extract only positive solutions

[Gabby Domingo]

hi all! how do you extract only positive solutions to a solver, like
NSolve?

sol={{-.532,.932},{.34,23}}

and extract {.34,.23}?

[Bill Rowe]

Here is an example of one way

In[3]:= p = (x + 3) (x - 4) (x - 5) // Expand;

In[4]:= Cases[Solve[p == 0, x], {_?(x > 0 /. # &)}]

Out[4]= {{x->4},{x->5}}

[Murray Eisenberg]

This sort of thing has been asked and answered many times before, and
you'll probably get lots of answers now. here's one way:

Select[pairs, Last@# > 0 &]

On 2/17/12 6:23 AM, Gabby Domingo wrote:
> hi all! how do you extract only positive solutions to a solver, like
> NSolve?
>
> sol={{-.532,.932},{.34,23}}
>
> and extract {.34,.23}?
>

--
Murray Eisenberg mur...@math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower phone 413 549-1020 (H)
University of Massachusetts 413 545-2859 (W)
710 North Pleasant Street fax 413 545-1801
Amherst, MA 01003-9305

[Bob Hanlon]

sol = {{-.532, .932}, {.34, 23}};

Select[sol, And @@ Positive[#] &]

{{0.34, 23}}

Cases[sol, _?(And @@ Positive[#] &)]

{{0.34, 23}}

Cases[sol, {_?Positive ..}]

{{0.34, 23}}

DeleteCases[sol, {___, _?NonPositive, ___}]

{{0.34, 23}}

DeleteCases[sol, _?(Or @@ NonPositive[#] &)]

{{0.34, 23}}


Bob Hanlon

[andres]

I don't fully understand, NSolve (or any solver) gives the solutions
as a set of rules, not as a list of numbers.
>From your example I guess you want to extract the pairs whose first
element is > 0. If that's the case I'd use Select:
Select[sol, #[[1]] > 0 &]
Andr=E9s

[Helen Read]

You can restrict the domain in Solve.

For example:

f[x_]:=150 - 15 x - 50 x^2 + 2 x^3 - 6 x^4 + x^5 + 2 x^6

Solve[f[x]==0,x]

Solve[f[x]==0,x,Reals]

Solve[f[x]==0,x,Rationals]

Solve[f[x]==0,x,Integers]

Solve[f[x] == 0 && x > 0, x]


So if I understand what you are trying to do, something like this:


{x, y} /.
Solve[{y == x^2 - 2 x - 5, y == 7 - x, x > 0, y > 0}, {x, y}]


--
Helen Read
University of Vermont