subsetting data frame by row index

Question

Why is my last step converting the data frame to a vector? I want to keep the first 6000 observations in the data frame key.

  set.seed(1)
  key         


        

Answer 1

It's being coerced to a vector basically because it can be and that's the default coercion when there's only 1 element. R is trying to be "helpful".

This will keep it as a dataframe:

set.seed(1)
key <- data.frame(matrix(NA, nrow = 10000, ncol = 1))
names(key) <- "ID"
key$ID <- replicate(10000, 
                      rawToChar(as.raw(sample(c(48:57,65:90,97:122), 8, replace=T))))
key <- unique(key)  
key <- as.data.frame(key[1:6000,]) # still a data frame

Answer 2

 key1 <- key[1:6000,,drop=F] #should prevent the data.frame from converting to a vector.

According to the documentation of ?Extract.data.frame

drop: logical. If ‘TRUE’ the result is coerced to the lowest possible dimension. The default is to drop if only one column is left, but not to drop if only one row is left.

Or, you could use subset, but usually, this is a bit slower. Here the row.names are numbers from 1 to 10000

 key2 <- subset(key, as.numeric(rownames(key)) <6000)

 is.data.frame(key2)
 #[1] TRUE

because,

 ## S3 method for class 'data.frame'
 subset(x, subset, select, drop = FALSE, ...) #by default it uses drop=F