I have a prime p, and I need to a list of all the primes upto it in descending order (the order is important for my situation). The obvious way to do this is to write [p, pred p ..] using the enum instance for primes, except that this sometimes does not do what one would expect; for example, if p = 7, this gives the list [7,5,3] instead of [7,5,3,2], It seems this is because the code sees that the two primes 7 and 5 are both 1 mod 2 and interprets this as asking for only the primes that are 1 mod 2. (Also, this "feature" that it will only generate primes with the same value mod their difference is really annoying in many cases when I dont want it, and i havent used it that much anyways.) The only way I see to do this is to generate the list of primes in ascending order, and then reverse it, which feels kind of wasteful, and there should be a more efficient way of generating primes in descending order IMO
I have a prime
p, and I need to a list of all the primes upto it in descending order (the order is important for my situation). The obvious way to do this is to write[p, pred p ..]using the enum instance for primes, except that this sometimes does not do what one would expect; for example, if p = 7, this gives the list[7,5,3]instead of[7,5,3,2], It seems this is because the code sees that the two primes 7 and 5 are both 1 mod 2 and interprets this as asking for only the primes that are 1 mod 2. (Also, this "feature" that it will only generate primes with the same value mod their difference is really annoying in many cases when I dont want it, and i havent used it that much anyways.) The only way I see to do this is to generate the list of primes in ascending order, and then reverse it, which feels kind of wasteful, and there should be a more efficient way of generating primes in descending order IMO