Sampling rows from dataframe and saving unselected ones

Question

I'm triying to randomly choose rows from a dataframe; however, I also need unselected ones. I proceeded as follows

#grades is data with size 100
half-grades = grades[sample(nrow(grades), 50), ]

How do I save unselected ones?

Answer 1

Assuming you're looking for an even 50% split, the code is as follows.

library(caTools)
split = sample.split(grades,SplitRatio=0.5)
half-grades1 = subset(grades, split == TRUE)
half-grades2 = subset(grades, split == FALSE)

Answer 2

This is easily accomplished with caret::createDataPartition().

library(caret)
theIndex <- createDataPartition(grades, p = .50,list=FALSE)
sampled <- grades[theIndex,]
notSampled <- grades[-theIndex,]

Answer 3

set.seed(1234)
half-grades=grades[-(sample(nrow(grades), 50)), ]