how to convert two lists into a dictionary (one list is the keys and the other is the values)?

Question

This is code in IDLE2 in python, and error.

I need to include each "data" element as key and value "otro", in an orderly manner. Well "data" and "otro" it's list with 38 string's, as for "dik" it's an dictionary.

>>> for i in range(len(otro)+1):
    dik[dato[i]] = otro[i]  

Traceback (most recent call last):
  File "<pyshell#206>", line 2, in <module>
    dik[dato[i]] = otro[i]
IndexError: list index out of range
>>>

this problem is range(0, 38) output -> (0, 1,2,3 ... 37) and it is all messy

Can you read out loud the last line and think about what the interpreter is telling you? — JBernardo, Nov 24 '12 at 01:57

mgilson · Accepted Answer · 2012-11-24T06:12:55.300

13

I think something like:

dik = dict(zip(dato,otro))

is a little cleaner...

If dik already exists and you're just updating it:

dik.update(zip(dato,otro))

If you don't know about zip, you should invest a little time learning it. It's super useful.

a = [ 1 , 2 , 3 , 4 ]
b = ['a','b','c','d']
zip(a,b)   #=>   [(1,'a'),(2,'b'),(3,'c'),(4,'d')] #(This is actually a zip-object on python 3.x)

zip can also take more arguments (zip(a,b,c)) for example will give you a list of 3-tuples, but that's not terribly important for the discussion here.

This happens to be exactly one of the things that the dict "constructor" (type) likes to initialize a set of key-value pairs. The first element in each tuple is the key and the second element is the value.

edited Nov 24 '12 at 06:12

answered Nov 24 '12 at 02:01

mgilson

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+1. This is the best solution, although RocketDonkey's answer does a good job of explaining why the current solution fails. – Gareth Latty Nov 24 '12 at 02:01
@Lattyware -- Agreed. +1 to his :) – mgilson Nov 24 '12 at 02:03
@mgilson +1's all around - this is definitely cleaner. – RocketDonkey Nov 24 '12 at 02:04
it's ok!!! thanks, gracias !! – opmeitle Nov 24 '12 at 02:04
muchachos es el poder de python! dictionary.update(zip(list1,list2)) – opmeitle Nov 24 '12 at 02:09

score 10 · Answer 2 · edited Nov 24 '12 at 06:15

10

The error comes from this: range(len(otro)+1). When you use range, the upper value isn't actually iterated, so when you say range(5) for instance, your iteration goes 0, 1, 2, 3, 4, where position 5 is the element 4. If we then took that list elements and said for i in range(len(nums)+1): print nums[i], the final i would be len(nums) + 1 = 6, which as you can see would cause an error.

The more 'Pythonic' way to iterate over something is to not use the len of the list - you iterate over the list itself, pulling out the index if necessary by using enumerate:

In [1]: my_list = ['one', 'two', 'three']

In [2]: for index, item in enumerate(my_list):
   ...:     print index, item
   ...:
   ...:
0 one
1 two
2 three

Applying this to your case, you can then say:

>>> for index, item in enumerate(otro):
...    dik[dato[index]] = item

However keeping with the Pythonicity theme, @mgilson's zip is the better version of this construct.

edited Nov 24 '12 at 06:15

mgilson

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answered Nov 24 '12 at 01:57

RocketDonkey

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-1. This may be the issue, but it doesn't really make the code better. Why loop over indices? – Gareth Latty Nov 24 '12 at 01:57
1

`for i, o in enumerate(otro):` is a little "neater". Or `for d, o in zip(dato, otro):`. Or `dik = dict(zip(dato,otro))` instead of the whole block. – millimoose Nov 24 '12 at 01:58
@Lattyware Ha, reading my mind - I'm augmenting now :) – RocketDonkey Nov 24 '12 at 01:58
@millimoose Why do you need the index here? ``zip()`` the iterables if you need to loop over two. – Gareth Latty Nov 24 '12 at 01:59
@Lattyware Because I thought of `zip()` about 0.5 seconds after submitting the comment ;) – millimoose Nov 24 '12 at 02:00
Downvote changed to an upvote, this is a good answer now. – Gareth Latty Nov 24 '12 at 02:00
@Lattyware Thanks - with the answer as it was, definitely a justified downvote :) – RocketDonkey Nov 24 '12 at 02:01
thanks!!! i don't speaking inglish expert, and sorry!! gracias – opmeitle Nov 24 '12 at 02:03
@opmeitle No problemo - que tengas suerte con todo! – RocketDonkey Nov 24 '12 at 02:08
@RocketDonkey -- estoy celoso. quiero a aprender espanol. – mgilson Nov 24 '12 at 02:11
@mgilson Jeje, en mi opinion, es mas facil para aprender cuando escuches musica y trates de traducir cada palabra. I'm garbage at it, but definitely good times :) – RocketDonkey Nov 24 '12 at 02:14
@mgilson Ha, good call on the edit :) – RocketDonkey Nov 24 '12 at 06:34

how to convert two lists into a dictionary (one list is the keys and the other is the values)?

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