[R] Lexical scoping for step and add1 functions

[Louisell, Paul T PW]

Hi,

I've run into a problem calling the step function from within a function; I sent this to the R development list first, but the moderator said it was better suited to R help. My OS is Windows 7 and I'm using R version 3.2.3.

Here's a simple function to help reproduce the error:
      > test.FN
      function(dfr, scope, k=2){
      temp.lm=lm(scope$lower, data=dfr)
      step(temp.lm, scope=scope$upper, k=k)
      }

And here's the code that gives the error when calling the function above:
      # Begin by setting the rng seed.
      > set.seed(523)

      # Generate a design matrix and response.
      > X.des=matrix(abs(rnorm(50*20, sd=4)), nrow=50)
      > Y=20 + X.des[, 1:3] %*% matrix(c(3, -4, 2), nrow=3) + rnorm(50)
      > X.des=cbind(as.data.frame(X.des), Y)

      # Create the lower and upper formula components of a list.
      > test.scope=list(lower=as.formula(Y ~ 1), upper=as.formula(paste("Y ~ ", paste(names(X.des)[1:20], collapse=" + "), sep="")))

      # Run 'test.FN'.
      > test.FN(dfr=X.des, scope=test.scope)
      Start:  AIC=257.58
      Y ~ 1

      Error in is.data.frame(data) : object 'dfr' not found
      > traceback()
      11: is.data.frame(data)
      10: model.frame.default(formula = Y ~ V1 + V2 + V3 + V4 + V5 + V6 +
              V7 + V8 + V9 + V10 + V11 + V12 + V13 + V14 + V15 + V16 +
              V17 + V18 + V19 + V20, data = dfr, drop.unused.levels = TRUE)
      9: stats::model.frame(formula = Y ~ V1 + V2 + V3 + V4 + V5 + V6 +
             V7 + V8 + V9 + V10 + V11 + V12 + V13 + V14 + V15 + V16 +
             V17 + V18 + V19 + V20, data = dfr, drop.unused.levels = TRUE)
      8: eval(expr, envir, enclos)
      7: eval(fcall, env)
      6: model.frame.lm(fob, xlev = object$xlevels)
      5: model.frame(fob, xlev = object$xlevels)
      4: add1.lm(fit, scope$add, scale = scale, trace = trace, k = k,
             ...)
      3: add1(fit, scope$add, scale = scale, trace = trace, k = k, ...)
      2: step(temp.lm, scope = scope$upper, k = k) at #3
      1: test.FN(dfr = X.des, scope = test.scope)

The call to the traceback function indicates add1 doesn't see the dataframe dfr  passed to test.FN. The step function runs fine when I do everything in the global environment without using test.FN. I know the lexical scoping rules are different for objects involving model formulae, but despite a fair amount of experimentation, I haven't found any way to make the step / add1 functions see the dataframe that's passed to test.FN. Any help would be greatly appreciated.

Thanks,

Paul Louisell
Statistical Specialist
Paul.Louisell at pw.utc.com<mailto:Paul.Louisell at pw.utc.com>
860-565-8104

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