Consider the following
abc = ["a", "b", "c", "d", "e"]
for x in abc:
print(x)
Output
a
b
c
d
e
["a", "b", "c", "d", "e"] is type of sequence called list. abc is assigned to the list.
The ordering of sequences starts at index 0 from the left.That is, "a" is at index 0.
for means for each element in the sequence. That is, the for loop will access each element of the sequence - in this example, the sequence is a list ["a", "b", "c", "d", "e"].
for each element x in the sequence ["a", "b", "c", "d", "e"]
- Begins at
index = 0, such that x = abc[0] = "a"
- print each element
x such that x = abc[index] (but we do not need to the indexing)
-
for loop indirectly increment the index by 1.
Suppose content_rating = [ [...], [...], [...]] = list of list
for row in content_rating
for each element row in the sequence content_rating.
Here row can be label as anything we want (Example, for x in content_rating but x is not meaningful as row.) However, we want to make the code readable. row informs the user that row contains the row data that is represent by a sequence list.
This is because content_rating is 2-dimensional list.
For illustration purpose (and not Python code):
-
content_rating = [ [...], [...], [...]] = list of list
-
row = sequence list = [...]
In your example, content_rating is a dictionary
content_rating is a dictionary {'4+': 4433, '12+': 1155, '9+': 987, '17+': 622}. And, for row in content_rating is not the appropriate way to iterate a dictionary.
for row in content_rating means for each element row of dictionary content_rating key values ['4+', '12+', '9+', '17+']. content_rating[row] access the value for particular key in the dictionary.
Label row for each element makes the code not as readable since there is no row but we actually meant key.
Edit code in your example
Use the following instead
for k, v in content_rating.items():
.items() converts into a sequence list of tuple (key, value) pair . That is, .items() = [(key1, value1), ...., (keyn, valuen)]
k, v gets assign to each element (key, value)
where k means key of the dictionary
and, where v means value of the dictionary
And replace
content_ratings[row]
with
v
You can simplified the code by using a dictionary comprehension
c_ratings_proportions = { k: v/total_number_of_apps for k, v in content_ratings.items()}
.items() converts into a sequence list of tuple (key, value) pair . That is, .items() = [(key1, value1), ...., (keyn, valuen)]
k, v gets assign to each element (key, value)
where k means key of the dictionary
and, where v means value of the dictionary
Reads mathematically as
dictionary[key] = value / total_number_of_apps, for each element (key, value) in dictionary content_ratings
Other questions on data structures
Dictionary { } are not sequences.
- Uses key to access the value
- When using a
for loop to iterate dictionary values, use .items() to convert into a sequence list of tuple (key, value) for for loop iterating each element.
Sequence, for example list [ ], or tuple ( , ) (tuples can contains 1 or more elements separated by a comma ,)
- Uses index to access elements in the sequences
- Sequences start index at 0.
There are different types of data structures. Read up on data structures in Python Tutorial.
You need to read up on Dictionary. Here’s the dictionary section in Python Tutorial.