Re: cannot work a "conditional If"

• To: mathgroup at smc.vnet.net
• Subject: [mg130574] Re: cannot work a "conditional If"
• From: Bob Hanlon <hanlonr357 at gmail.com>
• Date: Tue, 23 Apr 2013 00:03:04 -0400 (EDT)
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• References: <20130422071108.EC5C96AFD@smc.vnet.net>

```Your output of signy is a list of length 1 rather than a number.
Consequently, the test in the If does not evaluate as either True or False
and the If statement returns unevaluated. Change the definition of signy to
produce a number, i.e.,

signy[t_] := y[t] /. orbit[[1]]

However, rather than use an If statement, I recommend that you use
Piecewise.

Bob Hanlon

On Mon, Apr 22, 2013 at 3:11 AM, <patrice.haldemann at gmail.com> wrote:

> Hi everybody
>
> I am a newcomer in the mathematica world and I have been facing an issue
> for some days.
>
> I have used mathematica to solve an NDSolve and it has worked well
>
> I have tested  expressions with the solution of NDSolve and its has worked
> well.
>
> Now if I use an expression in an "Conditional If", it does not work.
>
> Thank you for your help
>
>
> See below
>
> signy[t_] := y[t] /. orbit   where y[t] is the solution of the NDSolve
>
> signy[1]
>
> {-1.96861}
>
>  ang12[t_] := If[signy[t] > 3, 10, 6]
>
>  ang12[1]
>
> If[{-1.96861} > 3, 10, 6]
>
>

```

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