Why does R drop attributes when subsetting?
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I had to spend about 1 hour yesterday because R did something completely unpredictable (for my taste). It dropped an attribute without a warning.
df <- data.frame(x = rep(c(1, 2), 20)) attr(df$x, "label") <- "This is clearly a label" df$x ## [1] 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 ## [36] 2 1 2 1 2 ## attr(,"label") ## [1] "This is clearly a label"
The label is clearly there. To my surprise, if I subset this data frame, R drops the attribute.
new_df <- df[df$x == 2, , drop = FALSE] new_df$x ## [1] 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
It doesn’t matter if it’s using bracket subsetting ([) or subset.
new_df <- subset(df, x == 2) new_df$x ## [1] 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2
That’s not good. R’s dropping attributes silently. For my specific purpose I ended up using dplyr::filter which safely enough preserves attributes.
library(dplyr) df %>% filter(df, x == 2) %>% pull(x) ## [1] 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 ## attr(,"label") ## [1] "This is clearly a label"
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