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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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