機型不能在

Question

預測考慮到可以通過創建模型列表：

fits = vector(mode="list",length=10) 
for(i in 1:10) 
{ 
    fits[[i]] = lm(nox~poly(dis,i),data=Boston) 
} 

其中，用於Boston數據集，可以在MASS庫中找到。現在
，爲了做出預測：

dislim = range(Boston$dis) 
dis.grid = seq(from = dislim[1],to = dislim[2],by = 0.1) 

這樣做是爲了給在其nox的值預測的dis值。 現在，爲了作出預測，我們可以做到以下幾點：

predict(fits[[1]],list(dis = dis.grid)) 

但是這將導致一個錯誤：

Error: variable 'poly(dis, i)' was fitted with type "nmatrix.1" but type "nmatrix.10" was supplied 
In addition: Warning message: 
In Z/rep(sqrt(norm2[-1L]), each = length(x)) : 
    longer object length is not a multiple of shorter object length 

但是，當我做到以下幾點：

lm.Boston = lm(nox~poly(dis,3),data=Boston) 
lm.Boston.pred = predict(lm.Boston,list(dis = dis.grid)) 

它工作正常。那麼，爲什麼我不能在列表中這樣做呢？

Answer 1

指定一個動態式正確的方法是使用paste和as.formula

library(MASS) 
data(Boston) 

dislim <- range(Boston$dis) 
dis.grid <- seq(from = dislim[1],to = dislim[2],by = 0.1) 

models <- lapply(1:10, function(i){ 
    form = as.formula(paste0("nox~", "poly(dis," , i, ")")) 
    lm(form, data=Boston) 
}) 

預測

lapply(models, function(x){ 
    predict(x, list(dis = dis.grid)) 
}) 

編輯：另一種方法來構建式（按照MrFlick評論）爲：

`lm(bquote(nox~poly(dis,.(i))), data=Boston)` 

models1 <- lapply(1:10, function(i){ 
    lm(bquote(nox~poly(dis,.(i))), data=Boston) 
}) 

此外（按照Natha Ñ韋斯評語）如果所述製劑：

models2 <- lapply(1:10, function(i){ 
    lm(nox~poly(dis,i),data=Boston) 
}) 

被使用時，i被視爲在模型中的變量，並且可以利用下列方式這樣的行爲：

predict(models2[[1]], list(dis = dis.grid, i = 1) 

library(purrr) 
models <- lapply(1:10, function(i){ 
    form = as.formula(paste0("nox~", "poly(dis," , i, ")")) 
    lm(form, data=Boston) 
}) 

models1 <- lapply(1:10, function(i){ 
    lm(bquote(nox~poly(dis,.(i))), data=Boston) 
}) 

models2 <- lapply(1:10, function(i){ 
    lm(nox~poly(dis,i),data=Boston) 
}) 

missuse <- lapply(models, function(x){ 
    predict(x,list(dis = dis.grid)) 
}) 

MrFlick <- lapply(models1, function(x){ 
    predict(x,list(dis = dis.grid)) 
}) 

NathanWerth <- purrr::map2(models2, 1:10, function(x, y){ 
    predict(x,list(dis = dis.grid, i = y)) 
}) 

purrr::pmap(list(missuse, MrFlick, NathanWerth), function(x, y, z) c(identical(x, y), identical(x, z)))