Sorting lists in dictionary

Question

Could someone please explain how I could sort a list in dictionary? For example:

B = {'Mary': [(850, 1000), (9, 10), (1000, 3000), (250, 550)], 'john': [(500, 1000), (800,3000), (20, 100), (5, 36)]}

Using the 'sorted' function, how do I sort it in ascending order based on the first value in the list? Likewise, how do I sort it in ascending order based on the second value in the list?

Many thanks

this seems like something they should probably have covered in whatever class this is from ... — Joran Beasley
– Joran Beasley, Commented Jul 15, 2014 at 19:17

Cory Kramer · Accepted Answer · 2014-07-15 19:15:19Z

2

I would iterate through your items, then in-place sort based on the first element of each tuple.

B = {
      'Mary': [(850, 1000), (9, 10), (1000, 3000), (250, 550)],
      'john': [(500, 1000), (800,3000), (20, 100), (5, 36)],
    }

for item in B:
    B[item].sort(key = lambda i: i[0])

Output

{
  'john': [(5, 36), (20, 100), (500, 1000), (800, 3000)],
  'Mary': [(9, 10), (250, 550), (850, 1000), (1000, 3000)]
}

edited Jul 15, 2014 at 19:15

answered Jul 15, 2014 at 18:53

Cory Kramer

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2 Comments

Casey Falk Over a year ago

I liked the brevity of the list comprehension, but this is much more Pythonic.

Casey Falk Over a year ago

Note that iterating over a dictionary in Python 2.7 will automatically iterate over the keys. Thus, B.keys() could just be B. (source: stackoverflow.com/questions/3294889/…)

Piotr Dabkowski · Accepted Answer · 2014-07-15 19:09:37Z

0

You have to use its key argument. Key is a function which takes the element of the iterable as an agrument and returns the value on which sorting is based:

for e in B:
    B[e] = sorted(B[e], key=lambda x: x[Element_ID])

Element ID is the index of the element on which you want to base your sort. So it will be 1 if you want to sort according to the second element and 0 if you want to sort according to the first element.

EDIT:

Also it would be faster to use list's sort method instead of sorted:

for e in B:
    B[e].sort(B[e], key=lambda x: x[Element_ID])

edited Jul 15, 2014 at 19:09

answered Jul 15, 2014 at 18:51

Piotr Dabkowski

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6 Comments

user2555451 Over a year ago

B[e].sort(key=lambda x: x[Element ID]) would be more efficient because it does not need to construct a new list.

Casey Falk Over a year ago

The question asks, "... Based on the first value in the list." [Element ID] probably should be [0].

Piotr Dabkowski Over a year ago

@iCodez He wanted to use sorted but anyway thank you I will add it.

ScottO Over a year ago

@user3302763 - It's the choice of which element of the tuple you want to sort on, e.g. x[0] first element, x[1] second element.

Piotr Dabkowski Over a year ago

@Casey Falk 'Likewise, how do I sort it in ascending order based on the second value in the list?' I think he wants a general method.

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