R Under development (unstable) (2024-08-16 r87026 ucrt) -- "Unsuffered Consequences"
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> suppressWarnings(RNGversion("3.5.2"))
> 
> library("partykit")
Loading required package: grid
Loading required package: libcoin
Loading required package: mvtnorm
> 
> set.seed(29)
> n <- 1000
> x <- runif(n)
> z <- runif(n)
> y <- rnorm(n, mean = x * c(-1, 1)[(z > 0.7) + 1], sd = 3)
> z_noise <- factor(sample(1:3, size = n, replace = TRUE))
> d <- data.frame(y = y, x = x, z = z, z_noise = z_noise)
> 
> 
> fmla <- as.formula("y ~ x | z + z_noise")
> 
> (m_mob <- mob(formula = fmla, data = d, fit = partykit:::lmfit))
Model-based recursive partitioning (partykit:::lmfit)

Model formula:
y ~ x | z + z_noise

Fitted party:
[1] root
|   [2] z <= 0.70311: n = 704
|       (Intercept)           x 
|        -0.1619978  -0.7896293 
|   [3] z > 0.70311: n = 296
|       (Intercept)           x 
|        0.08683535  0.65598287 

Number of inner nodes:    1
Number of terminal nodes: 2
Number of parameters per node: 2
Objective function: 9635.95
> (m_lm2 <- lmtree(formula = fmla, data = d))
Linear model tree

Model formula:
y ~ x | z + z_noise

Fitted party:
[1] root
|   [2] z <= 0.70311: n = 704
|       (Intercept)           x 
|        -0.1619978  -0.7896293 
|   [3] z > 0.70311: n = 296
|       (Intercept)           x 
|        0.08683535  0.65598287 

Number of inner nodes:    1
Number of terminal nodes: 2
Number of parameters per node: 2
Objective function (residual sum of squares): 9635.95
> 
> 
> mods <- nodeapply(m_lm2, ids = nodeids(m_lm2, terminal = TRUE), function(x) x$info$object)
> sum(sapply(mods, function(x) sum(x$residuals^2)))
[1] 9635.95
> 
> 
> proc.time()
   user  system elapsed 
   1.40    0.18    1.56