Re: Best way to determine # of entries in a list?
- To: mathgroup at smc.vnet.net
- Subject: [mg42036] Re: [mg42004] Best way to determine # of entries in a list?
- From: Selwyn Hollis <selwynh at earthlink.net>
- Date: Tue, 17 Jun 2003 05:43:10 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
Something like
alist = {1, 2, 3, 4, 5, 6};
poly[x_] := x^2 + x;
solns = x /. NSolve[Length[alist] == poly[x], x]
Or @@ IntegerQ /@ Rationalize[solns]
perhaps?
-----
Selwyn Hollis
http://www.math.armstrong.edu/faculty/hollis
On Monday, June 16, 2003, at 03:57 AM, cdj wrote:
> Hi,
>
> Example problem:
>
> To streamline the input of systems of linear equations, i'm setting up
> a function that accepts simply a flat list as argument. The first nxn
> entries are the rows of the matrix, and the last n are the target
> point (= b). This gives the length of the argument list as n^2 + n.
>
> I'd like to have a little bit of input error/type checking in the
> function. For this example, given a list, I'd like to test whether or
> not there exists an n such that length(list) = n^2 + n.
>
> The quickest/easiest way I know how to do it for this example problem
> is to test for integer-hood on sqrt[4*length(list) + 1] (yes = good
> data type).
>
> Q1: Does anybody know of a quicker/better/more elegant way to perform
> this test?
>
> Q2: In general, suppose i want to test (the length of) my list against
> an arbitrary polynomial P(n). What's the best way to do this?
>
> thanks for any insights,
>
> cdj
>
>