Poor mans count, is it off-topic?

Question

Recently we had the question "Project Euler Problem #12 - Highly divisible triangular number"

1. It was closed as the OP included how to use the function with the function's code.

   The code as posted fails to run due to NameError: name 'num' is not defined on the last line. – Peter Taylor Apr 25 at 7:30

   The function works, and so I don't think it should be closed as broken because of an NameError in the example usage, because num isn't declared. We allow this exception in other questions, even if half the program up for review is filled with them, not example input.

2. We shouldn't guess input to the function:

   @Peilonrayz, which is what? The problem which this is attempting to solve is quite small and specific, so the code which solves it should run standalone without requiring the user to guess a value for an undocumented parameter. – Peter Taylor Apr 25 at 8:38

   Another user in chat also held this view.

3. Because it takes a while on large inputs:

   -1 from me because the Code does not solve the problem. I was not able to find the result. for small values of num the program finishes without printing a result and for larger numthe program hangs up without printing a result. – miracle173 Apr 26 at 16:18

   Problems that exceed a time limit aren't off-topic on Code Review.

I don't think anyone's arguing for or against the first reason for closure. And the third has already been addressed. What should our views on the second be?

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Should we close questions for naive implementations implementing limits, when they're not needed?

If you replace for n in range(1,num) with for n in itertools.count(1), then there would be no problem. Or if a user entered float('inf') if they knew about it and if it worked with range. They're a novice and doing the next best thing they know of.

Answer 1

2. We shouldn't guess input to the function:

   @Peilonrayz, which is what? The problem which this is attempting to solve is quite small and specific, so the code which solves it should run standalone without requiring the user to guess a value for an undocumented parameter. – Peter Taylor Apr 25 at 8:38

"We shouldn't guess input to the function" is not a great summary of my comment, although maybe my phrasing was too subtle and should have been more explicit. A more accurate summary would be "We shouldn't have to guess how to invoke the function".

If the code had executed and solved the stated problem without any modifications then clearly there would have been no problem.

If, instead of a non-working invocation, the question had included text along the lines of

The function takes one parameter, which is an upper bound on the solution. It should be used interactively by guessing a value of the parameter, and then increasing the guess until a solution is found.

then there would also have been no problem. The fact that the code was not standalone would then have been a problem with the code (and therefore an issue to raise in the review) rather than a problem with the question. In fact, revisiting the question, I propose that adding that text might be a constructive way forward.

Answer 2

1. Is not an issue as that can be solved with just entering your own value. It is related to number 2, however.

2. This is actually a good point that the code itself does not solve the problem, one needs to manually enter various values of num in order to solve the problem. I don't think this is close worthy though, as it should still be possible to solve the problem by using the function in the question.

3. Not only does it take a while in large inputs, there is actually a bug in the code that probably causes it to never finish. This, is close worthy.

Because I am so nice though, I have added a comment indicating the bug with the code and suggested a solution:

The problem asks for What is the value of the first triangle number to have over five hundred divisors? but your code says if len(list) == 500:. Have you tried with if len(list) > 500: ?

Answer 3

I already stated that I was not able to find a value for num and it seems that the OP wasn't able, too.

• Code presented here must be working.

  If it's doubtful that the code is working it's up to the OP to show that the code is working. To see if a python code is working usually I run it. And if it does not produce the expected result or no result at all, then it does not work for me.

• time-limit-exceeded

  The description of the 'time-limit-exceeded' tag says:

  Time-limit-exceeded issues are a special kind of performance problem, where the code handles small test cases very well, but takes an unreasonable amount of time to complete for larger inputs. You may use this tag instead of performance when an online judge rejects your solution to a programming-challenge due to time constraints, or when you need to interrupt your computation because it appears to take much too long to finish.

  For programs rejected by an online judge i may be that it may happen that the program finished after some seconds and that may be to Long for the constes but isn't a Problem for a user who wants to see the result of this program. But if the program runs too long

• I don't think that it is our job to prove the correctness of a program, nor is it our job to find errors in a program.

In your post you cite my comment and say that "problems that exceed a time limit aren't off-topic on Code Review". But this makes no sense. In the description of the 'time-limit-exceeded' tag it is stated that such programs will run for small input values but not scale well and therefore do not finish for larger values in a reasonable time. So to make it on topic in the sense of the 'time-limit-exceeded' tag (which I don't like) it is at least necessary to write a function that reads a number and calculates the smallest integer where the the number of factors is larger or equal the input number. So the question is off topic here because it was not able to calculate any meaningful result in reasonable time for any input.

Answer 4

The question is not off-topic

1. This shouldn't be an issue as a we can just enter a number. It also isn't an error with the code but the example-usage code.

   Say we are asking for a review of the following function:

   def fn(num):
       return num ** 2 / num
   

   The following example usage results in a NameError, but the error doesn't originate from the posted code, it originates in the example usage. Added with the fact we allow snippets of code to be posted num may be defined, but the definition may not be posted in the question. And so the code isn't broken due to this NameError.

   fn(num)
   

   The following example usage results in a ZeroDivisionError which originates from within the code, not the example usage. And so the code may not be working as intended, which means that we then have to check the requirements whether this is off-topic.

   fn(0)
   

2. This is saying we shouldn't guess how to invoke the function. To know how to invoke the function we would need to know the input to the function. And so this is the same as saying we need example input for code to be on-topic on Code Review.

   We have never required example input for code to be on-topic on Code Review.

3. The code takes a while on large inputs. Optimizing the code so that the factors code runs in a fraction of the time shows the code works, and it runs in under a minute so you can check too. miracle173 agreed that the code works, and so his 'proof' it never ends isn't true.

   One 'requirement' (I've not seen enforced anywhere else) is for the code to work on small values. If you change the values in the code to be the same as the example on Project Euler, then the code will work too.

   And so with only a little amount of effort, changing the values to the ones on Project Euler, we would be able to tell the code works. Or we can optimise the code, and see that it still works.

4. @Peilonrayz - your gist code suffers from the same problem that the OP's code has, the break only exits the inner loop, so with euler_12(1000000) you will continue to execute even after finding the value at 12376 – rolfl♦ link

   The requirement of PE is to get the first triangle number with over 500 factors. The code gets this value and so the code works as intended.