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MathGroup Archive 2006

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Re: problem evaluating a function on a list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg69903] Re: problem evaluating a function on a list
  • From: Peter Pein <petsie at dordos.net>
  • Date: Wed, 27 Sep 2006 06:04:25 -0400 (EDT)
  • References: <efacrp$1vm$1@smc.vnet.net>

wtplasar at ehu.es schrieb:
> Hi everyone,
> 
> I have to ask for your advise once again.
> 
> I have this function
> 
> H[x_, om_, 
>     or_, H0_] := If[Im[om - 4*om^(2/3)*or^(1/3) + 2*
>           or + 2*Sqrt[or]*Sqrt[om + or]] == 0. && -1 + om - 4*om^(2/3)
> *or^(1/
>             3) + 2*or + 2*Sqrt[or]*Sqrt[om + or] < 0, H0*((1 - om - 
> 2*or - 2*
>       Sqrt[or]*Sqrt[om + or])*(1 + x)^2 + (Sqrt[or] + Sqrt[or + om*(1 
> + 
>           x)^3])^2),  I]
> 
> and I want to evaluate it at a list
> 
> xlist = {0.09, 0.17, 0.27, 0.4, 0.88, 1.3, 1.43, 1.53, 1.75};
> 
> When I choose parameters so that a list of real values should be 
> obtained everything seems to be fine as one can see if for instance we 
> evaluate
> 
> H[xlist, 0.4, 0.1, 70]
> 
> (my result is {81.138, 92.4217, 108.473, 132.803, 261.891, 435.859, 
> 503.238, 559.829, 699.774})
> 
> whereas in the cases I would expect the result to be 
> {I,I,I,I,I,I,I,I,I}
> 
> I get simply one single 
> 
> I 
> 
> instead, as one can see from evaluation of 
> 
> H[xlist, -0.4, 0.1, 70].
> 
> Can someone indicate why this happens, and how I can get the
> desired list which looks like {I,I,I,I,I,I,I,I,I}?
> 
> Thanks in advance for your time,
> 
> Ruth
> 
> 

Hi Ruth,

make use of the attribute "Listable":

In[3]:= SetAttributes[H, Listable]
In[4]:= H[xlist, 0.4, 0.1, 70]
Out[4]=
{81.1379736408618, 92.42173206211797, 108.47334023428536, 
132.80319561373202, 261.89137559629944, 435.85854432074075, 
503.2378317813145, 559.8294132419585, 699.7740480067278}
In[5]:= H[xlist, -0.4, 0.1, 70]
Out[5]=
{I, I, I, I, I, I, I, I, I}

Peter


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