python - what means "x=sorted(list(set(x for name,x in marksheet)))" in this code

Question

marksheet=[]
marksheet = [[input(), float(input())] for _ in range(int(input()))]
        

x=sorted(list(set(x for name,x in marksheet)))
print(x)

I've just started studying python,I'm not understand the "set(x for name,x in marksheet)" part.What is mean of that ?

question from:https://stackoverflow.com/questions/65923876/what-means-x-sortedlistsetx-for-name-x-in-marksheet-in-this-code

Answer 1

x = set(x for name,x in marksheet)

First, the x on the right has nothing to do with the x on the left. I'll call the one on the right x_i to distinguish them.

1. set: We're going to make a Python set. A set is a container (like a list) with the rule that each item may only appear once.

2. x_i for name,x_i in marksheet: This is a generator expression (like a list comprehension). It's a shortcut for doing a for loop.

3. name,x_i in marksheet: This is tuple unpacking. marksheet looks something like [(name1, x1), (name2, x2), ...]: it is a list of lists. The name,x_i there could also be written (name, x_i): more explicitly a tuple.

The same code could be written:

x = set([])  # start with an empty set
for name, x_i in marksheet:
    x.add(x_i)

So it selects the unique values of the second element of each pair in marksheet.

Since name isn't used, it's common to use the dummy variable _ for that. You could use a direct set comprehension, too, instead of a generator expression:

x = {x_i for _, x_i in marksheet}