How to use a custom function in max(x, key=custom_function) function?

Question

I have a custom data type, say: mytime, which represent hours and minutes, such as 29:45, it is 29 hours and 45 minutes.

I want to use max built-in function to find the item in a list of lists, whose sum of its elements is the greatest, where all lists contain values of mytime type.

x = [[a, b], [c, d]]

a,b,c,d are of mytime type.

max(x, key=sum)

won't work here, because a,b,c,d, are not integers.

If I type a + b at python command line, I get the sum of these two time values, result is of mytime type, without any errors.

How do I use max function here?

So given x = [[a, b], [c, d]], you want the sublist of x such that the sum of the mytime elements in that sublist is the greatest (in terms of hours and minutes)? Or do you want to know just the greatest sum of time from one of those sublists? — Simeon Visser
– Simeon Visser, Commented Jul 17, 2012 at 10:02
I want to have the sublist of x, whose sum of elements are greatest. For example if x = [[02:00, 03:00], [09:00, 04:00]] then I want to get [09:00,04:00]. — alwbtc
– alwbtc, Commented Jul 17, 2012 at 10:09

sloth · Accepted Answer · 2012-07-17 12:31:08Z

Let's say your class looks like this:

class mytime(object):

    def __init__(self, h, m):
        self.h = h
        self.m = m

    def __add__(self, other):
        return mytime(self.h + other.h, self.m + other.m)

    def __repr__(self):
        return '%i:%i' % (self.h, self.m)

and you use it like this:

a = mytime(10, 10)        
b = mytime(2, 22)
print a + b

and it will work as expect:

12:32

Problem:

What you want to do is:

l = [a, b]
print sum(l)

but it will fail:

TypeError: unsupported operand type(s) for +: 'int' and 'mytime'

The problem is that the sum function will start with 0 and will add up all values of the list. It will try to evaluate

0 + mytime(10, 10)

which will fail.

Solution:

The solution to your problem is implementing the __radd__ function, which represents "reverse add" and is called when the arguments can't be resolved in the "forward" direction. For example, x + y is evaluated as x.__add__(y) if possible, but if that doesn't exist then Python tries y.__radd__(x).

So you can add the following method to your class:

def __radd__(self, other):
    return mytime(self.h, self.m)

and the sum function will work for you (in this implementation ignoring the other value, which is probably fine in your case).

Simeon Visser · Accepted Answer · 2012-07-17 10:12:27Z

1

You can write your own sum function:

def my_sum(item):
    return sum(60 * e[0] + e[1] for e in item)

x = [[(2,0), (3,0)], [(9, 0), (4, 0)]]
print max(x, key=my_sum)

I have represented your mytime data structure as tuples (with hours and minutes) so you may need to adjust my_sum to your data structure. The only requirement is that the hours and minutes of a mytime can be filled in for e[0] and e[1] respectively.

The above code returns the greatest element (in this case [(9, 0), (4, 0)]).

answered Jul 17, 2012 at 10:12

Simeon Visser

123k19 gold badges192 silver badges185 bronze badges

Comments

Jon Clements · Accepted Answer · 2012-07-17 10:12:42Z

1

Are you sure using a + b works? All sum does is repeatedly apply + to adjacent elements (it's the same as reduce(operator.add, sequence) with a special case to break on strings)... So if it does work - then max(x, key=sum) should just work -- as long as mydate supports comparison operators - eg __gt__, __eq__, __lt__

Example

You need to have __gt__ defined for max to work...

class mydate(object):
    def __init__(self, num):
        self.num = num
    def __add__(self, other): # make sure sum works
        return self.num + other.num
    def __gt__(self, other): # make sure max can do > comparison
        return self.num > other.num
    def __repr__(self):
        return 'date: {}'.format(self.num)


x = mydate(3)
y = mydate(5)
z = mydate(2)

print max([x,y,z], key=sum)

edited Jul 17, 2012 at 10:12

answered Jul 17, 2012 at 10:02

Jon Clements

143k34 gold badges254 silver badges288 bronze badges

2 Comments

alwbtc Over a year ago

Yes a+b works. But max(x, key=sum) didn't work because I get: TypeError: unsupported operand type(s) for +: 'int' and 'mytime' error.

Jon Clements Over a year ago

@alwbtc have expanded answer - you might need to modify __add__ or add __iadd__ to work around +'ing with int`s

Collectives™ on Stack Overflow

How to use a custom function in max(x, key=custom_function) function?

3 Answers 3

Comments

Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related