R Under development (unstable) (2024-06-07 r86704 ucrt) -- "Unsuffered Consequences"
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Platform: x86_64-w64-mingw32/x64

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> library(leaps)
> set.seed(1)
> x = rnorm(100)
> e = rnorm(100)
> y = 1 + 1 * x + 1 * x^2 + 1 * x^3 + e
> fakedata <- y
> for (i in 1:10) {
+   newvar = x^i
+   fakedata <- cbind(fakedata,newvar)
+   colnames(fakedata)[i+1] <- paste0("x",i)
+ }
> fakedata <- data.frame(fakedata)
> colnames(fakedata)[1] <- c("y")
> 
> regfit.backward = regsubsets(y~.,data=fakedata,nvmax=10,method="backward")
> 
> rightwhich<-structure(c(TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, 
+ TRUE, FALSE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, 
+ TRUE, FALSE, FALSE, FALSE, FALSE, FALSE, FALSE, FALSE, FALSE, 
+ TRUE, TRUE, FALSE, FALSE, FALSE, FALSE, FALSE, FALSE, FALSE, 
+ FALSE, FALSE, TRUE, FALSE, FALSE, TRUE, TRUE, TRUE, TRUE, TRUE, 
+ TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, 
+ TRUE, TRUE, FALSE, FALSE, FALSE, TRUE, TRUE, TRUE, TRUE, TRUE, 
+ TRUE, TRUE, FALSE, FALSE, FALSE, FALSE, TRUE, TRUE, TRUE, TRUE, 
+ TRUE, TRUE, FALSE, FALSE, FALSE, FALSE, FALSE, TRUE, TRUE, TRUE, 
+ TRUE, TRUE, FALSE, FALSE, FALSE, FALSE, FALSE, FALSE, FALSE, 
+ TRUE, TRUE, TRUE, FALSE, FALSE, FALSE, FALSE, FALSE, FALSE, TRUE, 
+ TRUE, TRUE, TRUE), .Dim = 10:11, .Dimnames = list(c("1", "2", 
+ "3", "4", "5", "6", "7", "8", "9", "10"), c("(Intercept)", "x1", 
+ "x2", "x3", "x4", "x5", "x6", "x7", "x8", "x9", "x10")))
> 
> stopifnot(all.equal(summary(regfit.backward)$which, rightwhich))
> 
> m<-lm(y~x1+x4+x5,data=fakedata)
> 
> stopifnot(all.equal(coef(m),coef(regfit.backward,3)))
> stopifnot(all.equal(vcov(m),vcov(regfit.backward,3)))
> 
> 
> proc.time()
   user  system elapsed 
   0.14    0.06    0.18