这行代码做什么？

Question

I am a newbie to python. I am solving a problem on Hackerrank https://www.hackerrank.com/challenges/maximize-it/problem .

I wasn't able to solve the problem so I had opened the discussions. I found a code there -

from itertools import product

K,M = map(int,input().split())
N = (list(map(int, input().split()))[1:] for _ in range(K))
results = map(lambda x: sum(i**2 for i in x)%M, product(*N))
print(max(results))

I have 2 doubts:

1. First, if I try to print N, it throws "generator object is not subscriptable" error, but the value is already converted into list, so I am unable to understand the reason for error.
2. I am unable to understand what will be stored as a result of results=map(lambda x: sum(i**2 for i in x)%M, product(*N)) .

I understand what product(*N) does. But what would be the output of sum(i**2 for i in x)%M ? Is it adding the squares of the values of list and then performing the modulus operation? Or is it something else?

Answer 1

For your 1st doubt:

Why is N a generator?

The list function is being applied to the map(int, input().split()) statement and indeed that is a list being returned, however, the outermost pair of parentheses and for _ in range(K) means that a generator object is being returned to N and not the list. If you want to see the contents of N while debugging, change the outermost parentheses to square brackets [ and ] and add a print(N) statement after that as such:

# Debug & view contents of N like this:
N = [ list(map(int, input().split()))[1:] for _ in range(K) ]
print(N)

For your 2nd doubt:

How does the value of the variable results come about?

Just so everyone who's reading this answer is clear, in the below example, I'll let:

N = [[1, 2], [3, 4], [5, 6]]

product(*N) will return an iterator for a list of tuples containing the cartesian product of each of the sublists of N, eg product(*N) returns the tuples (1, 3, 5), (1, 3, 6), (1, 4, 5), (1, 4, 6), ..., (2, 4, 6) .

You can verify this by doing print([x for x in product(*N)]) or if you iterate through N in a for loop and print out each value as such:

N = [[1, 2], [3, 4], [5, 6]]

# You can verify the contents of a generator like this:
print([item for item in product(*N)])

# Or like this:
for item in product(*N):
    print(item)

Breaking down results = map(lambda x: sum(i**2 for i in x)%M, product(*N)) :

In (i**2 for i in x) , x refers to one of the tuples, eg (1, 3, 5) in product(*N) , and returns a generator that contains the values (1, 9, 25) . So it's basically squaring every integer in the tuple (1, 3, 5) and returning a tuple of their squares.

sum(i**2 for i in x) sums every integer in the tuple x to give you 1 + 9 + 25 = 35.

After that, the resultant sum, 35, is applied to the modulus operation % M .

Finally, results would contain the generator with all of the resultant modulus sums for every tuple. Similarly, you can verify the content of results by doing print([x for x in results]) or if you iterate results in a for loop and print out each value just like how it's done in the code snippet above.

print(max(results)) would then give you the largest value in results .

( I assume Python-3.x is used. )