[R] lm looking for weights outside of the user-defined function

[David Winsemius]

On Oct 22, 2010, at 12:17 PM, William Dunlap wrote:

>> -----Original Message-----
>> From: r-help-bounces at r-project.org
>> [mailto:r-help-bounces at r-project.org] On Behalf Of David Winsemius
>> Sent: Friday, October 22, 2010 6:25 AM
>> To: Dimitri Liakhovitski
>> Cc: r-help
>> Subject: Re: [R] lm looking for weights outside of the
>> user-defined function
>>
>>
>> On Oct 22, 2010, at 9:18 AM, Dimitri Liakhovitski wrote:
>>
>>> David,
>>> I undersand - and I am sure what you are suggesting should
>> work. But I
>>> just can't understand why it's not grabbing things INSIDE the
>>> environment of the formula first.
>>
>> I am not sure that either one of us understand what is meant by "the
>> environment of the formula".
>
> "The environment of the formula" is the output of
>   environment(formula)
> which is assigned to the current environment when the
> formula is created.  The modelling functions look for
> variables (in the formula, weights, and subset arguments)
> in the order
>   1) the data argument (usually an environment or a list)
>   2) environment of the formula
> When an environment is searched for a name, the search
> continues through all ancestral environments until the
> name is found or until you run out of ancestors.
>
> You can reassign the environment of a formula.  E.g.,
> compare the following two:
>
>> wr0 <- function(formula, MyData, WeightsVector) {
>  +     lm(formula, data=MyData, weights=WeightsVector)
>  + }
>> wr1 <- function(formula, MyData, WeightsVector) {
>  +     environment(formula) <- environment()
>  +     lm(formula, data=MyData, weights=WeightsVector)
>  + }
>> wr0(mpg~cyl, MyData=mtcars, WeightsVector=sqrt(1:32))
>  Error in eval(expr, envir, enclos) : object 'WeightsVector' not found

  The wr0 call created a formula but the weights vector was "outside"  
that environment? And that wss because the formula creation was at the  
stage of evaluation the function arguments when tehy wouldn't "see"  
each other?  Except this works:

 > xtoy <- function(x = 1:2, y=x){y}
 > xtoy()
[1] 1 2

I'm trying to figure out what makes the wr0 version fail and that  
xtoy() function succeed.

-- 
David.
>> wr1(mpg~cyl, MyData=mtcars, WeightsVector=sqrt(1:32))
>
>  Call:
>  lm(formula = formula, data = MyData, weights = WeightsVector)
>
>  Coefficients:
>  (Intercept)          cyl
>       38.567       -2.966
>
> Reassigning the environment can lead to the sort of
> surprises that dynamic scoping gives you.
>
> Bill Dunlap
> Spotfire, TIBCO Software
> wdunlap tibco.com
>
>>
>>> I've already tried to define the weights outside of the
>> function - and
>>> it finds them.
>>>
>>> But shouldn't it go in this order?
>>> 1. Look in the data frame
>>> 2. Look in the environment of the user-defined function
>>> 3. Look outside.
>>
>> Hey, I only work here, I don't make the rules, I just follow them. I
>> agree that one might guess that to be the search order, but
>> it is not
>> what is documented.
>>
>> -- 
>> David Winsemius, MD
>> West Hartford, CT
>>
>>>
>>> Dimitri
>>>
>>> On Fri, Oct 22, 2010 at 9:15 AM, David Winsemius
>> <dwinsemius at comcast.net
>>>> wrote:
>>>>
>>>> On Oct 22, 2010, at 9:01 AM, Dimitri Liakhovitski wrote:
>>>>
>>>>> Dear R'ers,
>>>>>
>>>>> I am fighting with a problem that is driving me crazy. I
>> use "lm" in
>>>>> my user-defined function, but it seems to be looking for weights
>>>>> outside of my function's environment:
>>>>>
>>>>> ### Generating example data:
>>>>> x<-data.frame(y=rnorm(100,0,1),a=rnorm(100,1,1),b=rnorm(100,2,1))
>>>>> myweights<-runif(100)
>>>>> data.for.regression<-x[1:3]
>>>>>
>>>>> ### Creating function "weighted.reg":
>>>>> weighted.reg=function(formula, MyData, filename,WeightsVector)
>>>>> {
>>>>>       print(dim(MyData))
>>>>>       print(filename)
>>>>>       print(length(WeightsVector))
>>>>>       regr.f<-
>>>>> lm(formula,MyData,weights=WeightsVector,na.action=na.omit)
>>>>>       results<-as.data.frame(round(summary(regr.f)$coeff,3))
>>>>>       write.csv(results,file=filename)
>>>>>       return(results)
>>>>> }
>>>>>
>>>>> ### Running "weighted.reg" with my data:
>>>>> reg2<-weighted.reg(y~., MyData=x, WeightsVector=myweights,
>>>>> filename="TEST.csv")
>>>>>
>>>>>
>>>>> I get an error: Error in eval(expr, envir, enclos) : object
>>>>> 'WeightsVector' not found
>>>>> Notice, that the function correctly prints
>> length(WeightsVector).
>>>>> But
>>>>> it looks like "lm" is looking for weights (in the 4th line of the
>>>>> function) OUTSIDE the function and does not see WeightsVector.
>>>>
>>>> Have you tried putting WeightsVector in the "x" dataframe? That
>>>> would seem
>>>> to reduce the potential for environmental conflation.
>>>>
>>>> From the details section of help(lm):
>>>> "All of weights, subset and offset are evaluated in the same way as
>>>> variables in formula, that is first in data and then in the
>>>> environment of
>>>> formula."
>>>>
>>>>
>>>>> Why is it looking outside the function for the object
>> that has just
>>>>> been defined inside the function?
>>
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