Question

I have a dictionary dict1['a'] = [ [1,2], [3,4] ] and need to generate a list out of it as l1 = [2, 4]. That is, a list out of the second element of each inner list. It can be a separate list or even the dictionary can be modified as dict1['a'] = [2,4].

Answer 1

Assuming that each value in the dictionary is a list of pairs, then this should do it for you:

[pair[1] for pairlist in dict1.values() for pair in pairlist]

As you can see:

• dict1.values() gets you just the values in your dict,
• for pairlist in dict1.values() gets you all the lists of pairs,
• for pair in pairlist gets you all the pairs in each of those lists,
• and pair[1] gets you the second value in each pair.

Try it out. The Python shell is your friend!…

>>> dict1 = {}
>>> dict1['a'] = [[1,2], [3,4]]
>>> dict1['b'] = [[5, 6], [42, 69], [220, 284]]
>>> 
>>> dict1.values()
[[[1, 2], [3, 4]], [[5, 6], [42, 69], [220, 284]]]
>>> 
>>> [pairlist for pairlist in dict1.values()]
[[[1, 2], [3, 4]], [[5, 6], [42, 69], [220, 284]]]
>>> # No real difference here, but we can refer to each list now.
>>> 
>>> [pair for pairlist in dict1.values() for pair in pairlist]
[[1, 2], [3, 4], [5, 6], [42, 69], [220, 284]]
>>> 
>>> # Finally...
>>> [pair[1] for pairlist in dict1.values() for pair in pairlist]
[2, 4, 6, 69, 284]

While I’m at it, I’ll just say: ipython loves you!