4 ms·

Can someone please explain how `for y in nums & {2020 - x} ` in his day 1 solution?

daveFNbuck 6y ago
{2020 - x} is a single item set. nums & {2020 - x} does set intersection with nums, which restricts nums to either the item that sums with x to 2020 or nothing if there is no such item in nums.
wodenokoto 6y ago
def day1_1(nums): "Find 2 distinct numbers that sum to 2020, and return their product." return first(x * y for x in nums for y in nums & {2020 - x} if x != y) `nums` is a set of integers `nums & {2020 - x}` Finds all the numbers that are in the set of `nums` and the set of `{2020 - x}` - this basically extracts a number, `y`, for which `x+y = 2020`. So for each x in nums, he extracts a number from nums that, added to x, equals 2020. If there is such a number y, he checks if it is the same as x, and only if it is different, does he yield in his generator, as the product of y and x. It's succint, but not exactly readable, imho.
sgk284 6y ago
It's a bit overly clever. I am a huge fan of Peter Norvig (and he was once my skip-level boss), but I'd call this code out in a code review for being obtuse. Notice that the input, `nums`, is a set. So he's taking the intersection of two sets. One set always has one item, so the result will be a collection with either 0 or 1 items. It could have also been written as: first(x * (2020 - x) for x in nums if (2020 - x) in nums and x != (2020 - x)) But that has a lot of duplication, so it'd probably be best to just use an imperative approach here and do something like: def day1_1(nums): "Find 2 distinct numbers that sum to 2020, and return their product." for x in nums: y = 2020 - x if x == y: continue if y in nums: return x * y
- philshem 6y ago
  when I saw the first() function, I thought it was some default python that was new to me. But he defines it above in his commonly used functions: def first(iterable, default=None) -> object: "Return first item in iterable, or default." return next(iter(iterable), default)
  - madhadron 6y ago
    How to tell he's a Lisper. :) Be glad it wasn't car().
    - norvig 6y ago
      madhadron, I'm a Common Lisper, so I'm good with `first` and `rest`.
- notretarded 6y ago
  abstruse Completely agree, your solution is more clear.
- Gupie 6y ago
  The "x == y" test looks like a bug to me, unless duplicate entries are not allowed. For example [1010,1010,1721,979,366,299,675,1456] should return 1010 * 1010, not 1721 * 299.
  - deleted 6y ago
    [deleted]
  - jstanley 6y ago
    Given that the comment says "Find 2 distinct numbers that sum to 2020", I expect duplicate entries are not allowed.
    - Gupie 6y ago
      Yes, but I cannot find the same in the problem definition: "they need you to find the two entries that sum to 2020 and then multiply those two numbers together" https://adventofcode.com/2020/day/1 https://adventofcode.com/2020/day/1

{2020 - x} is a single item set. nums & {2020 - x} does set intersection with nums, which restricts nums to either the item that sums with x to 2020 or nothing if there is no such item in nums.

wodenokoto 6y ago

def day1_1(nums): "Find 2 distinct numbers that sum to 2020, and return their product." return first(x * y for x in nums for y in nums & {2020 - x} if x != y) `nums` is a set of integers `nums & {2020 - x}` Finds all the numbers that are in the set of `nums` and the set of `{2020 - x}` - this basically extracts a number, `y`, for which `x+y = 2020`. So for each x in nums, he extracts a number from nums that, added to x, equals 2020. If there is such a number y, he checks if it is the same as x, and only if it is different, does he yield in his generator, as the product of y and x. It's succint, but not exactly readable, imho.

sgk284 6y ago

It's a bit overly clever. I am a huge fan of Peter Norvig (and he was once my skip-level boss), but I'd call this code out in a code review for being obtuse. Notice that the input, `nums`, is a set. So he's taking the intersection of two sets. One set always has one item, so the result will be a collection with either 0 or 1 items. It could have also been written as: first(x * (2020 - x) for x in nums if (2020 - x) in nums and x != (2020 - x)) But that has a lot of duplication, so it'd probably be best to just use an imperative approach here and do something like: def day1_1(nums): "Find 2 distinct numbers that sum to 2020, and return their product." for x in nums: y = 2020 - x if x == y: continue if y in nums: return x * y

philshem 6y ago
when I saw the first() function, I thought it was some default python that was new to me. But he defines it above in his commonly used functions: def first(iterable, default=None) -> object: "Return first item in iterable, or default." return next(iter(iterable), default)
- madhadron 6y ago
  How to tell he's a Lisper. :) Be glad it wasn't car().
  - norvig 6y ago
    madhadron, I'm a Common Lisper, so I'm good with `first` and `rest`.
notretarded 6y ago
abstruse Completely agree, your solution is more clear.
Gupie 6y ago
The "x == y" test looks like a bug to me, unless duplicate entries are not allowed. For example [1010,1010,1721,979,366,299,675,1456] should return 1010 * 1010, not 1721 * 299.
- deleted 6y ago
  [deleted]
- jstanley 6y ago
  Given that the comment says "Find 2 distinct numbers that sum to 2020", I expect duplicate entries are not allowed.
  - Gupie 6y ago
    Yes, but I cannot find the same in the problem definition: "they need you to find the two entries that sum to 2020 and then multiply those two numbers together" https://adventofcode.com/2020/day/1 https://adventofcode.com/2020/day/1