Understanding max function function in Python dictionary
I'm trying to understand the max operation of a function in a Python dictionary. Below is the code I am using:
tall_buildings = {
"Empire State": 381, "Sears Tower": 442,
"Burj Khalifa": 828, "Taipei 101": 509
}
# 1. find the height of the tallest building
print("Height of the tallest building: ", max(tall_buildings.values()))
# 2. find the name, height pair that is tallest
print(max(tall_buildings.items(), key=lambda b: b[1]))
# 3. find the tallest building
print(max(tall_buildings, key=tall_buildings.get))
All of the above printing instructions give correct results as noted in the comments in the code.
I figured out how #1
and #2
.
1:
tall_buildings.values()
gives the flow of heights and the functionmax
returns the maximum height.2:
tall_buildings.items()
gives a stream of pairs (name, height) and the max function returns a pair based onkey=pair height.
However, I am having a hard time understanding how it works # 3
. What key=tall_buildings.get
is the key to finding the tallest building?
I took the code from Ned Pycon Talk: https://youtu.be/EnSu9hHGq5o?t=12m42s
source to share
The max()
function iterates over the first argument, applies the key function to each item, and selects the item with the maximum key.
Iterating over a dictionary is the same as iterating over its keys. By doing
max(tall_buildings, key=tall_buildings.get)
we will first go through all the keys in tall_buildings
. For each key, a k
key function will be evaluated tall_buildings.get(k)
, which returns the height of the building indicated by k
. It will then be selected and returned k
with the maximum height.
source to share
How # 3 works, the method provided as key
just looks up the value from the dictionary tall_buildings
. Thus, for each key
repeating one, a corresponding one value
will be provided get
.
Method get
is synonymous with operator[]
>>> tall_buildings['Sears Tower']
442
>>> tall_buildings.get('Sears Tower')
442
Reason # 3 is for looping over the keys in the first place, is that by default iterating over dict
will only iterate over the keys
for i in tall_buildings:
print(i)
Taipei 101
Empire State
Burj Khalifa
Sears Tower
You can also explicitly loop through the keys
for i in tall_buildings.keys():
print(i)
Taipei 101
Empire State
Burj Khalifa
Sears Tower
Similarly, you can iterate over .values()
which are only the values in the dictionary, or .items()
that iterate over tuples of pairs (key,value)
.
source to share
The max concept requires a definition of the ordering of items.
So, here you provide a parameter key
just as you would do with sort : a function that is applied to each element of the dictionary to match pairs (key, val)
to values that have an inline ordering definition (e.g. numbers, strings). This way you will find the maximum value of the displayed values and the result will be the corresponding element in the original dictionary.
source to share