[R] term.formula error when updating an nls object

[Ken Knoblauch]

Thanks, I hadn't thought of that.  It works fine, now.
I'll look out for that in the future.

Ken

Quoting Peter Ehlers <ehlers at ucalgary.ca>:

> Ken,
>
> You just need to wrap the rhs of your formula in I() to
> get around update()'s parsing of terms.
>
>  m2 <- update(m1, . ~
>              I(Rm * Contrast^ex/(Contrast^fx + sig^fx)),
>              start = list(Rm = 30, sig = 0.05, ex = 3,
>              fx = 3.1))
>
>  -Peter Ehlers
>
> Ken Knoblauch wrote:
>> Hi,
>>
>> I'm getting an error that I don't understand when updating an nls
>> object.  Here is a toy example.
>>
>> dd <- structure(list(Contrast = c(0.00376, 0.03759, 0.12782, 0.25564,
>> 0.50376, 1), Response = c(0.29915, 6.13248, 29.01709, 30.0641,
>> 29.46581, 27.67094)), .Names = c("Contrast", "Response"), class =   
>> "data.frame", row.names = c(NA,
>> -6L))
>>
>> m1 <- nls(Response ~
>>            Rm * Contrast^ex/(Contrast^ex + sig^ex),
>>            data = dd,
>>            start = list(Rm = 30, sig = 0.05, ex = 3))
>>
>> m2 <- update(m1, . ~
>>            Rm * Contrast^ex/(Contrast^fx + sig^fx),
>>            start = list(Rm = 30, sig = 0.05, ex = 3,
>>            fx = 3.1))
>> Error in terms.formula(tmp, simplify = TRUE) : invalid power in formula
>>
>> but there is nothing wrong with fitting the second model directly
>> with nls
>>
>> m2a    <- nls(Response ~
>>            Rm * Contrast^ex/(Contrast^fx + sig^fx),
>>            data = dd,
>>            start = list(Rm = 30, sig = 0.05, ex = 3,
>>            fx = 3.1))
>>
>>
>> nor with fitting the model with the data transformed (though it is
>> not necessarily the way I would like to fit the model in this
>> case).
>>
>> m2b <- update(m1, log(.) ~ log(.))
>> m3 <- update(m2b, . ~
>>    log(Rm * Contrast^ex/(Contrast^fx + sig^fx)),
>>    start = list(Rm = 30, sig = 0.05, ex = 3,
>>            fx = 3.1))
>>
>> sessionInfo()
>> R version 2.10.1 Patched (2010-01-25 r51051)
>> i386-apple-darwin9.8.0
>>
>> locale:
>> [1] en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8
>>
>> attached base packages:
>> [1] stats     graphics  grDevices utils     datasets  methods
>> [7] base
>>
>> Thanks, in advance, for any help.
>>
>> Ken
>>
>
> -- 
> Peter Ehlers
> University of Calgary



-- 
Ken Knoblauch
Inserm U846
Stem-cell and Brain Research Institute
Department of Integrative Neurosciences
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69500 Bron
France
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