Shaun Overton - 1 year ago 275
R Question

# `predict()` error: no applicable method for 'predict' applied to an object of class "list"

I have a problem controlling the object types feeding into the predict function. Here's my simplified function that generates the

`glm`
object.

``````fitOneSample <- function(x,data,sampleSet)
{
#how big of a set are we going to analyze? Pick a number between 5,000 & 30,000, then select that many rows to study
sampleIndices <- 1:5000

#now randomly pick which columns to study
colIndices <- 1:10

xnames <- paste(names(data[,colIndices]),sep = "")
formula <- as.formula(paste("target ~ ", paste(xnames,collapse = "+")))
}

myFit <- fitOneSample(1,data,sampleSet)
fits <- sapply(1:2,fitOneSample,data,sampleSet)
all.equal(myFit,fits[,1]) #different object types

#this works
probability <- predict(myFit,newdata = data)

#this doesn't
probability2 <- predict(fits[,1],newdata = data)
# Error in UseMethod("predict") :
# no applicable method for 'predict' applied to an object of class "list"
``````

How do I access the column in
`fits[,1]`
so that I can use the predict function to get same result that I did with
`myFit`
?

I think I am now able to recover your situation.

``````fits <- sapply(names(trees),
function (y) do.call(lm, list(formula = paste0(y, " ~ ."), data = trees)))
``````

This uses built-in dataset `trees` as an example, fitting three linear models:

``````Girth ~ Height + Volume
Height ~ Girth + Volume
Volume ~ Height + Girth
``````

Since we have used `sapply`, and each iteration returns the same `lm` object, or a length-12 list, results will be simplified to a `12 * 3` matrix:

``````class(fits)
# "matrix"

dim(fits)
# 12  3
``````

Matrix indexing `fits[, 1]` is valid.

If you check `str(fits[, 1])`, it almost looks like a normal `lm` object. But if you further check:

``````class(fits[, 1])
# "list"
``````

Em? It does not have "lm" class! As a result, `S3` dispatch method will fails when you call generic function `predict`:

``````predict(x)
#Error in UseMethod("predict") :
#  no applicable method for 'predict' applied to an object of class "list"
``````

This can be seen as a good example that `sapply` is destructive. We want `lapply`, or at least, `sapply(..., simplify = FALSE)`:

``````fits <- lapply(names(trees),
function (y) do.call(lm, list(formula = paste0(y, " ~ ."), data = trees)))
``````

The results of `lapply` is easier to understood. It is a length-3 list, where each element is an `lm` object. We can access the first model via `fits[[1]]`. Now everything will work:

``````class(fits[[1]])
# "lm"

predict(fits[[1]])
#        1         2         3         4         5         6         7         8
# 9.642878  9.870295  9.941744 10.742507 10.801587 10.886282 10.859264 10.957380
#        9        10        11        12        13        14        15        16
#11.588754 11.289186 11.946525 11.458400 11.536472 11.835338 11.133042 11.783583
#       17        18        19        20        21        22        23        24
#13.547349 12.252715 12.603162 12.765403 14.002360 13.364889 14.535617 15.016944
#       25        26        27        28        29        30        31
#15.628799 17.945166 17.958236 18.556671 17.229448 17.131858 21.888147
``````

You can fix your code by

``````fits <- lapply(1:2,fitOneSample,data,sampleSet)
probability2 <-predict(fits[[1]],newdata = data)
``````
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