PCA : Can I reverse the axis of the first principal component in R?

Question

Here is a reproducible example :

set.seed(10)
pick <- sample(nrow(iris),nrow(iris)/2)
iris.training <- iris[pick,] 
iris.testing <- iris[-pick,]
pca.training <- prcomp(iris.training[-5])
pca.testing <- prcomp(iris.testing[-5])
autoplot(pca.training,loadings.label=T,loadings=T)
autoplot(pca.testing,loadings.label=T,loadings=T)

Which produces the following output :

As one can see, pca on data.training and on data.testing produces very similar biplots but the first principal components has reversed its sign, they are mirrored. Is it possible to force a 180 degree rotation on the two components ?

Answer 1

You are not returning the rotated variables. Changed code is as below. Notice retx=TRUE

set.seed(10)
pick <- sample(nrow(iris),nrow(iris)/2)
iris.training <- iris[pick,] 
iris.testing <- iris[-pick,]
pca.training <- prcomp(iris.training[-5], retx=TRUE)
pca.testing <- prcomp(iris.testing[-5], retx=TRUE)
autoplot(pca.training,loadings.label=TRUE,loadings=TRUE)
autoplot(pca.testing,loadings.label=TRUE,loadings=TRUE)

It produced the following outputs for training and testing.