Finding the max value of list in dictionary

Question

I have a dict and behind each key is a list stored. Looks like this:

dict with values:  {
u\'New_York\': [(u\'New_York\', u\'NY\', datetime.datetime(2014, 8, 13,         


        

Answer 1

max() takes a second parameter, a callable key that lets you specify how to calculate the maximum. It is called for each entry in the input iterable and its return value is used to find the highest value. You need to apply this against each value in your dictionary; you are finding the maximum of each individual list here, not the maximum of all values in a dictionary.

Use that against the values; the rest is just formatting for the output; I've used a dict comprehension here to process each of your key-value pairs from the input and produce a dictionary again for the output:

{k: max(v, key=lambda i: i[-1]) for k, v in somedic.iteritems()}

You could also use the operator.itemgetter() function to produce a callable for you, instead of using a lambda:

from operator import itemgetter

{k: max(v, key=itemgetter(-1)) for k, v in somedic.iteritems()}

Both grab the last element of each input tuple.

Demo:

>>> import datetime
>>> from pprint import pprint
>>> somedic = {
... u'New_York': (u'New_York', u'NY', datetime.datetime(2014, 8, 13, 0, 0), 10), (u'New_York', u'NY', datetime.datetime(2014, 8, 13, 0, 0), 4), (u'New_York', u'NY', datetime.datetime(2014, 8, 13, 0, 0), 3)], 
... u'Jersy': [(u'Jersy', u'JY', datetime.datetime(2014, 8, 13, 0, 0), 6), (u'Jersy', u'JY', datetime.datetime(2014, 8, 13, 0, 0), 7)], 
... u'Alameda': [(u'Alameda', u'CA', datetime.datetime(2014, 8, 13, 0, 0), 1), (u'Alameda', u'CA', datetime.datetime(2014, 8, 13, 0, 0), 2), (u'Alameda', u'CA', datetime.datetime(2014, 8, 13, 0, 0), 3), (u'Alameda', u'CA', datetime.datetime(2014, 8, 13, 0, 0), 1)]
... }
>>> {k: max(v, key=lambda i: i[-1]) for k, v in somedic.iteritems()}
{u'New_York': (u'New_York', u'NY', datetime.datetime(2014, 8, 13, 0, 0), 10), u'Jersy': (u'Jersy', u'JY', datetime.datetime(2014, 8, 13, 0, 0), 7), u'Alameda': (u'Alameda', u'CA', datetime.datetime(2014, 8, 13, 0, 0), 3)}
>>> pprint(_)
{u'Alameda': (u'Alameda', u'CA', datetime.datetime(2014, 8, 13, 0, 0), 3),
 u'Jersy': (u'Jersy', u'JY', datetime.datetime(2014, 8, 13, 0, 0), 7),
 u'New_York': (u'New_York', u'NY', datetime.datetime(2014, 8, 13, 0, 0), 10)}