How to exclude values from random selection-CodePudding

This is independent but related to this question Randomly take wrong answers of a quiz question from dataframe column, instead of doing by hand

Using the mtcars dataset I now have managed to randomly select one value from a certain column: In this example from cyl column.

mtcars[sample(1:nrow(mtcars), 1),2]

This code will give randomly [1] 6 or [1] 8 or ...

Now I want to exclude one certain value to be chosen, in this example say cyl==8. I would store the value in a vector like:

not_select <- 8

mtcars[sample(1:nrow(mtcars), 1),2]

My question: How can I integrate not_select in mtcars[sample(1:nrow(mtcars), 1),2]

Expected Output: The random sample should not include 8

UPDATE: e.g. the output should be: 6 or 4

UPDATE II due to unclear situation:

I want to select from column cyl one value randomly. This value should not be for example 8. So the value will be 4 or 6.

Explanation: 8 is the correct answer. And I am constructing randomly false answers with the other values (e.g. 4 and 6) from cyl column.

CodePudding user response：

Perhaps, another way -

tmp <- mtcars[, 2] 
sample(tmp[tmp != not_select], 1)

The above gives the probability of selecting each value based on their occurrence in the dataset. If you want the probability to be equal irrespective of how many times they occur you may only consider unique values.

tmp <- unique(mtcars[, 2])
sample(tmp[tmp != not_select], 1)

CodePudding user response：

Couldn‘t you just add a filtering condition based on not_select?

mtcars[sample(1:nrow(mtcars), 1) & mtcars$cyl != not_select, 2]

Update: how about:

not_select <- 8

draw_cyl <- sample(unique(mtcars$cyl[mtcars$cyl != not_select]), 1)

mtcars %>%
  filter(cyl == draw_cyl) %>%
  slice_sample(n = 1) %>%
  pull(cyl)

Or as suggested by TarJae themselve (so I don‘t own any credit for it!):

mtcars[sample(which (mtcars[,2] != not_select), 1), 2]

CodePudding user response：

This recursive function will run on itself again if the output matches not_selected.

exclude_not_selected <- function(not_selected) {
  value <- mtcars[sample(1:nrow(mtcars), 1),2] 
  if (value == not_selected) {
    exclude_not_selected(not_selected)
  } else {
    return(value)
  }
}

exclude_not_selected(8)
[1] 4