Using NumPy to Find Median of Second Element of List of Tuples

Question

Let's say I have a list of tuples, as follows:

list = [(a,1), (b,3), (c,5)]

My goal is to obtain the first element of the median of the list of tuples, using the tuples' second element. In the above case, I would want an output of b, as the median is 3. I tried using NumPy with the following code, to no avail:

import numpy as np

list = [('a',1), ('b',3), ('c',5)]
np.median(list, key=lambda x:x[1])

Answer 1

You could calculate the median like this:

np.median(dict(list).values()) 
# in Python 2.7; in Python 3.x it would be `np.median(list(dict(list_of_tuples).values()))`

That converts your list to a dictionary first and then calculates the median of its values.

When you want to get the actual key, you can do it like this:

dl = dict(list) #{'a': 1, 'b': 3, 'c': 5}

dl.keys()[dl.values().index(np.median(dl.values()))]

which will print 'b'. That assumes that the median is in the list, if not a ValueError will be thrown. You could therefore then use a try/except like this using the example from @Anand S Kumar's answer:

import numpy as np

l = [('a',1), ('b',3), ('c',5), ('d',22),('e',11),('f',3)]

# l = [('a',1), ('b',3), ('c',5)]

dl = dict(l)
try:
    print(dl.keys()[dl.values().index(np.median(dl.values()))])
except ValueError:
    print('The median is not in this list. Its value is ',np.median(dl.values()))
    print('The closest key is ', dl.keys()[min(dl.values(), key=lambda x:abs(x-np.median(dl.values())))])

For the first list you will then obtain:

The median is not in this list. Its value is 4.0

The closest key is f

for your example it just prints:

b

Answer 2

np.median does not accept any argument called key . Instead you can use a list comprehension, to take just the second elements from the inner list. Example -

In [3]: l = [('a',1), ('b',3), ('c',5)]

In [4]: np.median([x[1] for x in l])
Out[4]: 3.0

In [5]: l = [('a',1), ('b',3), ('c',5), ('d',22),('e',11),('f',3)]

In [6]: np.median([x[1] for x in l])
Out[6]: 4.0

Also, if its not for example purpose, do not use list as variable name, it shadows the builtin function list .