get output of a command in prompt

["arno." <arno.@no-log.org>]

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Hi,
I want to get the output of a command in my prompt 
(actually, I want the temperature of my cpu in RPS1 if it is superior 
to a some predifined value).
So I have something like :
 RPS1="%2v"
 and 

precmd() {
    gettemp
}

with gettemp that gets temperature and puts it in psvar[2] if needed.

Today, I've seen in zsh user guide :

> Some people make the mistake of using it [precmd function] to set up a 
> prompt, but there are enough ways of getting varying information into a 
> fixed prompt string that you shouldn't do that unless you have very odd 
> things in your prompt.
http://zsh.sunsite.dk/Guide/zshguide03.html#l51

But I didn't find a better way to do what I want. Did I miss something, 
or is my goal something "very odd".

thanks 
arno

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