstnames = ['good', 'bad', 'tall', 'big']
lastnames = ['boy', 'girl', 'guy', 'man']
list3 = [a + b for a, b in zip(firstnames, lastnames)]
l3 = sorted(list3)
n = len(l3)
l4 = zip(*[l3])
print l3
print l4

the zip* makes the elements into tuples, but how do I separate the elements again, and join them in a different order?

What output would you like? The code that you have *seems* like you want to just have another copy of the list, which you can accomplish using `l4 = l3[:]` or a host of other methods. — Wayne Werner, Sep 26 '13 at 15:29
How could you separate the items again? `l3` is `['badgirl', 'bigman', 'goodboy', 'tallguy']`: you've concatenated the strings together. How could the code know to separate `goodboy` into `good` and `boy`? — David Robinson, Sep 26 '13 at 15:30
possible duplicate of [A Transpose/Unzip Function in Python (inverse of zip)](http://stackoverflow.com/questions/19339/a-transpose-unzip-function-in-python-inverse-of-zip) — Saullo G. P. Castro, Sep 26 '13 at 15:32

Steve Jessop · Accepted Answer · 2013-09-26T17:06:57.077

2

Once you've done a+b you can't separate them. You've concatenated the strings together, and there's nothing in the result to indicate what part of the string is the firstname and what part the lastname.

Do your sorting on the tuples that come from zip:

print sorted(zip(firstnames, lastnames), key = lambda pair: pair[1])

One they're sorted, you can concatenate the strings.

Also, be aware that you could just zip them in the other order, then they'll sort on the lastname without needing to specify key:

print [b + a for a, b in sorted(zip(lastnames, firstnames))]

edited Sep 26 '13 at 17:06

answered Sep 26 '13 at 15:29

Steve Jessop

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`key = lambda pair: pair[1]` means it sort list of tuples-pairs using first element in pair? for `[('c', -1), ('a', 1), ('b': 0)]` becomes = `[('a', 1), ('b': 0), ('c', -1)]` Am I correct? – Grijesh Chauhan Sep 26 '13 at 15:43
This helped yes, I should sort after. – Kimi Lee Sep 26 '13 at 16:10
1

@GrijeshChauhan: almost. `pair[1]` is the *second* element in the pair, `pair[0]` would be the first. – Steve Jessop Sep 26 '13 at 16:45

score 1 · Answer 2 · edited Sep 26 '13 at 16:54

1

I think you just need to use zip once:

ans = ['%s %s' % (first, last) for (first, last) in zip(firstnames, sorted(lastnames))]

edited Sep 26 '13 at 16:54

Grijesh Chauhan

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answered Sep 26 '13 at 15:29

nofinator

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python how to unzip elements in a list, zip(*) doesn't work for me

2 Answers2