Finding lowest value in dictionary of objects, how?

Steven D'Aprano steven at REMOVE.THIS.cybersource.com.au
Mon Nov 19 10:19:37 CET 2007


On Mon, 19 Nov 2007 00:18:33 -0800, davenet wrote:

> Hi,
> 
> I'm new to Python and working on a school assignment.

Thank you for your honesty.


> I have setup a dictionary where the keys point to an object. Each object
> has two member variables. I need to find the smallest value contained in
> this group of objects.
> 
> The objects are defined as follows:
> 
> class Block:
>    def __init__(self,addr):
>       self.addr = addr
>       self.age = 0
> 
> My dictionary is defined as:
>    blocks = {}


One possible approach is to define your Block data-type so that it 
defines less-than and greater-than comparisons. Then you can just ask 
Python to find the minimum Block by passing a list (not a dictionary) of 
Blocks to the function min().

 
> I have added 1000 (will hold more after I get this working) objects. I
> need to find the lowest age in the dictionary. If there is more than one
> age that is lowest (like if several of them are '1', etc), then I can
> just pick randomly any that equal the lowest value. I don't care which
> one I get.

Question: you don't explain how you have added them to the dict. A dict 
has a key and a value. What are the keys, and what are the values?

 
> I saw the following code here but I don't know how to use this sample to
> get at the values I need in the blocks object.
> 
> def key_of_lowest(self,addr)
>    lowest = min(self.blocks.values())
>    return [k for k in self.blocks if self.blocks[k]==val][0]
> 
> This one returns the lowest value Object, but not the lowest value of
> age in all the Objects of the table.

That code doesn't make much sense. It looks like a method rather than a 
function (the argument "self" is the giveaway). What is self.blocks and 
what is val?


> I hope someone can help me figure out the syntax for what I'm trying to
> do.

The first step you should do is write down how YOU would solve the 
problem.

"Let's see now... if I had a list of objects, and I wanted to find the 
smallest one, I would look at the first object, and compare it to all the 
other objects. If it was smaller than or equal to every other object in 
the list, I've found the smallest object and I'm finished!

If not, I'd take the second object, and compare it to all the other 
objects. If it is smaller than or equal to everything else, I've found 
the smallest object, and I'm finished.

If not, I would do the same for the third, and fourth, and fifth, and so 
forth, until I've found the smallest object."

Now start converting it to Python, step by step:

# Start with English instructions:
with each item in the list of objects:
    if item is smaller than all the other items:
        item is the smallest, and we're done


# Turn it into a function:
function find the smallest(list of objects):
    with each item in the list of objects:
        if item is smaller than all the other items:
            item is the smallest, and we're done


# Now start using Python syntax:
def find_smallest(list_of_objects):
    for item in list_of_objects:
        if item is smaller than all the other items:
            return item


# And continue:
def find_smallest(list_of_objects):
    for item in list_of_objects:
        for x in list_of_objects:
            if item <= x:
                return item



What I've done there is re-write the min() function in one of the 
slowest, most inefficient ways possible. If you try doing it by hand, 
you'll quickly see it's VERY inefficient. The better ways should be 
obvious once you actually do it. Then go through the process of writing 
it as Python code.



Hope this helps,


-- 
Steven.



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