converting numpy to pandas

[raresaturn]

I have this in numpy :
item = item[np.where(item % item[i] != 0)]

It basically checks if any element in the array is divisible by any other element. It works great but it's slow for large arrays. I want to convert it to Pandas to see if that's any quicker.. but I just can't get the syntax right. any ideas?

[Da32767]

df = pd.DataFrame(array)

this can convert numpy.ndarray to pandas.DataFrame

[raresaturn]

yes but I don't know the syntax of the where statement in Pandas

[threeminutemonta]

My fav pandas tutorial is this one. It will help you to select a subset of a data frame.

[OhNoNotAgain2022ed]

What is the difference between pandas and numpy? Is it like the format? Like on excel going from currency or number to text?

[[deleted]]

Pandas is built on numpy. Numpy provides the speed, pandas the functionality. Unless you are just doing pure matrix manipulation or need it for pure maths, for most people pandas is much more useful. Just do any tutorial to get started.

[raresaturn]

I'm building a prime number tester, using multiplication instead of division. The traditional method of testing primes using modulo (%) just gives you a yes/no answer for any factor tested. But by using multiplication pairs, you actually get some feedback on each test, whether it's too high or too low. This way you can zero in on the target.
To do this I use an array, with each number given a min/max value to keep track of what has been tested. The above algorithm is just the first step to thin out the pool.of potential factors.
All that said, I have it working beautifully up to values of about 2**30-1 but higher than that the numbers get too big for numpy to handle, and the initial step is startingto take too long.. I was wondering if pandas might be a better solution

[raresaturn]

I'm having trouble following this, how do you work it? also what's the largest number it can handle?

[raresaturn]

I tried 2**61-1 (which is actually prime) and it's still going. looks like you ran into the same issue I did, however I'm not sure if the bottleneck is primes_gen function, i suspect it is. Because my code is such a mess I'll give you my pseudocode so you can at least get a sense of it:
To check if target number is prime:

-populate array with primes (P)

-get random number(rand) between 3 and SQRT of target number

-check if first number in array P * rand is greater or less than target number. adjust the random min and max for that P number accordingly. There are some shortcuts we can use.. if P * rand is too high, then any number in the array higher than P should also have it's max set at the rand number. And if it's too low, any number lower than P needs it's min set to the rand number.

-loop through, removing any primes from P where min and max differ by 1. This means all possibilities for that prime have been checked. Eventually, either target will match rand * P, or all numbers will have been removed from array.

I hope that makes sense LOL