r - 按行索引子集数据帧

Question

为什么我的最后一步是将数据框转换为向量？我想保留数据框中的前 6000 个观察值 key .

  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)  # still a data frame
  key <- key[1:6000,] # no longer a data frame

Answer 1

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

根据 ?Extract.data.frame 的文档

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.

或者，您可以使用 subset ，但通常情况下，这会慢一些。这里的 row.names 是来自 1 的数字至 10000

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

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

因为，

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