Sorting tuples in a list based on their product in Python

user_17234183 0
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Im trying to write a procedure to sort the tuples in a list based on their product value per tuple. I just tried sorting them first using a while loop but Im not able to integrate the multiplication of the numbers inside the tuple. The code I have so far is:

def sort_list (a):
    i = 0
    while i < len(a):
        key = i
        j = i + 1
        
        #first checking throuh the whole list if any number is bigger
        
        while j < len(a):
            if a[key] > a [j]:
                key = j
            j += 1
            
        #swap the numbers if the key one (i) is bigger than the j one the j one will be the new key
        # and swapped in the next section!
        
        a[i], a[key] = a[key], a[i]
        i += 1
    return a        

m = [(2, 3.0), (3, 1.0), (4, 2.5), (1, 1.0)]

sort_list(m)

print (m)

# should output : [(1, 1.0), (3, 1.0), (2, 3.0), (4, 2.5)]
product tuples sorting python
3 Answers

The list class has a method .sort which sorts in-place and has a key argument to specify custom comparison.

  • list.sort:

    key specifies a function of one argument that is used to extract a comparison key from each list element (for example, key=str.lower). The key corresponding to each item in the list is calculated once and then used for the entire sorting process. The default value of None means that list items are sorted directly without calculating a separate key value.

Python >= 3.8

In your case, we need to calculate the product of all the elements in the tuple. We can use math.prod1 similar to sum.

from math import prod

m = [(2, 3.0), (3, 1.0), (4, 2.5), (1, 1.0)]
m.sort(key=prod)
# list -> [(1, 1.0), (3, 1.0), (2, 3.0), (4, 2.5)]

Online Demo

I used math.prod to generalize the solution giving you the flexibility to use the solution on arbitrary length iterable.

Python < 3.8

We can use functools.reduce with operator.mul to generalize to make it work with arbitrary length iterable. The below code works in Python2.7 also.

from functools import reduce, partial
from operator import mul

prod = partial(reduce, mul)
m.sort(key=prod)

Online Demo

1. math.prod is available from python3.8

user_124164530

Use list.sort() or sorted() with key argument

Since Python 3.8 there is a function math.prod which can calculate the product of items in a tuple. You can use it as the key argument like that:

import math.prod
m = [(2, 3.0), (3, 1.0), (4, 2.5), (1, 1.0)]
list.sort(m, key=math.prod)
print(m)

Prior to Python 3.8 you can accomplish that with lambda (or any other function):

m = [(2, 3.0), (3, 1.0), (4, 2.5), (1, 1.0)]
list.sort(m, key=lambda x: x[0] * x[1])
print(m)

The difference between list.sort() and sorted() is that list.sort() is changing m in place so you don't have to assign it to m again. If you use sorted() on the other hand, the sorted list is beeing returned as a new list so you would have to assign it t m.

user_127317450

list.sort and sorted both let you specify a key argument, which should be a function giving a "score" which will be used for sorting. In your case, we just have to make this score the product of the tuple elements:

sorted(m, key=lambda x: x[0]*x[1])
user_131459540
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