Question

Assume we have a list of 5 tuple:

(a, b , c, d, e)

Let the list be student_tuples.

I wish to sort the list different orders for different fields.

The below mentioned command

sorted(student_tuples, key=itemgetter(2,4,0,1))

Will sort the list on ascending order for all the fields.

The below mentioned command

sorted(student_tuples, key=itemgetter(2,4,0,1), reverse=true)

Will sort the list on descending order for all the fields.
What I am looking for is sorting a list on different orders for different fields.
Is there a easy way to do so.

Based on the answers the technique could be used in any language

Thanks,
Gudge

Answer 1

If the values are numeric, you can do this easily using lambda:

sorted(student_tuples, key=lambda x: (x[2],x[4],-x[0],x[1]))
                                                #^ This field will be 
                                                #  in descending order 

If you can’t easily negate the order inside a lambda function, you need to rely on the stableness of python sorting and sort a few times:

s = sorted(student_tuples, key=itemgetter(1))
s.sort(key=itemgetter(0),reversed=True)
s.sort(key=itemgetter(2,4))

I explain it in more depth in this answer.

Proof that my answers above accomplish the same thing (with numeric input):

import random
def rand_tuple():
    """ Return a random 5-tuple """
    return tuple( random.random() for _ in range(5) )

#100 random 5-tuples
lst = [ rand_tuple() for _ in range(100) ] 

#sort the list using method 1
sorted_lst = sorted(lst, key = lambda x: (x[2],x[4],-x[0],x[1])) 

#sort the list in place using method 2
lst.sort(key = itemgetter(1))  #<- Rightmost tuple element first!!!
lst.sort(key = itemgetter(0), reversed = True)
lst.sort(key = itemgetter(2,4))

print (lst == sorted_lst) #True -- Results are the same :-)